Troubleshooting Problems with MySQL Programs - Installation-Related Issues

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A.3.1 Problems Linking to the MySQL Client Library

When you are linking an application program to use the MySQL client library, you might get undefined reference errors for symbols that start with mysql_, such as those shown here:

/tmp/ccFKsdPa.o: In function ´main':
/tmp/ccFKsdPa.o(.text+0xb): undefined reference to ´mysql_init'
/tmp/ccFKsdPa.o(.text+0x31): undefined reference to ´mysql_real_connect'
/tmp/ccFKsdPa.o(.text+0x57): undefined reference to ´mysql_real_connect'
/tmp/ccFKsdPa.o(.text+0x69): undefined reference to ´mysql_error'
/tmp/ccFKsdPa.o(.text+0x9a): undefined reference to ´mysql_close'

You should be able to solve this problem by adding -Ldir_path -lmysqlclient at the end of your link command, where dir_path represents the pathname of the directory where the client library is located. To determine the correct directory, try this command:

shell> mysql_config --libs

The output from mysql_config might indicate other libraries that should be specified on the link command as well.

If you get undefined reference errors for the uncompress or compress function, add -lz to the end of your link command and try again.

If you get undefined reference errors for a function 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.

You might get undefined reference errors such as the following for functions that don't exist on your system:

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

This usually means that your MySQL client library was compiled on a system that is not 100% compatible with yours. In this case, you should download the latest MySQL source distribution and compile MySQL yourself. See Section 2.3, "MySQL Installation Using a Source Distribution."

You might get undefined reference errors at runtime when you try to execute a MySQL program. If these errors specify symbols that start with mysql_ or indicate that the mysqlclient library can't be found, it means that your system can't find the shared libmysqlclient.so library. The fix for this is to tell your system to search for shared libraries where the library is located. Use whichever of the following methods is appropriate for your system:

• Add the path to the directory where libmysqlclient.so is located to the LD_LIBRARY_PATH environment variable.

• Add the path to the directory where libmysqlclient.so is located to the LD_LIBRARY environment variable.

• Copy libmysqlclient.so to some directory that is searched by your system, such as /lib, and update the shared library information by executing ldconfig.

Another way to solve this problem is by linking your program statically with the -static option, or by removing the dynamic MySQL libraries before linking your code. Before trying the second method, you should be sure that no other programs are using the dynamic libraries.

A.3.2 How to Run MySQL as a Normal User

On Windows, you can run the server as a Windows service using normal user accounts beginning with MySQL 4.0.17 and 4.1.2. (Older MySQL versions required you to have administrator rights. This was a bug introduced in MySQL 3.23.54.)

On Unix, the MySQL server mysqld can be started and run by any user. However, you should avoid running the server as the Unix root user for security reasons. In order to change mysqld to run as a normal unprivileged Unix user user_name, you must do the following:

1. Stop the server if it's running (use mysqladmin shutdown).

2. Change the database directories and files so that user_name has privileges to read and write files in them (you might need to do this as the Unix root user):

   shell> chown -R user_name /path/to/mysql/datadir

3. If you do not do this, the server will not be able to access databases or tables when it runs as user_name.

4. If directories or files within the MySQL data directory are symbolic links, you'll also need to follow those links and change the directories and files they point to. chown -R might not follow symbolic links for you.

5. Start the server as user user_name. If you are using MySQL 3.22 or later, another alternative is to start mysqld as the Unix root user and use the --user=user_name option. mysqld will start up, then switch to run as the Unix user user_name before accepting any connections.

6. To start the server as the given user automatically at system startup time, specify the username by adding a user option to the [mysqld] group of the /etc/my.cnf option file or the my.cnf option file in the server's data directory. For example:

   [mysqld]
   user=user_name

If your Unix machine itself isn't secured, you should assign passwords to the MySQL root accounts in the grant tables. Otherwise, any user with a login account on that machine can run the mysql client with a --user=root option and perform any operation. (It is a good idea to assign passwords to MySQL accounts in any case, but especially so when other login accounts exist on the server host.) See Section 2.4, "Post-Installation Setup and Testing."  

A.3.3 Problems with File Permissions

If you have problems with file permissions, the UMASK environment variable might be set incorrectly when mysqld starts. For example, MySQL might issue the following error message when you create a table:

ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)

The default UMASK value is 0660. You can change this behavior by starting mysqld_safe as follows:

shell> UMASK=384 # = 600 in octal
shell> export UMASK
shell> mysqld_safe &

By default, MySQL creates database and RAID directories with an access permission value of 0700. You can modify this behavior by setting the UMASK_DIR variable. If you set its value, new directories are created with the combined UMASK and UMASK_DIR values. For example, if you want to give group access to all new directories, you can do this:

shell> UMASK_DIR=504 # = 770 in octal
shell> export UMASK_DIR
shell> mysqld_safe &

In MySQL 3.23.25 and above, MySQL assumes that the value for UMASK and UMASK_DIR is in octal if it starts with a zero.

See Appendix B, "Environment Variables."   

This chapter is from MySQL Administrator's Guide, by MySQL AB. (Sams, 2004, ISBN: 0672326345). Check it out at your favorite bookstore today. Buy this book now.

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