## Chapter 34. 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 34.14, Section 34.15 and Section 34.18 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 34.21) 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.

## 34.1. 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`, `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

If untrusted users have access to a database that has not adopted a secure schema usage pattern, begin each session by removing publicly-writable schemas from `search_path`. One can set parameter key word `options` to value `-csearch_path=`. Alternately, one can issue `PQexec(`conn`, "SELECT pg_catalog.set_config('search_path', '', false)")` after connecting. This consideration is not specific to libpq; it applies to every interface for executing arbitrary SQL commands.

### 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 `exec` from the child process to load a new executable is safe.

### Note

On Windows, there is a way to improve performance if a single database connection is repeatedly started and shutdown. Internally, libpq calls `WSAStartup()` and `WSACleanup()` for connection startup and shutdown, respectively. `WSAStartup()` increments an internal Windows library reference count which is decremented by `WSACleanup()`. When the reference count is just one, calling `WSACleanup()` frees all resources and all DLLs are unloaded. This is an expensive operation. To avoid this, an application can manually call `WSAStartup()` 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 34.1.2.

  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 34.1.1.

  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 34.14) 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 34.1.1 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` parameter must be used appropriately to prevent DNS queries from being made. See the documentation of this parameter in Section 34.1.2 for details.

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

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

  To begin a nonblocking connection request, call `PQconnectStart` or `PQconnectStartParams`. If the result 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). Next call `PQstatus(conn)`. If the result is `CONNECTION_BAD`, the connection attempt has already failed, typically because of invalid connection parameters.

  If `PQconnectStart` or `PQconnectStartParams` 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. (Caution: do not assume that the socket remains the same across `PQconnectPoll` calls.) 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. On the first iteration, i.e. if you have yet to call `PQconnectPoll`, 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:

  `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.

  `CONNECTION_CHECK_WRITABLE`

    Checking if connection is able to handle write transactions.

  `CONNECTION_CONSUME`

    Consuming any remaining response messages on connection.

  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 when `PQconnectStart` or `PQconnectStartParams` 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:

  `PQPING_OK`

    The server is running and appears to be accepting connections.

  `PQPING_REJECT`

    The server is running but is in a state that disallows connections (startup, shutdown, or crash recovery).

  `PQPING_NO_RESPONSE`

    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).

  `PQPING_NO_ATTEMPT`

    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`.

### 34.1.1. Connection Strings

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 URIs. URIs generally follow RFC 3986, except that multi-host connection strings are allowed as further described below.

#### 34.1.1.1. Keyword/Value Connection Strings

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 34.1.2.

#### 34.1.1.2. Connection URIs

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
    postgresql://host1:123,host2:456/somedb?target_session_attrs=any&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, e.g. replace `=` with `%3D`.

Any connection parameters not corresponding to key words listed in Section 34.1.2 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

It is possible to specify multiple host components, each with an optional port component, in a single URI. A URI of the form `postgresql://host1:port1,host2:port2,host3:port3/` is equivalent to a connection string of the form `host=host1,host2,host3 port=port1,port2,port3`. Each host will be tried in turn until a connection is successfully established.

#### 34.1.1.3. Specifying Multiple Hosts

It is possible to specify multiple hosts to connect to, so that they are tried in the given order. In the Keyword/Value format, the `host`, `hostaddr`, and `port` options accept a comma-separated list of values. The same number of elements must be given in each option that is specified, such that e.g. the first `hostaddr` corresponds to the first host name, the second `hostaddr` corresponds to the second host name, and so forth. As an exception, if only one `port` is specified, it applies to all the hosts.

In the connection URI format, you can list multiple `host:port` pairs separated by commas, in the `host` component of the URI.

In either format, a single host name can translate to multiple network addresses. A common example of this is a host that has both an IPv4 and an IPv6 address.

When multiple hosts are specified, or when a single hostname is translated to multiple addresses, all the hosts and addresses will be tried in order, until one succeeds. If none of the hosts can be reached, the connection fails. If a connection is established successfully, but authentication fails, the remaining hosts in the list are not tried.

