# libpq - C Library

## Introduction

libpq is the C application programmer's interface to PostgreSQL. libpq is a set of library routines that allow client programs to pass queries to the PostgreSQL backend server and to receive the results of these queries. libpq is also the underlying engine for several other PostgreSQL application interfaces, including libpq++ (C++), `libpgtcl` (Tcl), Perl, and `ecpg`. So some aspects of libpq's behavior will be important to you if you use one of those packages.

Three short programs are included at the end of this section to show how to write programs that use `libpq`. There are several complete examples of `libpq` applications in the following directories:

- `src/test/examples`
- `src/bin/psql`

Frontend programs that use `libpq` must include the header file `libpq-fe.h` and must link with the `libpq` library.

## Database Connection Functions

The following routines deal with making a connection to a PostgreSQL backend server. The application program can have several backend connections open at one time. (One reason to do that is to access more than one database.) Each connection is represented by a PGconn object which is obtained from `PQconnectdb` or `PQsetdbLogin`. Note that these functions will always return a non-null object pointer, unless perhaps there is too little memory even to allocate the PGconn object. The `PQstatus` function should be called to check whether a connection was successfully made before queries are sent via the connection object.

- `PQconnectdb` Makes a new connection to the database server.

      PGconn *PQconnectdb(const char *conninfo)

  This routine opens a new database connection using the parameters taken from the string `conninfo`. Unlike `PQsetdbLogin` below, the parameter set can be extended without changing the function signature, so use either of this routine or the nonblocking analogues `PQconnectStart` and `PQconnectPoll` is preferred for application programming. The passed string can be empty to use all default parameters, or it can contain one or more parameter settings separated by whitespace.

  Each parameter setting is in the form `keyword = value`. (To write an empty value or a value containing spaces, surround it with single quotes, e.g., `keyword = 'a value'`. Single quotes and backslashes within the value must be escaped with a backslash, e.g., `\'` or `\\`.) Spaces around the equal sign are optional. The currently recognized parameter keywords are:

  `host`

- Name of host to connect to. If this begins with a slash, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored. The default is to connect to a Unix-domain socket in `/tmp`.

`hostaddr`

- IP address of host to connect to. This should be in standard numbers-and-dots form, as used by the BSD functions `inet_aton` et al. If a nonzero-length string is specified, TCP/IP communication is used.

  Using `hostaddr` instead of host allows the application to avoid a host name look-up, which may be important in applications with time constraints. However, Kerberos authentication requires the host name. The following therefore applies. If host is specified without `hostaddr`, a host name lookup is forced. If `hostaddr` is specified without host, the value for `hostaddr` gives the remote address; if Kerberos is used, this causes a reverse name query. If both host and `hostaddr` are specified, the value for `hostaddr` gives the remote address; the value for host is ignored, unless Kerberos is used, in which case that value is used for Kerberos authentication. Note that authentication is likely to fail if libpq is passed a host name that is not the name of the machine at `hostaddr`.

  Without either a host name or host address, libpq will connect using a local Unix domain socket.

`port`

- Port number to connect to at the server host, or socket file name extension for Unix-domain connections.

`dbname`

- The database name.

`user`

- User name to connect as.

`password`

- Password to be used if the server demands password authentication.

`connect_timeout`

- Time space in seconds given to connect routine. Zero or not set means infinite.

`options`

- Trace/debug options to be sent to the server.

`tty`

- A file or tty for optional debug output from the backend.

`requiressl`

- Set to 1 to require SSL connection to the server. Libpq will then refuse to connect if the server does not accept an SSL connection. Set to 0 (default) to negotiate with server. This option is only available if PostgreSQL is compiled with SSL support.

If any parameter is unspecified, then the corresponding environment variable (see libpq-envars) is checked. If the environment variable is not set either, then hardwired defaults are used. The return value is a pointer to an abstract `struct` representing the connection to the backend.

- `PQsetdbLogin` Makes a new connection to the database server.

      PGconn *PQsetdbLogin(const char *pghost,
                           const char *pgport,
                           const char *pgoptions,
                           const char *pgtty,
                           const char *dbName,
                           const char *login,
                           const char *pwd)

  This is the predecessor of `PQconnectdb` with a fixed number of parameters but the same functionality.

- `PQsetdb` Makes a new connection to the database server.

      PGconn *PQsetdb(char *pghost,
                      char *pgport,
                      char *pgoptions,
                      char *pgtty,
                      char *dbName)

  This is a macro that calls `PQsetdbLogin` with null pointers for the `login` and `pwd` parameters. It is provided primarily for backward compatibility with old programs.

- `PQconnectStart`, `PQconnectPoll`
  Make a connection to the database server in a nonblocking manner.

      PGconn *PQconnectStart(const char *conninfo)

      PostgresPollingStatusType PQconnectPoll(PGconn *conn)

  These two routines are used to open a connection to a database server such that your application's thread of execution is not blocked on remote I/O whilst doing so.

  The database connection is made using the parameters taken from the string `conninfo`, passed to `PQconnectStart`. This string is in the same format as described above for `PQconnectdb`.

  Neither `PQconnectStart` nor `PQconnectPoll` will block, as long as a number of restrictions are met:

  - The `hostaddr` and `host` parameters are used appropriately to ensure that name and reverse name queries are not made. See the documentation of these parameters under `PQconnectdb` above for details.

  - If you call `PQtrace`, ensure that the stream object into which you trace will not block.

  - You ensure for yourself that the socket is in the appropriate state before calling `PQconnectPoll`, as described below.

  To begin, call `conn=PQconnectStart("`connection_info_string`")`. If `conn` is NULL, then libpq has been unable to allocate a new PGconn structure. Otherwise, a valid PGconn pointer is returned (though not yet representing a valid connection to the database). On return from `PQconnectStart`, call `status=PQstatus(conn)`. If status equals CONNECTION_BAD, `PQconnectStart` has failed.

  If `PQconnectStart` succeeds, the next stage is to poll libpq so that it may proceed with the connection sequence. Loop thus: Consider a connection inactive by default. If `PQconnectPoll` last returned PGRES_POLLING_ACTIVE, consider it active instead. If `PQconnectPoll(conn)` last returned PGRES_POLLING_READING, perform a `select()` for reading on `PQsocket(conn)`. If it last returned PGRES_POLLING_WRITING, perform a `select()` for writing on `PQsocket(conn)`. If you have yet to call `PQconnectPoll`, i.e. after the call to `PQconnectStart`, behave as if it last returned PGRES_POLLING_WRITING. If the `select()` shows that the socket is ready, consider it active. If it has been decided that this connection is active, call `PQconnectPoll(conn)` again. If this call returns PGRES_POLLING_FAILED, the connection procedure has failed. If this call returns PGRES_POLLING_OK, the connection has been successfully made.

  Note that the use of `select()` to ensure that the socket is ready is merely a (likely) example; those with other facilities available, such as a `poll()` call, may of course use that instead.

  At any time during connection, the status of the connection may be checked, by calling `PQstatus`. If this is CONNECTION_BAD, then the connection procedure has failed; if this is `CONNECTION_OK`, then the connection is ready. Either of these states should be equally detectable from the return value of `PQconnectPoll`, as above. Other states may be shown during (and only during) an asynchronous connection procedure. These indicate the current stage of the connection procedure, and may be useful to provide feedback to the user for example. These statuses may include:

  CONNECTION_STARTED

- Waiting for connection to be made.

