PostgreSQL 9.5.3 Documentation | |||
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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
,
PQconnectdbParams
, 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
the return value for a successful connection before queries are sent
via the connection object.
Warning |
On Unix, forking a process with open libpq connections can lead to
unpredictable results because the parent and child processes share
the same sockets and operating system resources. For this reason,
such usage is not recommended, though doing an |
Note: On Windows, there is a way to improve performance if a single database connection is repeatedly started and shutdown. Internally, libpq calls
WSAStartup()
andWSACleanup()
for connection startup and shutdown, respectively.WSAStartup()
increments an internal Windows library reference count which is decremented byWSACleanup()
. When the reference count is just one, callingWSACleanup()
frees all resources and all DLLs are unloaded. This is an expensive operation. To avoid this, an application can manually callWSAStartup()
so resources will not be freed when the last database connection is closed.
PQconnectdbParams
Makes a new connection to the database server.
PGconn *PQconnectdbParams(const char * const *keywords, const char * const *values, int expand_dbname);
This function opens a new database connection using the parameters taken
from two NULL-terminated arrays. The first,
keywords, is defined as an array of strings, each one
being a key word. The second, values, gives the value
for each key word. Unlike PQsetdbLogin
below, the parameter
set can be extended without changing the function signature, so use of
this function (or its nonblocking analogs PQconnectStartParams
and PQconnectPoll
) is preferred for new application
programming.
The currently recognized parameter key words are listed in Section 31.1.2.
When expand_dbname is non-zero, the dbname key word value is allowed to be recognized as a connection string. Only the first occurrence of dbname is expanded this way, any subsequent dbname value is processed as plain database name. More details on the possible connection string formats appear in Section 31.1.1.
The passed arrays can be empty to use all default parameters, or can contain one or more parameter settings. They should be matched in length. Processing will stop at the first NULL element in the keywords array.
If any parameter is NULL or an empty string, the corresponding environment variable (see Section 31.14) is checked. If the environment variable is not set either, then the indicated built-in defaults are used.
In general key words are processed from the beginning of these arrays in index order. The effect of this is that when key words are repeated, the last processed value is retained. Therefore, through careful placement of the dbname key word, it is possible to determine what may be overridden by a conninfo string, and what may not.
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.
The passed string can be empty to use all default parameters, or it can contain one or more parameter settings separated by whitespace, or it can contain a URI. See Section 31.1.1 for details.
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.
If the dbName contains
an = sign or has a valid connection URI prefix, it
is taken as a conninfo string in exactly the same way as
if it had been passed to PQconnectdb
, and the remaining
parameters are then applied as specified for PQconnectdbParams
.
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.
PQconnectStartParams
PQconnectStart
PQconnectPoll
Make a connection to the database server in a nonblocking manner.
PGconn *PQconnectStartParams(const char * const *keywords, const char * const *values, int expand_dbname); PGconn *PQconnectStart(const char *conninfo); PostgresPollingStatusType PQconnectPoll(PGconn *conn);
These three 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
PQconnectdbParams
or PQconnectdb
, and so the
application can manage this operation in parallel with other activities.
With PQconnectStartParams
, the database connection is made
using the parameters taken from the keywords and
values arrays, and controlled by expand_dbname,
as described above for PQconnectdbParams
.
With PQconnectStart
, the database connection is made
using the parameters taken from the string conninfo as
described above for PQconnectdb
.
Neither PQconnectStartParams
nor 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 in Section 31.1.2 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.
Note: use of PQconnectStartParams
is analogous to
PQconnectStart
shown 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 can 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 can be
checked by calling PQstatus
. If this call returns CONNECTION_BAD, then the
connection procedure has failed; if the call returns CONNECTION_OK
, then the
connection is ready. Both of these states are equally detectable
from the return value of PQconnectPoll
, described above. Other states might also occur
during (and only during) an asynchronous connection procedure. These
indicate the current stage of the connection procedure and might be useful
to provide feedback to the user for example. These statuses are:
Waiting for connection to be made.
Connection OK; waiting to send.
Waiting for a response from the server.
Received authentication; waiting for backend start-up to finish.
Negotiating SSL encryption.
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; /* Indicates how to display 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 can 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. A
missing or invalid service file will be silently ignored. 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
.
PQconninfo
Returns the connection options used by a live connection.
PQconninfoOption *PQconninfo(PGconn *conn);
Returns a connection options array. This can be used to determine
all possible PQconnectdb
options and the
values that were used to connect to the server. The return
value points to an array of PQconninfoOption
structures, which ends with an entry having a null keyword
pointer. All notes above for PQconndefaults
also
apply to the result of PQconninfo
.
PQconninfoParse
Returns parsed connection options from the provided connection string.
PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);
Parses a connection string and returns the resulting options as an
array; or returns NULL if there is a problem with the connection
string. This function can be used to extract
the PQconnectdb
options in the provided
connection string. The return value points to an array of
PQconninfoOption structures, which ends
with an entry having a null keyword pointer.
