The C API provides low-level access to the MySQL client/server
protocol and enables C programs to access database contents. The C
API code is distributed with MySQL and implemented in the
libmysqlclient library. See
Chapter 2, MySQL C API Implementations.
Most other client APIs use the
library to communicate with the MySQL server. (Exceptions are
Connector/J and Connector/NET.) This means that, for example, you can
take advantage of many of the same environment variables that are
used by other client programs because they are referenced from the
library. For a list of these variables, see
Overview of MySQL Programs.
For instructions on building client programs using the C API, see Section 3.2, “Building C API Client Programs”. For programming with threads, see Section 3.4, “Writing C API Threaded Client Programs”. To create a standalone application which includes the "server" and "client" in the same program (and does not communicate with an external MySQL server), see libmysqld, the Embedded MySQL Server Library.
If, after an upgrade, you experience problems with compiled
client programs, such as
Commands out of sync
or unexpected core dumps, the programs were probably compiled
using old header or library files. In this case, check the date
mysql.h file and
libmysqlclient.a library used for
compilation to verify that they are from the new MySQL
distribution. If not, recompile the programs with the new
headers and libraries. Recompilation might also be necessary for
programs compiled against the shared client library if the
library major version number has changed (for example, from
libmysqlclient.so.18). For additional
compatibility information, see
Section 3.5, “Running C API Client Programs”.
Clients have a maximum communication buffer size. The size of the buffer that is allocated initially (16KB) is automatically increased up to the maximum size (16MB by default). Because buffer sizes are increased only as demand warrants, simply increasing the maximum limit does not in itself cause more resources to be used. This size check is mostly a precaution against erroneous statements and communication packets.
The communication buffer must be large enough to contain a single
SQL statement (for client-to-server traffic) and one row of
returned data (for server-to-client traffic). Each session's
communication buffer is dynamically enlarged to handle any query
or row up to the maximum limit. For example, if you have
BLOB values that contain up to 16MB
of data, you must have a communication buffer limit of at least
16MB (in both server and client). The default maximum built into
the client library is 1GB, but the default maximum in the server
is 1MB. You can increase this by changing the value of the
max_allowed_packet parameter at
server startup. See Configuring the Server.
The MySQL server shrinks each communication buffer to
net_buffer_length bytes after
each query. For clients, the size of the buffer associated with a
connection is not decreased until the connection is closed, at
which time client memory is reclaimed.