CONNECTION_MADE

- Connection OK; waiting to send.

CONNECTION_AWAITING_RESPONSE

- Waiting for a response from the server.

CONNECTION_AUTH_OK

- Received authentication; waiting for connection start-up to continue.

CONNECTION_SETENV

- Negotiating environment (part of the connection start-up).

Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these appearing in a particular order, or at all, or on the status always being one of these documented values. An application may do something like this:

    switch(PQstatus(conn))
    {
        case CONNECTION_STARTED:
            feedback = "Connecting...";
            break;

        case CONNECTION_MADE:
            feedback = "Connected to server...";
            break;
    .
    .
    .
        default:
            feedback = "Connecting...";
    }

Note that if `PQconnectStart` returns a non-NULL pointer, you must call `PQfinish` when you are finished with it, in order to dispose of the structure and any associated memory blocks. This must be done even if a call to `PQconnectStart` or `PQconnectPoll` failed.

`PQconnectPoll` will currently block if libpq is compiled with USE_SSL defined. This restriction may be removed in the future.

These functions leave the socket in a nonblocking state as if `PQsetnonblocking` had been called.

- `PQconndefaults` Returns the default connection options.

      PQconninfoOption *PQconndefaults(void)

      struct PQconninfoOption
      {
          char   *keyword;   /* The keyword of the option */
          char   *envvar;    /* Fallback environment variable name */
          char   *compiled;  /* Fallback compiled in default value */
          char   *val;       /* Option's current value, or NULL */
          char   *label;     /* Label for field in connect dialog */
          char   *dispchar;  /* Character to display for this field
                                in a connect dialog. Values are:
                                ""        Display entered value as is
                                "*"       Password field - hide value
                                "D"       Debug option - don't show by default */
          int     dispsize;  /* Field size in characters for dialog */
      }

  Returns a connection options array. This may be used to determine all possible `PQconnectdb` options and their current default values. The return value points to an array of PQconninfoOption `struct`s, which ends with an entry having a NULL keyword pointer. Note that the default values (val fields) will depend on environment variables and other context. Callers must treat the connection options data as read-only.

  After processing the options array, free it by passing it to `PQconninfoFree`. If this is not done, a small amount of memory is leaked for each call to `PQconndefaults`.

  In PostgreSQL versions before 7.0, `PQconndefaults` returned a pointer to a static array, rather than a dynamically allocated array. That was not thread-safe, so the behavior has been changed.

- `PQfinish` Close the connection to the backend. Also frees memory used by the PGconn object.

      void PQfinish(PGconn *conn)

  Note that even if the backend connection attempt fails (as indicated by `PQstatus`), the application should call `PQfinish` to free the memory used by the PGconn object. The PGconn pointer should not be used after `PQfinish` has been called.

- `PQreset` Reset the communication port with the backend.

      void PQreset(PGconn *conn)

  This function will close the connection to the backend and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost.

- `PQresetStart` `PQresetPoll` Reset the communication port with the backend, in a nonblocking manner.

      int PQresetStart(PGconn *conn);

      PostgresPollingStatusType PQresetPoll(PGconn *conn);

  These functions will close the connection to the backend and attempt to reestablish a new connection to the same server, using all the same parameters previously used. This may be useful for error recovery if a working connection is lost. They differ from `PQreset` (above) in that they act in a nonblocking manner. These functions suffer from the same restrictions as `PQconnectStart` and `PQconnectPoll`.

  Call `PQresetStart`. If it returns 0, the reset has failed. If it returns 1, poll the reset using `PQresetPoll` in exactly the same way as you would create the connection using `PQconnectPoll`.

libpq application programmers should be careful to maintain the PGconn abstraction. Use the accessor functions below to get at the contents of PGconn. Avoid directly referencing the fields of the PGconn structure because they are subject to change in the future. (Beginning in PostgreSQL release 6.4, the definition of `struct PGconn` is not even provided in `libpq-fe.h`. If you have old code that accesses PGconn fields directly, you can keep using it by including `libpq-int.h` too, but you are encouraged to fix the code soon.)

- `PQdb` Returns the database name of the connection.

      char *PQdb(const PGconn *conn)

  `PQdb` and the next several functions return the values established at connection. These values are fixed for the life of the PGconn object.

- `PQuser` Returns the user name of the connection.

      char *PQuser(const PGconn *conn)

- `PQpass` Returns the password of the connection.

      char *PQpass(const PGconn *conn)

- `PQhost` Returns the server host name of the connection.

      char *PQhost(const PGconn *conn)

- `PQport` Returns the port of the connection.

      char *PQport(const PGconn *conn)

- `PQtty` Returns the debug tty of the connection.

      char *PQtty(const PGconn *conn)

- `PQoptions` Returns the backend options used in the connection.

      char *PQoptions(const PGconn *conn)

- `PQstatus` Returns the status of the connection.

      ConnStatusType PQstatus(const PGconn *conn)

  The status can be one of a number of values. However, only two of these are seen outside of an asynchronous connection procedure - `CONNECTION_OK` or `CONNECTION_BAD`. A good connection to the database has the status `CONNECTION_OK`. A failed connection attempt is signaled by status `CONNECTION_BAD`. Ordinarily, an OK status will remain so until `PQfinish`, but a communications failure might result in the status changing to `CONNECTION_BAD` prematurely. In that case the application could try to recover by calling `PQreset`.

  See the entry for `PQconnectStart` and `PQconnectPoll` with regards to other status codes that might be seen.

- `PQerrorMessage`
  Returns the error message most recently generated by an operation on the connection.

      char *PQerrorMessage(const PGconn* conn);

  Nearly all libpq functions will set `PQerrorMessage` if they fail. Note that by libpq convention, a non-empty `PQerrorMessage` will include a trailing newline.

- `PQbackendPID` Returns the process ID of the backend server handling this connection.

      int PQbackendPID(const PGconn *conn);

  The backend PID is useful for debugging purposes and for comparison to NOTIFY messages (which include the PID of the notifying backend). Note that the PID belongs to a process executing on the database server host, not the local host!

- `PQgetssl`
  Returns the SSL structure used in the connection, or NULL if SSL is not in use.

      SSL *PQgetssl(const PGconn *conn);

  This structure can be used to verify encryption levels, check server certificate and more. Refer to the SSL documentation for information about this structure.

  You must define `USE_SSL` in order to get the prototype for this function. Doing this will also automatically include `ssl.h` from OpenSSL.

## Command Execution Functions

Once a connection to a database server has been successfully established, the functions described here are used to perform SQL queries and commands.

### Main Routines

- `PQexec` Submit a command to the server and wait for the result.

      PGresult *PQexec(PGconn *conn,
                       const char *query);

  Returns a PGresult pointer or possibly a NULL pointer. A non-NULL pointer will generally be returned except in out-of-memory conditions or serious errors such as inability to send the command to the backend. If a NULL is returned, it should be treated like a PGRES_FATAL_ERROR result. Use `PQerrorMessage` to get more information about the error.

The `PGresult` structure encapsulates the result returned by the backend. `libpq` application programmers should be careful to maintain the PGresult abstraction. Use the accessor functions below to get at the contents of PGresult. Avoid directly referencing the fields of the PGresult structure because they are subject to change in the future. (Beginning in PostgreSQL 6.4, the definition of `struct PGresult` is not even provided in `libpq-fe.h`. If you have old code that accesses PGresult fields directly, you can keep using it by including `libpq-int.h` too, but you are encouraged to fix the code soon.)