All legal options will be present in the result array, but the PQconninfoOption for any option not present in the connection string will have val set to NULL; default values are not inserted.
If errmsg is not NULL, then *errmsg is set
to NULL on success, else to a malloc
'd error string explaining
the problem. (It is also possible for *errmsg to be
set to NULL and the function to return NULL;
this indicates an out-of-memory condition.)
After processing the options array, free it by passing it to
PQconninfoFree
. If this is not done, some memory
is leaked for each call to PQconninfoParse
.
Conversely, if an error occurs and errmsg is not NULL,
be sure to free the error string using PQfreemem
.
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 might 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 can 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 PQconnectStartParams
, 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
.
PQpingParams
PQpingParams
reports the status of the
server. It accepts connection parameters identical to those of
PQconnectdbParams
, described above. It is not
necessary to supply correct user name, password, or database name
values to obtain the server status; however, if incorrect values
are provided, the server will log a failed connection attempt.
PGPing PQpingParams(const char * const *keywords, const char * const *values, int expand_dbname);
The function returns one of the following values:
The server is running and appears to be accepting connections.
The server is running but is in a state that disallows connections (startup, shutdown, or crash recovery).
The server could not be contacted. This might indicate that the server is not running, or that there is something wrong with the given connection parameters (for example, wrong port number), or that there is a network connectivity problem (for example, a firewall blocking the connection request).
No attempt was made to contact the server, because the supplied parameters were obviously incorrect or there was some client-side problem (for example, out of memory).
PQping
PQping
reports the status of the
server. It accepts connection parameters identical to those of
PQconnectdb
, described above. It is not
necessary to supply correct user name, password, or database name
values to obtain the server status; however, if incorrect values
are provided, the server will log a failed connection attempt.
PGPing PQping(const char *conninfo);
The return values are the same as for PQpingParams
.
Several libpq functions parse a user-specified string to obtain connection parameters. There are two accepted formats for these strings: plain keyword = value strings and RFC 3986 URIs.
In the first format, 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 \\.
Example:
host=localhost port=5432 dbname=mydb connect_timeout=10
The recognized parameter key words are listed in Section 31.1.2.
The general form for a connection URI is:
postgresql://[user[:password]@][netloc][:port][/dbname][?param1=value1&...]
The URI scheme designator can be either postgresql:// or postgres://. Each of the URI parts is optional. The following examples illustrate valid URI syntax uses:
postgresql:// postgresql://localhost postgresql://localhost:5433 postgresql://localhost/mydb postgresql://user@localhost postgresql://user:secret@localhost postgresql://other@localhost/otherdb?connect_timeout=10&application_name=myapp
Components of the hierarchical part of the URI can also be given as parameters. For example:
postgresql:///mydb?host=localhost&port=5433
Percent-encoding may be used to include symbols with special meaning in any of the URI parts.
Any connection parameters not corresponding to key words listed in Section 31.1.2 are ignored and a warning message about them is sent to stderr.
For improved compatibility with JDBC connection URIs, instances of parameter ssl=true are translated into sslmode=require.
The host part may be either host name or an IP address. To specify an IPv6 host address, enclose it in square brackets:
postgresql://[2001:db8::1234]/database
The host component is interpreted as described for the parameter host. In particular, a Unix-domain socket connection is chosen if the host part is either empty or starts with a slash, otherwise a TCP/IP connection is initiated. Note, however, that the slash is a reserved character in the hierarchical part of the URI. So, to specify a non-standard Unix-domain socket directory, either omit the host specification in the URI and specify the host as a parameter, or percent-encode the path in the host component of the URI:
postgresql:///dbname?host=/var/lib/postgresql postgresql://%2Fvar%2Flib%2Fpostgresql/dbname
The currently recognized parameter key words are:
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.
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 might be important in applications with time constraints. However, a host name is required for GSSAPI or SSPI authentication methods, as well as for verify-full SSL certificate verification. The following rules are used:
If host is specified without hostaddr, a host name lookup occurs.
If hostaddr is specified without host, the value for hostaddr gives the server network address. The connection attempt will fail if the authentication method requires a host name.
If both host and hostaddr are specified, the value for hostaddr gives the server network address. The value for host is ignored unless the authentication method requires it, in which case it will be used as the host name.
Note that authentication is likely to fail if host is not the name of the server at network address hostaddr. Also, note that host rather than hostaddr is used to identify the connection in ~/.pgpass (see Section 31.15).
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 number to connect to at the server host, or socket file name extension for Unix-domain connections.
The database name. Defaults to be the same as the user name. In certain contexts, the value is checked for extended formats; see Section 31.1.1 for more details on those.
PostgreSQL user name to connect as. Defaults to be the same as the operating system name of the user running the application.
Password to be used if the server demands password authentication.
