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MySQL Reference Manual by Kaj Arno, David Axmark, Michael Widenius

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Installation-Related Issues

Problems When Linking with the MySQL Client Library

If you are linking your program and you get errors for unreferenced symbols that start with mysql_, like the following:

/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 by adding -Lpath-to-the-mysql-library -lmysqlclient last on your link line.

If you get undefined reference errors for the uncompress or compress function, add -lz last on your link line and try again!

If you get undefined reference errors for functions that should exist on your system, like connect, check the manpage for the function in question for which libraries you should add to the link line!

If you get undefined reference errors for functions that don’t exist on your system, like the following:

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

it usually means that your library is compiled on a system that is not 100% compatible with yours. In this case you should download the latest MySQL source distribution and compile this yourself. See Section 2.3.

If you are trying to run a program and you then get errors for unreferenced symbols that start with mysql_ or that the mysqlclient library can’t be found, this means that your system can’t find the share libmysqlclient.so library.

The fix for this is to tell your system to search after shared libraries where the library is located by one of the following methods:

  • Add the path to the directory where you have libmysqlclient.so to the LD_LIBRARY_PATH environment variable.

  • Add the path to the directory where you have libmysqlclient.so to the LD_LIBRARY environment variable.

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

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

How to Run MySQL as a Normal User

The MySQL server mysqld can be started and run by any user. In order to change mysqld to run as a 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 may need to do this as the Unix root user):

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

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

  3. Start the server as user user_name, or if you are using MySQL Version 3.22 or later, start mysqld as the Unix root user and use the --user=user_name option. mysqld will switch to run as the Unix user user_name before accepting any connections.

  4. To start the server as the given username automatically at system startup time, add a user line that specifies the username 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

At this point, your mysqld process should be running fine and dandy as the Unix user user_name. One thing hasn’t changed, though: the contents of the permissions tables. By default (right after running the permissions table install script mysql_install_db), the MySQL user root is the only user with permission to access the mysql database or to create or drop databases. Unless you have changed those permissions, they still hold. This shouldn’t stop you from accessing MySQL as the MySQL root user when you’re logged in as a Unix user other than root; just specify the -u root option to the client program.

Note that accessing MySQL as root, by supplying -u root on the command-line, has nothing to do with MySQL running as the Unix root user or, indeed, as another Unix user. The access permissions and usernames of MySQL are completely separate from Unix usernames. The only connection with Unix usernames is that if you don’t provide a -u option when you invoke a client program, the client will try to connect using your Unix login name as your MySQL user name.

If your Unix box itself isn’t secured, you should probably at least put a password on the MySQL root users in the access tables. Otherwise, any user with an account on that machine can run mysql -u root db_name and do whatever he likes.

Problems with File Permissions

If you have problems with file permissions—for example, if mysql issues the following error message when you create a table:

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

the environment variable UMASK might be set incorrectly when mysqld starts up. The default umask value is 0660. You can change this behavior by starting safe_mysqld as follows:

shell> UMASK=384  # = 600 in octal
shell> export UMASK
shell> /path/to/safe_mysqld &

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

shell> UMASK_DIR=504  # = 770 in octal
shell> export UMASK_DIR
shell> /path/to/safe_mysqld &

In MySQL Versions 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 E.

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