- `PQresultStatus` Returns the result status of the command.

      ExecStatusType PQresultStatus(const PGresult *res)

  `PQresultStatus` can return one of the following values:

  - `PGRES_EMPTY_QUERY` -- The string sent to the backend was empty.

  - `PGRES_COMMAND_OK` -- Successful completion of a command returning no data

  - `PGRES_TUPLES_OK` -- The query successfully executed

  - `PGRES_COPY_OUT` -- Copy Out (from server) data transfer started

  - `PGRES_COPY_IN` -- Copy In (to server) data transfer started

  - `PGRES_BAD_RESPONSE` -- The server's response was not understood

  - `PGRES_NONFATAL_ERROR`

  - `PGRES_FATAL_ERROR`

  If the result status is `PGRES_TUPLES_OK`, then the routines described below can be used to retrieve the rows returned by the query. Note that a SELECT command that happens to retrieve zero rows still shows `PGRES_TUPLES_OK`. `PGRES_COMMAND_OK` is for commands that can never return rows (INSERT, UPDATE, etc.). A response of `PGRES_EMPTY_QUERY` often exposes a bug in the client software.

- `PQresStatus` Converts the enumerated type returned by `PQresultStatus` into a string constant describing the status code.

      char *PQresStatus(ExecStatusType status);

- `PQresultErrorMessage` returns the error message associated with the query, or an empty string if there was no error.

      char *PQresultErrorMessage(const PGresult *res);

  Immediately following a `PQexec` or `PQgetResult` call, `PQerrorMessage` (on the connection) will return the same string as `PQresultErrorMessage` (on the result). However, a PGresult will retain its error message until destroyed, whereas the connection's error message will change when subsequent operations are done. Use `PQresultErrorMessage` when you want to know the status associated with a particular PGresult; use `PQerrorMessage` when you want to know the status from the latest operation on the connection.

- `PQclear` Frees the storage associated with the PGresult. Every query result should be freed via `PQclear` when it is no longer needed.

      void PQclear(PQresult *res);

  You can keep a PGresult object around for as long as you need it; it does not go away when you issue a new query, nor even if you close the connection. To get rid of it, you must call `PQclear`. Failure to do this will result in memory leaks in the frontend application.

- `PQmakeEmptyPGresult` Constructs an empty PGresult object with the given status.

      PGresult* PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);

  This is libpq's internal routine to allocate and initialize an empty PGresult object. It is exported because some applications find it useful to generate result objects (particularly objects with error status) themselves. If `conn` is not NULL and status indicates an error, the connection's current error message is copied into the PGresult. Note that `PQclear` should eventually be called on the object, just as with a PGresult returned by libpq itself.

### Escaping strings for inclusion in SQL queries

`PQescapeString` Escapes a string for use within an SQL query.

    size_t PQescapeString (char *to, const char *from, size_t length);

If you want to include strings that have been received from a source that is not trustworthy (for example, because a random user entered them), you cannot directly include them in SQL queries for security reasons. Instead, you have to quote special characters that are otherwise interpreted by the SQL parser.

`PQescapeString` performs this operation. The `from` points to the first character of the string that is to be escaped, and the `length` parameter counts the number of characters in this string (a terminating zero byte is neither necessary nor counted). `to` shall point to a buffer that is able to hold at least one more character than twice the value of `length`, otherwise the behavior is undefined. A call to `PQescapeString` writes an escaped version of the `from` string to the `to` buffer, replacing special characters so that they cannot cause any harm, and adding a terminating zero byte. The single quotes that must surround PostgreSQL string literals are not part of the result string.

`PQescapeString` returns the number of characters written to `to`, not including the terminating zero byte. Behavior is undefined when the `to` and `from` strings overlap.

### Escaping binary strings for inclusion in SQL queries

`PQescapeBytea` Escapes a binary string (`bytea` type) for use within an SQL query.

        unsigned char *PQescapeBytea(unsigned char *from,
                                             size_t from_length,
                                             size_t *to_length);

Certain ASCII characters must be escaped (but all characters may be escaped) when used as part of a `bytea` string literal in an SQL statement. In general, to escape a character, it is converted into the three digit octal number equal to the decimal ASCII value, and preceded by two backslashes. The single quote (') and backslash (\) characters have special alternate escape sequences. See the User's Guide for more information. `PQescapeBytea` performs this operation, escaping only the minimally required characters.

The `from` parameter points to the first character of the string that is to be escaped, and the `from_length` parameter reflects the number of characters in this binary string (a terminating zero byte is neither necessary nor counted). The `to_length` parameter shall point to a buffer suitable to hold the resultant escaped string length. The result string length includes the terminating zero byte of the result.

`PQescapeBytea` returns an escaped version of the `from` parameter binary string, to a caller-provided buffer. The return string has all special characters replaced so that they can be properly processed by the PostgreSQL string literal parser, and the `bytea` input function. A terminating zero byte is also added. The single quotes that must surround PostgreSQL string literals are not part of the result string.

`PQunescapeBytea` Converts an escaped string representation of binary data into binary data - the reverse of `PQescapeBytea`.

        unsigned char *PQunescapeBytea(unsigned char *from, size_t *to_length);

The `from` parameter points to an escaped string such as might be returned by `PQgetvalue` of a `BYTEA` column. `PQunescapeBytea` converts this string representation into its binary representation, filling the supplied buffer. It returns a pointer to the buffer which is NULL on error, and the size of the buffer in `to_length`. The pointer may subsequently be used as an argument to the function `free(3)`.

### Retrieving SELECT Result Information

- `PQntuples` Returns the number of tuples (rows) in the query result.

      int PQntuples(const PGresult *res);

- `PQnfields` Returns the number of fields (columns) in each row of the query result.

      int PQnfields(const PGresult *res);

- `PQfname` Returns the field (column) name associated with the given field index. Field indices start at 0.

      char *PQfname(const PGresult *res,
                          int field_index);

- `PQfnumber` Returns the field (column) index associated with the given field name.

      int PQfnumber(const PGresult *res,
                    const char *field_name);

  -1 is returned if the given name does not match any field.

- `PQftype` Returns the field type associated with the given field index. The integer returned is an internal coding of the type. Field indices start at 0.

      Oid PQftype(const PGresult *res,
                  int field_index);

  You can query the system table `pg_type` to obtain the name and properties of the various data types. The OIDs of the built-in data types are defined in `src/include/catalog/pg_type.h` in the source tree.

- `PQfmod` Returns the type-specific modification data of the field associated with the given field index. Field indices start at 0.

      int PQfmod(const PGresult *res,
                 int field_index);

- `PQfsize` Returns the size in bytes of the field associated with the given field index. Field indices start at 0.

      int PQfsize(const PGresult *res,
                  int field_index);

  `PQfsize` returns the space allocated for this field in a database tuple, in other words the size of the server's binary representation of the data type. -1 is returned if the field is variable size.

- `PQbinaryTuples` Returns 1 if the PGresult contains binary tuple data, 0 if it contains ASCII data.

      int PQbinaryTuples(const PGresult *res);

  Currently, binary tuple data can only be returned by a query that extracts data from a binary cursor.