If a password file is used, you can have different passwords for different hosts. All the other connection options are the same for every host in the list; it is not possible to e.g. specify different usernames for different hosts.

### 34.1.2. Parameter Key Words

The currently recognized parameter key words are:

`host`

  Name of host to connect to. If a host name 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, or is empty, 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`.

  A comma-separated list of host names is also accepted, in which case each host name in the list is tried in order; an empty item in the list selects the default behavior as explained above. See Section 34.1.1.3 for details.

`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 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. (When using `PQconnectPoll`, the lookup occurs when `PQconnectPoll` first considers this host name, and it may cause `PQconnectPoll` to block for a significant amount of time.)

  - 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, when both `host` and `hostaddr` are specified, `host` is used to identify the connection in a password file (see Section 34.15).

  A comma-separated list of `hostaddr` values is also accepted, in which case each host in the list is tried in order. An empty item in the list causes the corresponding host name to be used, or the default host name if that is empty as well. See Section 34.1.1.3 for details.

  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. If multiple hosts were given in the `host` or `hostaddr` parameters, this parameter may specify a comma-separated list of ports of the same length as the host list, or it may specify a single port number to be used for all hosts. An empty string, or an empty item in a comma-separated list, specifies the default port number established when PostgreSQL was built.

`dbname`

  The database name. Defaults to be the same as the user name. In certain contexts, the value is checked for extended formats; see Section 34.1.1 for more details on those.

`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.

`passfile`

  Specifies the name of the file used to store passwords (see Section 34.15). Defaults to `~/.pgpass`, or `%APPDATA%\postgresql\pgpass.conf` on Microsoft Windows. (No error is reported if this file does not exist.)

`connect_timeout`

  Maximum wait for connection, in seconds (write as a decimal integer, e.g. `10`). Zero, negative, or not specified means wait indefinitely. The minimum allowed timeout is 2 seconds, therefore a value of `1` is interpreted as `2`. This timeout applies separately to each host name or IP address. For example, if you specify two hosts and `connect_timeout` is 5, each host will time out if no connection is made within 5 seconds, so the total time spent waiting for a connection might be up to 10 seconds.

`client_encoding`

  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).

`options`

  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 20.

`application_name`

  Specifies a value for the application_name configuration parameter.

`fallback_application_name`

  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.

`keepalives`

  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.

`keepalives_idle`

  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 `TCP_KEEPIDLE` or an equivalent socket option is available, and on Windows; on other systems, it has no effect.

`keepalives_interval`

  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 `TCP_KEEPINTVL` or an equivalent socket option is available, and on Windows; on other systems, it has no effect.

`keepalives_count`

  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 `TCP_KEEPCNT` or an equivalent socket option is available; on other systems, it has no effect.

`tty`

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

`sslmode`

  This option determines whether or with what priority a secure SSL TCP/IP connection will be negotiated with the server. There are six modes:

  `disable`

    only try a non-SSL connection

  `allow`

    first try a non-SSL connection; if that fails, try an SSL connection

  `prefer` (default)

    first try an SSL connection; if that fails, try a non-SSL connection

  `require`

    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

  `verify-ca`

    only try an SSL connection, and verify that the server certificate is issued by a trusted certificate authority (CA)

  `verify-full`

    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 34.18 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.

`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.

`sslcompression`

  If set to 1 (default), data sent over SSL connections will be compressed. 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.

`sslcert`

  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.

`sslkey`

  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.

`sslrootcert`

  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`.

`sslcrl`

  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`.

`requirepeer`

  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 21.3.6.

`krbsrvname`

  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 21.3.3.)

`gsslib`

  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`

  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 34.16.

`target_session_attrs`

  If this parameter is set to `read-write`, only a connection in which read-write transactions are accepted by default is considered acceptable. The query `SHOW transaction_read_only` will be sent upon any successful connection; if it returns `on`, the connection will be closed. If multiple hosts were specified in the connection string, any remaining servers will be tried just as if the connection attempt had failed. The default value of this parameter, `any`, regards all connections as acceptable.