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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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