### Retrieving SELECT Result Values

- `PQgetvalue` Returns a single field (column) value of one tuple (row) of a PGresult. Tuple and field indices start at 0.

      char* PQgetvalue(const PGresult *res,
                       int tup_num,
                       int field_num);

  For most queries, the value returned by `PQgetvalue` is a null-terminated character string representation of the attribute value. But if `PQbinaryTuples()` is 1, the value returned by `PQgetvalue` is the binary representation of the type in the internal format of the backend server (but not including the size word, if the field is variable-length). It is then the programmer's responsibility to cast and convert the data to the correct C type. The pointer returned by `PQgetvalue` points to storage that is part of the PGresult structure. One should not modify it, and one must explicitly copy the value into other storage if it is to be used past the lifetime of the PGresult structure itself.

- `PQgetisnull` Tests a field for a NULL entry. Tuple and field indices start at 0.

      int PQgetisnull(const PGresult *res,
                      int tup_num,
                      int field_num);

  This function returns 1 if the field contains a NULL, 0 if it contains a non-null value. (Note that `PQgetvalue` will return an empty string, not a null pointer, for a NULL field.)

- `PQgetlength` Returns the length of a field (attribute) value in bytes. Tuple and field indices start at 0.

      int PQgetlength(const PGresult *res,
                      int tup_num,
                      int field_num);

  This is the actual data length for the particular data value, that is the size of the object pointed to by `PQgetvalue`. Note that for character-represented values, this size has little to do with the binary size reported by `PQfsize`.

- `PQprint` Prints out all the tuples and, optionally, the attribute names to the specified output stream.

      void PQprint(FILE* fout,      /* output stream */
                   const PGresult *res,
                   const PQprintOpt *po);

      struct {
          pqbool  header;      /* print output field headings and row count */
          pqbool  align;       /* fill align the fields */
          pqbool  standard;    /* old brain dead format */
          pqbool  html3;       /* output html tables */
          pqbool  expanded;    /* expand tables */
          pqbool  pager;       /* use pager for output if needed */
          char    *fieldSep;   /* field separator */
          char    *tableOpt;   /* insert to HTML table ... */
          char    *caption;    /* HTML caption */
          char    **fieldName; /* null terminated array of replacement field names */
      } PQprintOpt;

  This function was formerly used by psql to print query results, but this is no longer the case and this function is no longer actively supported.

### Retrieving Non-SELECT Result Information

- `PQcmdStatus` Returns the command status string from the SQL command that generated the PGresult.

      char * PQcmdStatus(PGresult *res);

- `PQcmdTuples` Returns the number of rows affected by the SQL command.

      char * PQcmdTuples(PGresult *res);

  If the SQL command that generated the PGresult was INSERT, UPDATE or DELETE, this returns a string containing the number of rows affected. If the command was anything else, it returns the empty string.

- `PQoidValue` Returns the object ID of the inserted row, if the SQL command was an INSERT that inserted exactly one row into a table that has OIDs. Otherwise, returns `InvalidOid`.

      Oid PQoidValue(const PGresult *res);

  The type `Oid` and the constant `InvalidOid` will be defined if you include the libpq header file. They will both be some integer type.

- `PQoidStatus` Returns a string with the object ID of the inserted row, if the SQL command was an INSERT. (The string will be `0` if the INSERT did not insert exactly one row, or if the target table does not have OIDs.) If the command was not an INSERT, returns an empty string.

      char * PQoidStatus(const PGresult *res);

  This function is deprecated in favor of `PQoidValue` and is not thread-safe.

## Asynchronous Query Processing

The `PQexec` function is adequate for submitting commands in simple synchronous applications. It has a couple of major deficiencies however:

- `PQexec` waits for the command to be completed. The application may have other work to do (such as maintaining a user interface), in which case it won't want to block waiting for the response.

- Since control is buried inside `PQexec`, it is hard for the frontend to decide it would like to try to cancel the ongoing command. (It can be done from a signal handler, but not otherwise.)

- `PQexec` can return only one PGresult structure. If the submitted command string contains multiple SQL commands, all but the last PGresult are discarded by `PQexec`.

Applications that do not like these limitations can instead use the underlying functions that `PQexec` is built from: `PQsendQuery` and `PQgetResult`.

Older programs that used this functionality as well as `PQputline` and `PQputnbytes` could block waiting to send data to the backend. To address that issue, the function `PQsetnonblocking` was added.

Old applications can neglect to use `PQsetnonblocking` and get the older potentially blocking behavior. Newer programs can use `PQsetnonblocking` to achieve a completely nonblocking connection to the backend.

- `PQsetnonblocking` Sets the nonblocking status of the connection.

      int PQsetnonblocking(PGconn *conn, int arg)

  Sets the state of the connection to nonblocking if `arg` is 1, blocking if `arg` is 0. Returns 0 if OK, -1 if error.

  In the nonblocking state, calls to `PQputline`, `PQputnbytes`, `PQsendQuery` and `PQendcopy` will not block but instead return an error if they need to be called again.

  When a database connection has been set to nonblocking mode and `PQexec` is called, it will temporarily set the state of the connection to blocking until the `PQexec` completes.

  More of libpq is expected to be made safe for `PQsetnonblocking` functionality in the near future.

- `PQisnonblocking` Returns the blocking status of the database connection.

      int PQisnonblocking(const PGconn *conn)

  Returns 1 if the connection is set to nonblocking mode, 0 if blocking.

- `PQsendQuery` Submit a command to the server without waiting for the result(s). 1 is returned if the command was successfully dispatched, 0 if not (in which case, use `PQerrorMessage` to get more information about the failure).

      int PQsendQuery(PGconn *conn,
                      const char *query);

  After successfully calling `PQsendQuery`, call `PQgetResult` one or more times to obtain the results. `PQsendQuery` may not be called again (on the same connection) until `PQgetResult` has returned NULL, indicating that the command is done.

- `PQgetResult` Wait for the next result from a prior `PQsendQuery`, and return it. NULL is returned when the query is complete and there will be no more results.

      PGresult *PQgetResult(PGconn *conn);

  `PQgetResult` must be called repeatedly until it returns NULL, indicating that the command is done. (If called when no command is active, `PQgetResult` will just return NULL at once.) Each non-NULL result from `PQgetResult` should be processed using the same PGresult accessor functions previously described. Don't forget to free each result object with `PQclear` when done with it. Note that `PQgetResult` will block only if a query is active and the necessary response data has not yet been read by `PQconsumeInput`.

Using `PQsendQuery` and `PQgetResult` solves one of `PQexec`'s problems: If a command string contains multiple SQL commands, the results of those commands can be obtained individually. (This allows a simple form of overlapped processing, by the way: the frontend can be handling the results of one query while the backend is still working on later queries in the same command string.) However, calling `PQgetResult` will still cause the frontend to block until the backend completes the next SQL command. This can be avoided by proper use of three more functions:

- `PQconsumeInput` If input is available from the backend, consume it.

      int PQconsumeInput(PGconn *conn);

  `PQconsumeInput` normally returns 1 indicating no error, but returns 0 if there was some kind of trouble (in which case `PQerrorMessage` is set). Note that the result does not say whether any input data was actually collected. After calling `PQconsumeInput`, the application may check `PQisBusy` and/or `PQnotifies` to see if their state has changed.

  `PQconsumeInput` may be called even if the application is not prepared to deal with a result or notification just yet. The routine will read available data and save it in a buffer, thereby causing a `select()` read-ready indication to go away. The application can thus use `PQconsumeInput` to clear the `select()` condition immediately, and then examine the results at leisure.

