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Commandlibpq 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 29.12, “Environment Variables”, Section 29.13, “The Password File” and Section 29.16, “SSL Support” 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 29.19, “Example Programs”) 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.
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 29.13, “The Password File”).
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.
krbsrvname
Kerberos service name to use when authenticating with Kerberos 5. This must match the service name specified in the server configuration for Kerberos authentication to succeed. (See also Section 20.2.3, “Kerberos authentication”.)
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 Section 29.14, “The Connection Service File”.
If any parameter is unspecified, then the corresponding environment variable (see Section 29.12, “Environment Variables”) 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("
.
If connection_info_string
")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. The
null pointer is returned if memory could not be allocated. 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
.