MySQL 8.0 C API Developer Guide 概览

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"MySQL8.0CAPIDeveloperGuide是一份详细的文档，主要介绍如何使用C语言接口来与MySQL8.0数据库进行交互。这份指南旨在为开发者提供低级别访问MySQL客户端/服务器协议的方法，使C语言程序能够访问数据库内容。C API的实现包含在MySQL的libmysqlclient库中。文档包含了法律信息、论坛支持以及一系列关于如何编写基于C API的客户端应用程序的章节，如构建程序、处理线程安全和利用API的各种特性，如加密连接、多语句执行、日期和时间值的预处理语句处理等。" 在MySQL8.0的C API中，开发者可以： 1. **低级别访问协议**：C API提供了直接操作MySQL客户端/服务器通信协议的能力，这使得开发者能够自定义更复杂的数据访问逻辑。 2. **libmysqlclient库**：C API的核心实现存在于这个库中，它包含了所有必要的函数和数据结构，供C程序员在他们的应用程序中使用MySQL服务。 3. **函数说明**：文档详细介绍了每个API函数的用途、参数和返回值，帮助开发者理解和使用这些函数进行数据库操作，例如连接、查询、结果集处理和断开连接等。 4. **构建C API客户端程序**：开发者可以学习如何配置和构建C API客户端程序，包括使用pkg-config工具来管理依赖项。 5. **线程安全**：指南涵盖了编写线程安全的C API程序的方法，这对于多线程应用尤其重要，确保在并发环境中正确地管理和同步资源。 6. **特性支持**： - **加密连接**：C API支持加密连接，确保数据在传输过程中的安全性。 - **多语句执行**：允许在一个网络往返中发送和执行多个SQL语句，提高效率。 - **日期和时间值的预处理语句处理**：优化了对日期和时间类型的预处理语句，提高了性能和准确性。 - **预处理CALL语句支持**：支持预处理存储过程调用，增强了代码复用和执行效率。 7. **错误处理和调试**：文档还应包括如何处理运行时错误和调试信息，帮助开发者解决可能出现的问题。通过这个开发者指南，C程序员可以深入理解MySQL的内部工作原理，并利用C API创建高效、可靠的数据库应用。无论是新手还是经验丰富的开发者，这份指南都是一个宝贵的资源，可以帮助他们充分利用MySQL8.0的功能。

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Building C API Client Programs Using pkg-config

mysql_config --libs

The output from mysql_config might indicate other libraries that should be specified on the link

command as well. You can include mysql_config output directly in your compile or link command using

backticks. For example:

gcc -o progname progname.o `mysql_config --libs`

If an error occurs at link time that the floor symbol is undefined, link to the math library by adding -lm

to the end of the compile/link line. Similarly, if you get undefined-reference errors for other functions that

should exist on your system, such as connect(), check the manual page for the function in question to

determine which libraries you should add to the link command.

If you get undefined-reference errors such as the following for functions that do not exist on your system, it

usually means that your MySQL client library was compiled on a system that is not 100% compatible with

yours:

mf_format.o(.text+0x201): undefined reference to `__lxstat'

In this case, you should download a source distribution for the latest version of MySQL and compile the

MySQL client library yourself. See Installing MySQL from Source.

3.3 Building C API Client Programs Using pkg-config

MySQL distributions contain a mysqlclient.pc file that provides information about MySQL configuration

for use by the pkg-config command. This enables pkg-config to be used as an alternative to

mysql_config for obtaining information such as compiler flags or link libraries required to compile

MySQL applications. For example, the following pairs of commands are equivalent:

mysql_config --cflags

pkg-config --cflags mysqlclient

mysql_config --libs

pkg-config --libs mysqlclient

The last pkg-config command produces flags for dynamic linking. To produce flags for static linking, use

this command:

pkg-config --static --libs mysqlclient

On some platforms, the output with and without --static might be the same.

Note

If pkg-config does not find MySQL information, it might be necessary to

set the PKG_CONFIG_PATH environment variable to the directory in which the

mysqlclient.pc file is located, which by default is usually the pkgconfig

directory under the MySQL library directory. For example (adjust the location

appropriately):

# For sh, bash, ...

export PKG_CONFIG_PATH=/usr/local/mysql/lib/pkgconfig

# For csh, tcsh, ...

setenv PKG_CONFIG_PATH /usr/local/mysql/lib/pkgconfig

The mysqlconfig.pc installation location can be controlled using the

INSTALL_PKGCONFIGDIR CMake option. See MySQL Source-Configuration

Options.