- `PQisBusy` Returns 1 if a query is busy, that is, `PQgetResult` would block waiting for input. A 0 return indicates that `PQgetResult` can be called with assurance of not blocking.

      int PQisBusy(PGconn *conn);

  `PQisBusy` will not itself attempt to read data from the backend; therefore `PQconsumeInput` must be invoked first, or the busy state will never end.

- `PQflush` Attempt to flush any data queued to the backend, returns 0 if successful (or if the send queue is empty) or EOF if it failed for some reason.

      int PQflush(PGconn *conn);

  `PQflush` needs to be called on a nonblocking connection before calling `select()` to determine if a response has arrived. If 0 is returned it ensures that there is no data queued to the backend that has not actually been sent. Only applications that have used `PQsetnonblocking` have a need for this.

- `PQsocket` Obtain the file descriptor number for the backend connection socket. A valid descriptor will be >= 0; a result of -1 indicates that no backend connection is currently open.

      int PQsocket(const PGconn *conn);

  `PQsocket` should be used to obtain the backend socket descriptor in preparation for executing `select()`. This allows an application using a blocking connection to wait for either backend responses or other conditions. If the result of `select()` indicates that data can be read from the backend socket, then `PQconsumeInput` should be called to read the data; after which, `PQisBusy`, `PQgetResult`, and/or `PQnotifies` can be used to process the response.

  Nonblocking connections (that have used `PQsetnonblocking`) should not use `select()` until `PQflush` has returned 0 indicating that there is no buffered data waiting to be sent to the backend.

A typical frontend using these functions will have a main loop that uses `select` to wait for all the conditions that it must respond to. One of the conditions will be input available from the backend, which in `select`'s terms is readable data on the file descriptor identified by `PQsocket`. When the main loop detects input ready, it should call `PQconsumeInput` to read the input. It can then call `PQisBusy`, followed by `PQgetResult` if `PQisBusy` returns false (0). It can also call `PQnotifies` to detect NOTIFY messages (see libpq-notify).

A frontend that uses `PQsendQuery`/`PQgetResult` can also attempt to cancel a command that is still being processed by the backend.

- `PQrequestCancel` Request that PostgreSQL abandon processing of the current command.

      int PQrequestCancel(PGconn *conn);

  The return value is 1 if the cancel request was successfully dispatched, 0 if not. (If not, `PQerrorMessage` tells why not.) Successful dispatch is no guarantee that the request will have any effect, however. Regardless of the return value of `PQrequestCancel`, the application must continue with the normal result-reading sequence using `PQgetResult`. If the cancellation is effective, the current command will terminate early and return an error result. If the cancellation fails (say, because the backend was already done processing the command), then there will be no visible result at all.

Note that if the current command is part of a transaction, cancellation will abort the whole transaction.

`PQrequestCancel` can safely be invoked from a signal handler. So, it is also possible to use it in conjunction with plain `PQexec`, if the decision to cancel can be made in a signal handler. For example, psql invokes `PQrequestCancel` from a `SIGINT` signal handler, thus allowing interactive cancellation of queries that it issues through `PQexec`. Note that `PQrequestCancel` will have no effect if the connection is not currently open or the backend is not currently processing a command.

## The Fast-Path Interface

PostgreSQL provides a fast-path interface to send function calls to the backend. This is a trapdoor into system internals and can be a potential security hole. Most users will not need this feature.

- `PQfn` Request execution of a backend function via the fast-path interface.

      PGresult* PQfn(PGconn* conn,
                     int fnid,
                     int *result_buf,
                     int *result_len,
                     int result_is_int,
                     const PQArgBlock *args,
                     int nargs);

  The `fnid` argument is the object identifier of the function to be executed. `result_buf` is the buffer in which to place the return value. The caller must have allocated sufficient space to store the return value (there is no check!). The actual result length will be returned in the integer pointed to by `result_len`. If a 4-byte integer result is expected, set `result_is_int` to 1; otherwise set it to 0. (Setting `result_is_int` to 1 tells libpq to byte-swap the value if necessary, so that it is delivered as a proper int value for the client machine. When `result_is_int` is 0, the byte string sent by the backend is returned unmodified.) `args` and `nargs` specify the arguments to be passed to the function.

      typedef struct {
          int len;
          int isint;
          union {
              int *ptr;
              int integer;
          } u;
      } PQArgBlock;

  `PQfn` always returns a valid PGresult*. The result status should be checked before the result is used. The caller is responsible for freeing the PGresult with `PQclear` when it is no longer needed.

## Asynchronous Notification

PostgreSQL supports asynchronous notification via the `LISTEN` and `NOTIFY` commands. A backend registers its interest in a particular notification condition with the `LISTEN` command (and can stop listening with the `UNLISTEN` command). All backends listening on a particular condition will be notified asynchronously when a `NOTIFY` of that condition name is executed by any backend. No additional information is passed from the notifier to the listener. Thus, typically, any actual data that needs to be communicated is transferred through a database relation. Commonly the condition name is the same as the associated relation, but it is not necessary for there to be any associated relation.

`libpq` applications submit `LISTEN` and `UNLISTEN` commands as ordinary SQL command. Subsequently, arrival of `NOTIFY` messages can be detected by calling `PQnotifies`.

- `PQnotifies` Returns the next notification from a list of unhandled notification messages received from the backend. Returns NULL if there are no pending notifications. Once a notification is returned from `PQnotifies`, it is considered handled and will be removed from the list of notifications.

      PGnotify* PQnotifies(PGconn *conn);

      typedef struct pgNotify {
          char *relname;              /* name of relation containing data */
          int  be_pid;                /* process id of backend */
      } PGnotify;

  After processing a PGnotify object returned by `PQnotifies`, be sure to free it with `free()` to avoid a memory leak.

  In PostgreSQL 6.4 and later, the `be_pid` is that of the notifying backend, whereas in earlier versions it was always the PID of your own backend.

The second sample program gives an example of the use of asynchronous notification.

`PQnotifies()` does not actually read backend data; it just returns messages previously absorbed by another libpq function. In prior releases of libpq, the only way to ensure timely receipt of NOTIFY messages was to constantly submit queries, even empty ones, and then check `PQnotifies()` after each `PQexec()`. While this still works, it is deprecated as a waste of processing power.

A better way to check for NOTIFY messages when you have no useful queries to make is to call `PQconsumeInput()`, then check `PQnotifies()`. You can use `select()` to wait for backend data to arrive, thereby using no CPU power unless there is something to do. (See `PQsocket()` to obtain the file descriptor number to use with `select()`.) Note that this will work OK whether you submit queries with `PQsendQuery`/`PQgetResult` or simply use `PQexec`. You should, however, remember to check `PQnotifies()` after each `PQgetResult` or `PQexec`, to see if any notifications came in during the processing of the query.

## Functions Associated with the COPY Command

The COPY command in PostgreSQL has options to read from or write to the network connection used by `libpq`. Therefore, functions are necessary to access this network connection directly so applications may take advantage of this capability.

These functions should be executed only after obtaining a `PGRES_COPY_OUT` or `PGRES_COPY_IN` result object from `PQexec` or `PQgetResult`.

- `PQgetline` Reads a newline-terminated line of characters (transmitted by the backend server) into a buffer string of size length.

      int PQgetline(PGconn *conn,
                    char *string,
                    int length)

  Like `fgets`, this routine copies up to length-1 characters into string. It is like `gets`, however, in that it converts the terminating newline into a zero byte. `PQgetline` returns EOF at the end of input, 0 if the entire line has been read, and 1 if the buffer is full but the terminating newline has not yet been read.