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.
This sets the client_encoding configuration parameter for this connection. In addition to the values accepted by the corresponding server option, you can use auto to determine the right encoding from the current locale in the client (LC_CTYPE environment variable on Unix systems).
Specifies command-line options to send to the server at connection start. For example, setting this to -c geqo=off sets the session's value of the geqo parameter to off. Spaces within this string are considered to separate command-line arguments, unless escaped with a backslash (\); write \\ to represent a literal backslash. For a detailed discussion of the available options, consult Chapter 18.
Specifies a value for the application_name configuration parameter.
Specifies a fallback value for the application_name configuration parameter. This value will be used if no value has been given for application_name via a connection parameter or the PGAPPNAME environment variable. Specifying a fallback name is useful in generic utility programs that wish to set a default application name but allow it to be overridden by the user.
Controls whether client-side TCP keepalives are used. The default value is 1, meaning on, but you can change this to 0, meaning off, if keepalives are not wanted. This parameter is ignored for connections made via a Unix-domain socket.
Controls the number of seconds of inactivity after which TCP should send a keepalive message to the server. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where the TCP_KEEPIDLE or TCP_KEEPALIVE socket option is available, and on Windows; on other systems, it has no effect.
Controls the number of seconds after which a TCP keepalive message that is not acknowledged by the server should be retransmitted. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where the TCP_KEEPINTVL socket option is available, and on Windows; on other systems, it has no effect.
Controls the number of TCP keepalives that can be lost before the client's connection to the server is considered dead. A value of zero uses the system default. This parameter is ignored for connections made via a Unix-domain socket, or if keepalives are disabled. It is only supported on systems where the TCP_KEEPCNT socket option is available; on other systems, it has no effect.
Ignored (formerly, this specified where to send server debug output).
This option determines whether or with what priority a secure SSL TCP/IP connection will be negotiated with the server. There are six modes:
only try a non-SSL connection
first try a non-SSL connection; if that fails, try an SSL connection
first try an SSL connection; if that fails, try a non-SSL connection
only try an SSL connection. If a root CA file is present, verify the certificate in the same way as if verify-ca was specified
only try an SSL connection, and verify that the server certificate is issued by a trusted certificate authority (CA)
only try an SSL connection, verify that the server certificate is issued by a trusted CA and that the requested server host name matches that in the certificate
See Section 31.18 for a detailed description of how these options work.
sslmode is ignored for Unix domain socket communication. If PostgreSQL is compiled without SSL support, using options require, verify-ca, or verify-full will cause an error, while options allow and prefer will be accepted but libpq will not actually attempt an SSL connection.
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.
If set to 1 (default), data sent over SSL connections will be compressed (this requires OpenSSL version 0.9.8 or later). If set to 0, compression will be disabled (this requires OpenSSL 1.0.0 or later). This parameter is ignored if a connection without SSL is made, or if the version of OpenSSL used does not support it.
Compression uses CPU time, but can improve throughput if the network is the bottleneck. Disabling compression can improve response time and throughput if CPU performance is the limiting factor.
This parameter specifies the file name of the client SSL certificate, replacing the default ~/.postgresql/postgresql.crt. This parameter is ignored if an SSL connection is not made.
This parameter specifies the location for the secret key used for the client certificate. It can either specify a file name that will be used instead of the default ~/.postgresql/postgresql.key, or it can specify a key obtained from an external "engine" (engines are OpenSSL loadable modules). An external engine specification should consist of a colon-separated engine name and an engine-specific key identifier. This parameter is ignored if an SSL connection is not made.
This parameter specifies the name of a file containing SSL certificate authority (CA) certificate(s). If the file exists, the server's certificate will be verified to be signed by one of these authorities. The default is ~/.postgresql/root.crt.
This parameter specifies the file name of the SSL certificate revocation list (CRL). Certificates listed in this file, if it exists, will be rejected while attempting to authenticate the server's certificate. The default is ~/.postgresql/root.crl.
This parameter specifies the operating-system user name of the server, for example requirepeer=postgres. When making a Unix-domain socket connection, if this parameter is set, the client checks at the beginning of the connection that the server process is running under the specified user name; if it is not, the connection is aborted with an error. This parameter can be used to provide server authentication similar to that available with SSL certificates on TCP/IP connections. (Note that if the Unix-domain socket is in /tmp or another publicly writable location, any user could start a server listening there. Use this parameter to ensure that you are connected to a server run by a trusted user.) This option is only supported on platforms for which the peer authentication method is implemented; see Section 19.3.6.
Kerberos service name to use when authenticating with GSSAPI. This must match the service name specified in the server configuration for Kerberos authentication to succeed. (See also Section 19.3.3.)
GSS library to use for GSSAPI authentication. Only used on Windows. Set to gssapi to force libpq to use the GSSAPI library for authentication instead of the default SSPI.
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 31.16.