libpq - C Library

PostgreSQL 8.0.1 Documentation
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# Chapter 27. libpq - C Library

libpq is the C application programmer's interface to PostgreSQL. libpq is a set of library functions 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 those written for C++, Perl, Python, Tcl and ECPG. So some aspects of libpq's behavior will be important to you if you use one of those packages. In particular, Section 27.11, Section 27.12 and Section 27.13 describe behavior that is visible to the user of any application that uses libpq.

Some short programs are included at the end of this chapter (Section 27.16) to show how to write programs that use libpq. There are also several complete examples of libpq applications in the directory `src/test/examples` in the source code distribution.

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

# 27.1. Database Connection Control Functions

The following functions deal with making a connection to a PostgreSQL backend server. An 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 the function `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 function 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 of this function (or its nonblocking analogues `PQconnectStart` and `PQconnectPoll`) is preferred for new 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`. Spaces around the equal sign are optional. 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, i.e., `\'` and `\\`.

  The currently recognized parameter key words 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 behavior when `host` is not specified is to connect to a Unix-domain socket in `/tmp` (or whatever socket directory was specified when PostgreSQL was built). On machines without Unix-domain sockets, the default is to connect to `localhost`.

  `hostaddr`

    Numeric IP address of host to connect to. This should be in the standard IPv4 address format, e.g., `172.28.40.9`. If your machine supports IPv6, you can also use those addresses. TCP/IP communication is always used when a nonempty string is specified for this parameter.

    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 occurs. If `hostaddr` is specified without `host`, the value for `hostaddr` gives the remote address. When Kerberos is used, a reverse name query occurs to obtain the host name for Kerberos. 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`.) Also, `host` rather than `hostaddr` is used to identify the connection in `~/.pgpass` (see Section 27.12).

    Without either a host name or host address, libpq will connect using a local Unix-domain socket; or on machines without Unix-domain sockets, it will attempt to connect to `localhost`.

  `port`

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

  `dbname`

    The database name. Defaults to be the same as the user name.

  `user`

    PostgreSQL user name to connect as. Defaults to be the same as the operating system name of the user running the application.

  `password`

    Password to be used if the server demands password authentication.

  `connect_timeout`

    Maximum wait for connection, in seconds (write as a decimal integer string). Zero or not specified means wait indefinitely. It is not recommended to use a timeout of less than 2 seconds.

  `options`

    Command-line options to be sent to the server.

  `tty`

    Ignored (formerly, this specified where to send server debug output).

  `sslmode`

    This option determines whether or with what priority an SSL connection will be negotiated with the server. There are four modes: `disable` will attempt only an unencrypted SSL connection; `allow` will negotiate, trying first a non-SSL connection, then if that fails, trying an SSL connection; `prefer` (the default) will negotiate, trying first an SSL connection, then if that fails, trying a regular non-SSL connection; `require` will try only an SSL connection.

    If PostgreSQL is compiled without SSL support, using option `require` will cause an error, while options `allow` and `prefer` will be accepted but libpq will not in fact attempt an SSL connection.

  `requiressl`

    This option is deprecated in favor of the `sslmode` setting.

    If set to 1, an SSL connection to the server is required (this is equivalent to `sslmode` `require`). libpq will then refuse to connect if the server does not accept an SSL connection. If set to 0 (default), libpq will negotiate the connection type with the server (equivalent to `sslmode` `prefer`). This option is only available if PostgreSQL is compiled with SSL support.

  `service`

    Service name to use for additional parameters. It specifies a service name in `pg_service.conf` that holds additional connection parameters. This allows applications to specify only a service name so connection parameters can be centrally maintained. See `share/pg_service.conf.sample` in the installation directory for information on how to set up the file.

  If any parameter is unspecified, then the corresponding environment variable (see Section 27.11) is checked. If the environment variable is not set either, then the indicated built-in defaults are used.

`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 set of parameters. It has the same functionality except that the missing parameters will always take on default values. Write `NULL` or an empty string for any one of the fixed parameters that is to be defaulted.

`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 for backward compatibility with very 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 functions 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 point of this approach is that the waits for I/O to complete can occur in the application's main loop, rather than down inside `PQconnectdb`, and so the application can manage this operation in parallel with other activities.

  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, so 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 that the socket is in the appropriate state before calling `PQconnectPoll`, as described below.

  To begin a nonblocking connection request, 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. Use `PQsocket(conn)` to obtain the descriptor of the socket underlying the database connection. Loop thus: If `PQconnectPoll(conn)` last returned `PGRES_POLLING_READING`, wait until the socket is ready to read (as indicated by `select()`, `poll()`, or similar system function). Then call `PQconnectPoll(conn)` again. Conversely, if `PQconnectPoll(conn)` last returned `PGRES_POLLING_WRITING`, wait until the socket is ready to write, then call `PQconnectPoll(conn)` again. If you have yet to call `PQconnectPoll`, i.e., just after the call to `PQconnectStart`, behave as if it last returned `PGRES_POLLING_WRITING`. Continue this loop until `PQconnectPoll(conn)` returns `PGRES_POLLING_FAILED`, indicating the connection procedure has failed, or `PGRES_POLLING_OK`, indicating the connection has been successfully made.

  At any time during connection, the status of the connection may be checked by calling `PQstatus`. If this gives `CONNECTION_BAD`, then the connection procedure has failed; if it gives `CONNECTION_OK`, then the connection is ready. Both of these states are equally detectable from the return value of `PQconnectPoll`, described above. Other states may also occur 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 are:

  `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 backend start-up to finish.

  `CONNECTION_SSL_STARTUP`

    Negotiating SSL encryption.

  `CONNECTION_SETENV`

    Negotiating environment-driven parameter settings.

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

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

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

  The `connect_timeout` connection parameter is ignored when using `PQconnectPoll`; it is the application's responsibility to decide whether an excessive amount of time has elapsed. Otherwise, `PQconnectStart` followed by a `PQconnectPoll` loop is equivalent to `PQconnectdb`.

  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 the connection attempt fails or is abandoned.

`PQconndefaults`

  Returns the default connection options.

      PQconninfoOption *PQconndefaults(void);

      typedef struct
      {
          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 */
      } PQconninfoOption;

  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` structures, which ends with an entry having a null `keyword` pointer. Note that the current 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`.

`PQfinish`

  Closes the connection to the server. Also frees memory used by the `PGconn` object.

      void PQfinish(PGconn *conn);

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

`PQreset`

  Resets the communication channel to the server.

      void PQreset(PGconn *conn);

  This function will close the connection to the server 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 channel to the server, in a nonblocking manner.

      int PQresetStart(PGconn *conn);

      PostgresPollingStatusType PQresetPoll(PGconn *conn);

  These functions will close the connection to the server 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`.

  To initiate a connection reset, 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`.

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