  Notice that the application must check to see if a new line consists of the two characters `\.`, which indicates that the backend server has finished sending the results of the copy command. If the application might receive lines that are more than length-1 characters long, care is needed to be sure one recognizes the `\.` line correctly (and does not, for example, mistake the end of a long data line for a terminator line). The code in `src/bin/psql/copy.c` contains example routines that correctly handle the copy protocol.

- `PQgetlineAsync` Reads a newline-terminated line of characters (transmitted by the backend server) into a buffer without blocking.

      int PQgetlineAsync(PGconn *conn,
                         char *buffer,
                         int bufsize)

  This routine is similar to `PQgetline`, but it can be used by applications that must read COPY data asynchronously, that is without blocking. Having issued the COPY command and gotten a `PGRES_COPY_OUT` response, the application should call `PQconsumeInput` and `PQgetlineAsync` until the end-of-data signal is detected. Unlike `PQgetline`, this routine takes responsibility for detecting end-of-data. On each call, `PQgetlineAsync` will return data if a complete newline- terminated data line is available in libpq's input buffer, or if the incoming data line is too long to fit in the buffer offered by the caller. Otherwise, no data is returned until the rest of the line arrives.

  The routine returns -1 if the end-of-copy-data marker has been recognized, or 0 if no data is available, or a positive number giving the number of bytes of data returned. If -1 is returned, the caller must next call `PQendcopy`, and then return to normal processing. The data returned will not extend beyond a newline character. If possible a whole line will be returned at one time. But if the buffer offered by the caller is too small to hold a line sent by the backend, then a partial data line will be returned. This can be detected by testing whether the last returned byte is `\n` or not. The returned string is not null-terminated. (If you want to add a terminating null, be sure to pass a `bufsize` one smaller than the room actually available.)

- `PQputline` Sends a null-terminated string to the backend server. Returns 0 if OK, EOF if unable to send the string.

      int PQputline(PGconn *conn,
                    const char *string);

  Note the application must explicitly send the two characters `\.` on a final line to indicate to the backend that it has finished sending its data.

- `PQputnbytes` Sends a non-null-terminated string to the backend server. Returns 0 if OK, EOF if unable to send the string.

      int PQputnbytes(PGconn *conn,
                      const char *buffer,
                      int nbytes);

  This is exactly like `PQputline`, except that the data buffer need not be null-terminated since the number of bytes to send is specified directly.

- `PQendcopy` Synchronizes with the backend. This function waits until the backend has finished the copy. It should either be issued when the last string has been sent to the backend using `PQputline` or when the last string has been received from the backend using `PGgetline`. It must be issued or the backend may get out of sync with the frontend. Upon return from this function, the backend is ready to receive the next SQL command. The return value is 0 on successful completion, nonzero otherwise.

      int PQendcopy(PGconn *conn);

  As an example:

      PQexec(conn, "CREATE TABLE foo (a int4, b char(16), d double precision)");
      PQexec(conn, "COPY foo FROM STDIN");
      PQputline(conn, "3\thello world\t4.5\n");
      PQputline(conn,"4\tgoodbye world\t7.11\n");
      ...
      PQputline(conn,"\\.\n");
      PQendcopy(conn);

When using `PQgetResult`, the application should respond to a `PGRES_COPY_OUT` result by executing `PQgetline` repeatedly, followed by `PQendcopy` after the terminator line is seen. It should then return to the `PQgetResult` loop until `PQgetResult` returns NULL. Similarly a `PGRES_COPY_IN` result is processed by a series of `PQputline` calls followed by `PQendcopy`, then return to the `PQgetResult` loop. This arrangement will ensure that a copy in or copy out command embedded in a series of SQL commands will be executed correctly.

Older applications are likely to submit a copy in or copy out via `PQexec` and assume that the transaction is done after `PQendcopy`. This will work correctly only if the copy in/out is the only SQL command in the command string.

## libpq Tracing Functions

- `PQtrace` Enable tracing of the frontend/backend communication to a debugging file stream.

      void PQtrace(PGconn *conn
                   FILE *debug_port)

- `PQuntrace` Disable tracing started by `PQtrace`.

      void PQuntrace(PGconn *conn)

## libpq Control Functions

- `PQsetNoticeProcessor`
  Control reporting of notice and warning messages generated by libpq.

      typedef void (*PQnoticeProcessor) (void *arg, const char *message);

      PQnoticeProcessor
      PQsetNoticeProcessor(PGconn *conn,
                           PQnoticeProcessor proc,
                           void *arg);

By default, libpq prints notice messages from the backend on `stderr`, as well as a few error messages that it generates by itself. This behavior can be overridden by supplying a callback function that does something else with the messages. The callback function is passed the text of the error message (which includes a trailing newline), plus a void pointer that is the same one passed to `PQsetNoticeProcessor`. (This pointer can be used to access application-specific state if needed.) The default notice processor is simply

    static void
    defaultNoticeProcessor(void * arg, const char * message)
    {
        fprintf(stderr, "%s", message);
    }

To use a special notice processor, call `PQsetNoticeProcessor` just after creation of a new PGconn object.

The return value is the pointer to the previous notice processor. If you supply a callback function pointer of NULL, no action is taken, but the current pointer is returned.

Once you have set a notice processor, you should expect that that function could be called as long as either the PGconn object or PGresult objects made from it exist. At creation of a PGresult, the PGconn's current notice processor pointer is copied into the PGresult for possible use by routines like `PQgetvalue`.

## Environment Variables

The following environment variables can be used to select default connection parameter values, which will be used by `PQconnectdb`, `PQsetdbLogin` and `PQsetdb` if no value is directly specified by the calling code. These are useful to avoid hard-coding database connection information into simple client applications.

- `PGHOST` sets the default server name. If this begins with a slash, it specifies Unix-domain communication rather than TCP/IP communication; the value is the name of the directory in which the socket file is stored (default `/tmp`).

- `PGPORT` sets the default TCP port number or Unix-domain socket file extension for communicating with the PostgreSQL backend.

- `PGDATABASE` sets the default PostgreSQL database name.

- `PGUSER` sets the user name used to connect to the database and for authentication.

- `PGPASSWORD` sets the password used if the backend demands password authentication. This functionality is deprecated for security reasons; consider migrating to use the `$HOME/.pgpass` file.

- `PGREALM` sets the Kerberos realm to use with PostgreSQL, if it is different from the local realm. If `PGREALM` is set, PostgreSQL applications will attempt authentication with servers for this realm and use separate ticket files to avoid conflicts with local ticket files. This environment variable is only used if Kerberos authentication is selected by the backend.

- `PGOPTIONS` sets additional run-time options for the PostgreSQL backend.

- `PGTTY` sets the file or tty on which debugging messages from the backend server are displayed.

- `PGREQUIRESSL` sets whether or not the connection must be made over SSL. If set to 1, libpq will refuse to connect if the server does not accept an SSL connection. This option is only available if PostgreSQL is compiled with SSL support.

- `PGCONNECT_TIMEOUT` sets the maximum number of seconds that libpq will wait when attempting to connect to the PostgreSQL server. This option should be set to at least 2 seconds.

The following environment variables can be used to specify user-level default behavior for every PostgreSQL session:

- `PGDATESTYLE` sets the default style of date/time representation.

- `PGTZ` sets the default time zone.

- `PGCLIENTENCODING` sets the default client encoding (if multibyte support was selected when configuring PostgreSQL).