The --variable option takes a configuration variable name and displays the variable value:

Writing C API Threaded Client Programs

# installation prefix directory

pkg-config --variable=prefix mysqlclient

# header file directory

pkg-config --variable=includedir mysqlclient

# library directory

pkg-config --variable=libdir mysqlclient

To see which variable values pkg-config can display using the --variable option, use this command:

pkg-config --print-variables mysqlclient

You can use pkg-config within a command line using backticks to include the output that it produces for

particular options. For example, to compile and link a MySQL client program, use pkg-config as follows:

gcc -c `pkg-config --cflags mysqlclient` progname.c

gcc -o progname progname.o `pkg-config --libs mysqlclient`

3.4 Writing C API Threaded Client Programs

This section provides guidance for writing client programs that use the thread-related functions in the

MySQL C API. For further information about these functions, see Section 8.2, “C API Threaded Function

Descriptions”. For examples of source code that uses them, look in the client directory of a MySQL

source distribution:

• The source for mysqlimport uses threading in the code associated with the --use-threads option.

• The source for mysqlslap uses threads to set up simultaneous workloads, to test server operation

under high load.

As an alternative to thread programming, applications may find the asynchronous (nonblocking) C API

functions useful. These functions enable applications to submit multiple outstanding requests to the

server and determine when each has finished using polling. For more information, see Chapter 7, C API

Asynchronous Interface.

If undefined-reference errors occur when linking a threaded program against the MySQL client library, the

most likely cause is that you did not include the thread libraries on the link/compile command.

The client library is almost thread-safe. The biggest problem is that the subroutines in sql/net_serv.cc

that read from sockets are not interrupt-safe. This was done with the thought that you might want to have

your own alarm that can break a long read to a server. If you install interrupt handlers for the SIGPIPE

interrupt, socket handling should be thread-safe.

To avoid aborting the program when a connection terminates, MySQL blocks SIGPIPE on the first call to

mysql_library_init(), mysql_init(), or mysql_connect(). To use your own SIGPIPE handler,

first call mysql_library_init(), then install your handler.

The client library is thread-safe per connection. Two threads can share the same connection with the

following caveats:

• Unless you are using the asynchronous C API functions mentioned previously, multiple threads

cannot send a query to the MySQL server at the same time on the same connection. In particular,

you must ensure that between calls to mysql_real_query() (or mysql_query()) and

mysql_store_result() in one thread, no other thread uses the same connection. To do

this, use a mutex lock around your pair of mysql_real_query() (or mysql_query()) and

mysql_store_result() calls. After mysql_store_result() returns, the lock can be released and

other threads may query the same connection.

If you use POSIX threads, you can use pthread_mutex_lock() and pthread_mutex_unlock() to

establish and release a mutex lock.

Running C API Client Programs

Note

If you examine programs in a MySQL source distribution, instead of calls to

pthread_mutex_lock() and pthread_mutex_unlock(), you will see

calls to native_mutex_lock() and native_mutex_unlock(). The latter

functions are defined in the thr_mutex.h header file and map to platform-

specific mutex functions.

• Multiple threads can access different result sets that are retrieved with mysql_store_result().

• To use mysql_use_result(), you must ensure that no other thread uses the same connection until

the result set is closed. However, it really is best for threaded clients that share the same connection to

use mysql_store_result().

If a thread does not create the connection to the MySQL database but calls MySQL functions, take the

following into account:

When you call mysql_init(), MySQL creates a thread-specific variable for the thread that is used

by the debug library (among other things). If you call a MySQL function before the thread has called

mysql_init(), the thread does not have the necessary thread-specific variables in place and you are

likely to end up with a core dump sooner or later. To avoid problems, you must do the following:

1. Call mysql_library_init() before any other MySQL functions. It is not thread-safe, so call it

before threads are created, or protect the call with a mutex.

2. Arrange for mysql_thread_init() to be called early in the thread handler before calling any MySQL

function. (If you call mysql_init(), it calls mysql_thread_init() for you.)

3. In the thread, call mysql_thread_end() before calling pthread_exit(). This frees the memory

used by MySQL thread-specific variables.

The preceding notes regarding mysql_init() also apply to mysql_connect(), which calls

mysql_init().

3.5 Running C API Client Programs

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 of the mysql.h header 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.17

to libmysqlclient.so.18).

The major shared client library version determines compatibility. (For example, for

libmysqlclient.so.18.1.0, the major version is 18.) Libraries shipped with newer versions of MySQL

are drop-in replacements for older versions that have the same major number. As long as the major library

version is the same, you can upgrade the library and old applications should continue to work with it.

Undefined-reference errors might occur at runtime when you try to execute a MySQL program. If these

errors specify symbols that start with mysql_ or indicate that the libmysqlclient library cannot be

found, it means that your system cannot find the shared libmysqlclient.so library. The solution to this

problem is to tell your system to search for shared libraries in the directory where that library is located.

Use whichever of the following methods is appropriate for your system:

• Add the path of the directory where libmysqlclient.so is located to the LD_LIBRARY_PATH or

LD_LIBRARY environment variable.

Using C API Features

• On macOS, add the path of the directory where libmysqlclient.dylib is located to the

DYLD_LIBRARY_PATH environment variable.