The following environment variables can be used to specify default internal behavior for every PostgreSQL session:

- `PGGEQO` sets the default mode for the genetic optimizer.

Refer to the `SET` SQL command for information on correct values for these environment variables.

## Files

The file `.pgpass` in the home directory is a file that can contain passwords to be used if the connection requires a password. This file should have the format:

    hostname:port:database:username:password

Any of these may be a literal name, or `*`, which matches anything. The first matching entry will be used, so put more-specific entries first. When an entry contains `:` or `\`, it must be escaped with `\`.

The permissions on `.pgpass` must disallow any access to world or group; achieve this by the command `chmod 0600 .pgpass`. If the permissions are less strict than this, the file will be ignored.

## Threading Behavior

`libpq` is thread-safe as of PostgreSQL 7.0, so long as no two threads attempt to manipulate the same PGconn object at the same time. In particular, you cannot issue concurrent queries from different threads through the same connection object. (If you need to run concurrent queries, start up multiple connections.)

PGresult objects are read-only after creation, and so can be passed around freely between threads.

The deprecated functions `PQoidStatus` and `fe_setauthsvc` are not thread-safe and should not be used in multithread programs. `PQoidStatus` can be replaced by `PQoidValue`. There is no good reason to call `fe_setauthsvc` at all.

`Libpq` clients using the `crypt` encryption method rely on the `crypt()` operating system function, which is often not thread-safe. It is better to use `MD5` encryption, which is thread-safe on all platforms.

## Building Libpq Programs

To build (i.e., compile and link) your libpq programs you need to do all of the following things:

- Include the `libpq-fe.h` header file:

      #include <libpq-fe.h>

  If you failed to do that then you will normally get error messages from your compiler similar to

      foo.c: In function `main':
      foo.c:34: `PGconn' undeclared (first use in this function)
      foo.c:35: `PGresult' undeclared (first use in this function)
      foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
      foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
      foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)

- Point your compiler to the directory where the PostgreSQL header files were installed, by supplying the `-I`directory`` option to your compiler. (In some cases the compiler will look into the directory in question by default, so you can omit this option.) For instance, your compile command line could look like:

      cc -c -I/usr/local/pgsql/include testprog.c

  If you are using makefiles then add the option to the `CPPFLAGS` variable:

      CPPFLAGS += -I/usr/local/pgsql/include

  If there is any chance that your program might be compiled by other users then you should not hardcode the directory location like that. Instead, you can run the utility `pg_config` to find out where the header files are on the local system:

      $ pg_config --includedir
      /usr/local/include

  Failure to specify the correct option to the compiler will result in an error message such as

      testlibpq.c:8:22: libpq-fe.h: No such file or directory

- When linking the final program, specify the option `-lpq` so that the libpq library gets pulled in, as well as the option `-L`directory`` to point it to the directory where the libpq library resides. (Again, the compiler will search some directories by default.) For maximum portability, put the `-L` option before the `-lpq` option. For example:

      cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq

  You can find out the library directory using `pg_config` as well:

      $ pg_config --libdir
      /usr/local/pgsql/lib

  Error messages that point to problems in this area could look like the following.

      testlibpq.o: In function `main':
      testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
      testlibpq.o(.text+0x71): undefined reference to `PQstatus'
      testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'

  This means you forgot `-lpq`.

      /usr/bin/ld: cannot find -lpq

  This means you forgot the `-L` or did not specify the right path.

If your codes references the header file `libpq-int.h` and you refuse to fix your code to not use it, starting in PostgreSQL 7.2, this file will be found in ``includedir`/postgresql/internal/libpq-int.h`, so you need to add the appropriate `-I` option to your compiler command line.

## Example Programs

### libpq Example Program 1

    /*
     * testlibpq.c
     *
     * Test the C version of libpq, the PostgreSQL frontend
     * library.
     */
    #include <stdio.h>
    #include <libpq-fe.h>

    void
    exit_nicely(PGconn *conn)
    {
        PQfinish(conn);
        exit(1);
    }

    main()
    {
        char       *pghost,
                   *pgport,
                   *pgoptions,
                   *pgtty;
        char       *dbName;
        int         nFields;
        int         i,
                    j;

        /* FILE *debug; */

        PGconn     *conn;
        PGresult   *res;

        /*
         * begin, by setting the parameters for a backend connection if the
         * parameters are null, then the system will try to use reasonable
         * defaults by looking up environment variables or, failing that,
         * using hardwired constants
         */
        pghost = NULL;              /* host name of the backend server */
        pgport = NULL;              /* port of the backend server */
        pgoptions = NULL;           /* special options to start up the backend
                                     * server */
        pgtty = NULL;               /* debugging tty for the backend server */
        dbName = "template1";

        /* make a connection to the database */
        conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);

        /*
         * check to see that the backend connection was successfully made
         */
        if (PQstatus(conn) == CONNECTION_BAD)
        {
            fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
            fprintf(stderr, "%s", PQerrorMessage(conn));
            exit_nicely(conn);
        }

        /* debug = fopen("/tmp/trace.out","w"); */
        /* PQtrace(conn, debug);  */

        /* start a transaction block */
        res = PQexec(conn, "BEGIN");
        if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
        {
            fprintf(stderr, "BEGIN command failed\n");
            PQclear(res);
            exit_nicely(conn);
        }

        /*
         * should PQclear PGresult whenever it is no longer needed to avoid
         * memory leaks
         */
        PQclear(res);

        /*
         * fetch rows from the pg_database, the system catalog of
         * databases
         */
        res = PQexec(conn, "DECLARE mycursor CURSOR FOR SELECT * FROM pg_database");
        if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
        {
            fprintf(stderr, "DECLARE CURSOR command failed\n");
            PQclear(res);
            exit_nicely(conn);
        }
        PQclear(res);
        res = PQexec(conn, "FETCH ALL in mycursor");
        if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
        {
            fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
            PQclear(res);
            exit_nicely(conn);
        }

        /* first, print out the attribute names */
        nFields = PQnfields(res);
        for (i = 0; i < nFields; i++)
            printf("%-15s", PQfname(res, i));
        printf("\n\n");

        /* next, print out the rows */
        for (i = 0; i < PQntuples(res); i++)
        {
            for (j = 0; j < nFields; j++)
                printf("%-15s", PQgetvalue(res, i, j));
            printf("\n");
        }
        PQclear(res);

        /* close the cursor */
        res = PQexec(conn, "CLOSE mycursor");
        PQclear(res);

        /* commit the transaction */
        res = PQexec(conn, "COMMIT");
        PQclear(res);

        /* close the connection to the database and cleanup */
        PQfinish(conn);

        /* fclose(debug); */
        return 0;

    }

### libpq Example Program 2

    /*
     * testlibpq2.c
     *  Test of the asynchronous notification interface
     *
     * Start this program, then from psql in another window do
     *   NOTIFY TBL2;
     *
     * Or, if you want to get fancy, try this:
     * Populate a database with the following:
     *
     *   CREATE TABLE TBL1 (i int4);
     *
     *   CREATE TABLE TBL2 (i int4);
     *
     *   CREATE RULE r1 AS ON INSERT TO TBL1 DO
     *     (INSERT INTO TBL2 values (new.i); NOTIFY TBL2);
     *
     * and do
     *
     *   INSERT INTO TBL1 values (10);
     *
     */
    #include <stdio.h>
    #include "libpq-fe.h"

    void
    exit_nicely(PGconn *conn)
    {
        PQfinish(conn);
        exit(1);
    }

    main()
    {
        char       *pghost,
                   *pgport,
                   *pgoptions,
                   *pgtty;
        char       *dbName;
        int         nFields;
        int         i,
                    j;

        PGconn     *conn;
        PGresult   *res;
        PGnotify   *notify;

        /*
         * begin, by setting the parameters for a backend connection if the
         * parameters are null, then the system will try to use reasonable
         * defaults by looking up environment variables or, failing that,
         * using hardwired constants
         */
        pghost = NULL;              /* host name of the backend server */
        pgport = NULL;              /* port of the backend server */
        pgoptions = NULL;           /* special options to start up the backend
                                     * server */
        pgtty = NULL;               /* debugging tty for the backend server */
        dbName = getenv("USER");    /* change this to the name of your test
                                     * database */

        /* make a connection to the database */
        conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);

        /*
         * check to see that the backend connection was successfully made
         */
        if (PQstatus(conn) == CONNECTION_BAD)
        {
            fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
            fprintf(stderr, "%s", PQerrorMessage(conn));
            exit_nicely(conn);
        }

        res = PQexec(conn, "LISTEN TBL2");
        if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
        {
            fprintf(stderr, "LISTEN command failed\n");
            PQclear(res);
            exit_nicely(conn);
        }

        /*
         * should PQclear PGresult whenever it is no longer needed to avoid
         * memory leaks
         */
        PQclear(res);

        while (1)
        {

            /*
             * wait a little bit between checks; waiting with select()
             * would be more efficient.
             */
            sleep(1);
            /* collect any asynchronous backend messages */
            PQconsumeInput(conn);
            /* check for asynchronous notify messages */
            while ((notify = PQnotifies(conn)) != NULL)
            {
                fprintf(stderr,
                     "ASYNC NOTIFY of '%s' from backend pid '%d' received\n",
                        notify->relname, notify->be_pid);
                free(notify);
            }
        }

        /* close the connection to the database and cleanup */
        PQfinish(conn);

        return 0;
    }

### libpq Example Program 3

    /*
     * testlibpq3.c Test the C version of Libpq, the PostgreSQL frontend
     * library. tests the binary cursor interface
     *
     *
     *
     * populate a database by doing the following:
     *
     * CREATE TABLE test1 (i int4, d real, p polygon);
     *
     * INSERT INTO test1 values (1, 3.567, polygon '(3.0, 4.0, 1.0, 2.0)');
     *
     * INSERT INTO test1 values (2, 89.05, polygon '(4.0, 3.0, 2.0, 1.0)');
     *
     * the expected output is:
     *
     * tuple 0: got i = (4 bytes) 1, d = (4 bytes) 3.567000, p = (4
     * bytes) 2 points   boundbox = (hi=3.000000/4.000000, lo =
     * 1.000000,2.000000) tuple 1: got i = (4 bytes) 2, d = (4 bytes)
     * 89.050003, p = (4 bytes) 2 points   boundbox =
     * (hi=4.000000/3.000000, lo = 2.000000,1.000000)
     *
     *
     */
    #include <stdio.h>
    #include "libpq-fe.h"
    #include "utils/geo_decls.h"    /* for the POLYGON type */

    void
    exit_nicely(PGconn *conn)
    {
        PQfinish(conn);
        exit(1);
    }

    main()
    {
        char       *pghost,
                   *pgport,
                   *pgoptions,
                   *pgtty;
        char       *dbName;
        int         nFields;
        int         i,
                    j;
        int         i_fnum,
                    d_fnum,
                    p_fnum;
        PGconn     *conn;
        PGresult   *res;

        /*
         * begin, by setting the parameters for a backend connection if the
         * parameters are null, then the system will try to use reasonable
         * defaults by looking up environment variables or, failing that,
         * using hardwired constants
         */
        pghost = NULL;              /* host name of the backend server */
        pgport = NULL;              /* port of the backend server */
        pgoptions = NULL;           /* special options to start up the backend
                                     * server */
        pgtty = NULL;               /* debugging tty for the backend server */

        dbName = getenv("USER");    /* change this to the name of your test
                                     * database */

        /* make a connection to the database */
        conn = PQsetdb(pghost, pgport, pgoptions, pgtty, dbName);

        /*
         * check to see that the backend connection was successfully made
         */
        if (PQstatus(conn) == CONNECTION_BAD)
        {
            fprintf(stderr, "Connection to database '%s' failed.\n", dbName);
            fprintf(stderr, "%s", PQerrorMessage(conn));
            exit_nicely(conn);
        }

        /* start a transaction block */
        res = PQexec(conn, "BEGIN");
        if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
        {
            fprintf(stderr, "BEGIN command failed\n");
            PQclear(res);
            exit_nicely(conn);
        }

        /*
         * should PQclear PGresult whenever it is no longer needed to avoid
         * memory leaks
         */
        PQclear(res);

        /*
         * fetch rows from the pg_database, the system catalog of
         * databases
         */
        res = PQexec(conn, "DECLARE mycursor BINARY CURSOR FOR SELECT * FROM test1");
        if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
        {
            fprintf(stderr, "DECLARE CURSOR command failed\n");
            PQclear(res);
            exit_nicely(conn);
        }
        PQclear(res);

        res = PQexec(conn, "FETCH ALL in mycursor");
        if (!res || PQresultStatus(res) != PGRES_TUPLES_OK)
        {
            fprintf(stderr, "FETCH ALL command didn't return tuples properly\n");
            PQclear(res);
            exit_nicely(conn);
        }

        i_fnum = PQfnumber(res, "i");
        d_fnum = PQfnumber(res, "d");
        p_fnum = PQfnumber(res, "p");

        for (i = 0; i < 3; i++)
        {
            printf("type[%d] = %d, size[%d] = %d\n",
                   i, PQftype(res, i),
                   i, PQfsize(res, i));
        }
        for (i = 0; i < PQntuples(res); i++)
        {
            int        *ival;
            float      *dval;
            int         plen;
            POLYGON    *pval;

            /* we hard-wire this to the 3 fields we know about */
            ival = (int *) PQgetvalue(res, i, i_fnum);
            dval = (float *) PQgetvalue(res, i, d_fnum);
            plen = PQgetlength(res, i, p_fnum);

            /*
             * plen doesn't include the length field so need to
             * increment by VARHDSZ
             */
            pval = (POLYGON *) malloc(plen + VARHDRSZ);
            pval->size = plen;
            memmove((char *) &pval->npts, PQgetvalue(res, i, p_fnum), plen);
            printf("tuple %d: got\n", i);
            printf(" i = (%d bytes) %d,\n",
                   PQgetlength(res, i, i_fnum), *ival);
            printf(" d = (%d bytes) %f,\n",
                   PQgetlength(res, i, d_fnum), *dval);
            printf(" p = (%d bytes) %d points \tboundbox = (hi=%f/%f, lo = %f,%f)\n",
                   PQgetlength(res, i, d_fnum),
                   pval->npts,
                   pval->boundbox.xh,
                   pval->boundbox.yh,
                   pval->boundbox.xl,
                   pval->boundbox.yl);
        }
        PQclear(res);

        /* close the cursor */
        res = PQexec(conn, "CLOSE mycursor");
        PQclear(res);

        /* commit the transaction */
        res = PQexec(conn, "COMMIT");
        PQclear(res);

        /* close the connection to the database and cleanup */
        PQfinish(conn);

        return 0;
    }