• Copy the shared-library files (such as libmysqlclient.so) to some directory that is searched by your

system, such as /lib, and update the shared library information by executing ldconfig. Be sure to

copy all related files. A shared library might exist under several names, using symlinks to provide the

alternate names.

3.6 Using C API Features

The following sections dicsuss techniques for working with several features of the C API into your

applications. It also covers some restrictions and troubleshooting topics.

3.6.1 Support for Encrypted Connections

This section describes how C applications use the C API capabilities for encrypted connections. By default,

MySQL programs attempt to connect using encryption if the server supports encrypted connections, falling

back to an unencrypted connection if an encrypted connection cannot be established (see Configuring

MySQL to Use Encrypted Connections). For applications that require control beyond the default behavior

over how encrypted connections are established, the C API provides these capabilities:

• The mysql_options() function enables applications to set the appropriate SSL/TLS options before

calling mysql_real_connect(). For example, to require the use of an encrypted connection, see

Enforcing an Encrypted Connection.

• The mysql_get_ssl_cipher() function enables applications to determine, after a connection has

been established, whether the connection uses encryption. A NULL return value indicates that encryption

is not being used. A non-NULL return value indicates an encrypted connection and names the encryption

cipher. See Section 5.4.35, “mysql_get_ssl_cipher()”.

• Options for Encrypted Connections

• Enforcing an Encrypted Connection

• Improving Security of Encrypted Connections

Options for Encrypted Connections

mysql_options() provides the following options for control over use of encrypted connections. For

option details, see Section 5.4.51, “mysql_options()”.

• MYSQL_OPT_SSL_CA: The path name of the Certificate Authority (CA) certificate file. This option, if used,

must specify the same certificate used by the server.

• MYSQL_OPT_SSL_CAPATH: The path name of the directory that contains trusted SSL CA certificate files.

• MYSQL_OPT_SSL_CERT: The path name of the client public key certificate file.

• MYSQL_OPT_SSL_CIPHER: The list of encryption ciphers the client permits for connections that use TLS

protocols up through TLSv1.2.

• MYSQL_OPT_SSL_CRL: The path name of the file containing certificate revocation lists.

• MYSQL_OPT_SSL_CRLPATH: The path name of the directory that contains certificate revocation list files.

• MYSQL_OPT_SSL_KEY: The path name of the client private key file.

• MYSQL_OPT_SSL_MODE: The connection security state.

Multiple Statement Execution Support

• MYSQL_OPT_TLS_CIPHERSUITES: The list of encryption ciphersuites the client permits for connections

that use TLSv1.3.

• MYSQL_OPT_TLS_VERSION: The encryption protocols the client permits.

mysql_ssl_set() can be used as a convenience routine that is equivalent to a set of

mysql_options() calls that specify certificate and key files, encryption ciphers, and so forth. See

Section 5.4.79, “mysql_ssl_set()”.

Enforcing an Encrypted Connection

mysql_options() options for information such as SSL certificate and key files are used to establish

an encrypted connection if such connections are available, but do not enforce any requirement that the

connection obtained be encrypted. To require an encrypted connection, use the following technique:

1. Call mysql_options() as necessary supply the appropriate SSL parameters (certificate and key

files, encryption ciphers, and so forth).

2. Call mysql_options() to pass the MYSQL_OPT_SSL_MODE option with a value of

SSL_MODE_REQUIRED or one of the more-restrictive option values.

3. Call mysql_real_connect() to connect to the server. The call fails if an encrypted connection

cannot be obtained; exit with an error.

Improving Security of Encrypted Connections

For additional security relative to that provided by the default encryption, clients can supply a CA certificate

matching the one used by the server and enable host name identity verification. In this way, the server and

client place their trust in the same CA certificate and the client verifies that the host to which it connected is

the one intended:

• To specify the CA certificate, call mysql_options() to pass the MYSQL_OPT_SSL_CA (or

MYSQL_OPT_SSL_CAPATH) option, and call mysql_options() to pass the MYSQL_OPT_SSL_MODE

option with a value of SSL_MODE_VERIFY_CA.

• To enable host name identity verification as well, call mysql_options() to pass the

MYSQL_OPT_SSL_MODE option with a value of SSL_MODE_VERIFY_IDENTITY rather than

SSL_MODE_VERIFY_CA.

Note

Host name identity verification with SSL_MODE_VERIFY_IDENTITY does not

work with self-signed certificates created automatically by the server, or manually

using mysql_ssl_rsa_setup (see Creating SSL and RSA Certificates and Keys

using MySQL). Such self-signed certificates do not contain the server name as the

Common Name value.

Host name identity verification also does not work with certificates that specify the

Common Name using wildcards because that name is compared verbatim to the

server name.

3.6.2 Multiple Statement Execution Support

By default, mysql_real_query() and mysql_query() interpret their statement string argument as a

single statement to be executed, and you process the result according to whether the statement produces

a result set (a set of rows, as for SELECT) or an affected-rows count (as for INSERT, UPDATE, and so

forth).

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