安装好MySQL后没有/usr/local/mysql 目录
源程序如下：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "sql.h "
#include "sqlext.h "

void
check_return (RETCODE rc,
HENV henv,
HDBC hdbc,
HSTMT hstmt )
{
UCHAR state_str [SQL_MAX_MESSAGE_LENGTH];
SDWORD native_error;
UCHAR error_msg [SQL_MAX_MESSAGE_LENGTH];
SWORD error_msg_avail = SQL_MAX_MESSAGE_LENGTH - 1;
SWORD error_msg_len;
RETCODE local_rc;

if (rc != SQL_ERROR && rc != SQL_SUCCESS_WITH_INFO )
{
return;
}


local_rc = SQL_Error (henv,hdbc,hstmt,state_str,&native_error,error_msg,error_msg_avail,&error_msg_len);
if (local_rc != SQL_SUCCESS && rc !=SQL_SUCCESS_WITH_INFO)
{
fprintf(stderr, "Uninterpretable error; exiting \n ");
exit (EXIT_FAILURE);
}

if (rc == SQL_SUCCESS_WITH_INFO)
{
fprintf(stderr, "Fatal Error %s: %s\n ", state_str,error_msg);
return;
}

fprintf(stderr, "Fatal Error %s: %s\n ",state_str,error_msg);
exit (EXIT_FAILURE);
}

/*----------------------------------*/
int main (void)
{
HENV henv = SQL_NULL_HENV;
HDBC hdbc = SQL_NULL_HDBC;
HSTMT hstmt = SQL_NULL_HSTMT;
RETCODE rc;

char buf[257];
short buflen;

printf ( "Initialize the environment structure.\n ");
SQLAllocEnv (&henv);

printf ( "Initialize the connection structure.\n ");
SQLAllocConnect (henv,&hdbc);

printf( "Load the ODBC driver.\n ");
rc = SQLDriverConnect (hdbc,0, "DSN=baseball;UID=myloginWD=mypassword ",SQL_NTS,(UCHAR*) buf,sizeof (buf),&buflen,SQL_DRIVER_COMPLETE);
check_return (rc,henv,hdbc,hstmt);

printf ( "Initialize the statement structure.\n ");

SQLAllocStmt (hdbc,&hstmt);

/* now do something*/
printf ( "Creat table table \ "foo\ ".\n ");
rc = SQLExecdirect (hstmt, "CREATE TABLE foo (bar INTEGER) ", SQL_NTS);
check_return (rc, henv, hdbc, hstmt);

printf ( "Insert values into table \ "foo\ ".\n ");
rc = SQLExecdirect (hstmt, "INSERT INTO foo(bar) VALUES (1) ", SQL_NTS);
check_return (rc, henv, hdbc, hstmt);

rc = SQLExecdirect (hstmt, "INSERT INTO foo(bar) VALUES (2) ", SQL_NTS);
check_return (rc, henv, hdbc, hstmt);

rc = SQLExecdirect (hstmt, "INSERT INTO foo(bar) VALUES (3) ", SQL_NTS);
check_return (rc, henv, hdbc, hstmt);

printf ( "Drop table \ "foo\ ".\n ");
rc = SQLExecDirect (hstmt, "DROP TABLE foo ", SQL_NTS);
check_return (rc,henv,hdbc,hstmt);

/* We 're done:free resources and exit*/

printf ( "Free the statement handle.\n ");
SQLFreeStmt (hstmt,SQL_DROP);

printf ( "Disconnect from the data source.\n ");
SQLDisconnect (hdbc);

printf ( "Free the connection structure.\n ");
SQLFreeConnect (hdbc);

printf ( "Free the environment structure.\n ");
SQLFreeEnv (henv);

printf ( "Goodbye!\n ");
exit (EXIT_SUCCESS);
}


我的程序编译、连接的结果是：
[root@localhost database_experiment]# gcc odbc.c
/tmp/ccldHxyK.o(.text+0x5a): In function `check_return ':
: undefined reference to `SQLError '
/tmp/ccldHxyK.o(.text+0x130): In function `main ':
: undefined reference to `SQLAllocEnv '
/tmp/ccldHxyK.o(.text+0x152): In function `main ':
: undefined reference to `SQLAllocConnect '
/tmp/ccldHxyK.o(.text+0x18b): In function `main ':
: undefined reference to `SQLDriverConnect '
/tmp/ccldHxyK.o(.text+0x1c7): In function `main ':
: undefined reference to `SQLAllocStmt '
/tmp/ccldHxyK.o(.text+0x1ec): In function `main ':
: undefined reference to `SQLExecdirect '

/tmp/ccldHxyK.o(.text+0x22b): In function `main ':
: undefined reference to `SQLExecdirect '
/tmp/ccldHxyK.o(.text+0x25a): In function `main ':
: undefined reference to `SQLExecdirect '
/tmp/ccldHxyK.o(.text+0x289): In function `main ':
: undefined reference to `SQLExecdirect '
/tmp/ccldHxyK.o(.text+0x2c8): In function `main ':
: undefined reference to `SQLExecDirect '
/tmp/ccldHxyK.o(.text+0x302): In function `main ':
: undefined reference to `SQLFreeStmt '
/tmp/ccldHxyK.o(.text+0x320): In function `main ':
: undefined reference to `SQLDisconnect '
/tmp/ccldHxyK.o(.text+0x33e): In function `main ':
: undefined reference to `SQLFreeConnect '
/tmp/ccldHxyK.o(.text+0x35c): In function `main ':
: undefined reference to `SQLFreeEnv '
collect2: ld returned 1 exit status

网上说要执行
ln -s /usr/local/mysql/lib /usr/lib/mysql

ln -s /usr/local/mysql/include /usr/include/mysql

然后执行
gcc -g connect_db.c -L/usr/lib/mysql -lmysqlclient -lz

但是我发现没有/usr/local/mysql/lib 这个目录，是什么原因，应该怎么办？

[解决办法]
一般都是再/usr/local/include/mysql
[解决办法]
你的mysql是不是装到/usr/share下面去了?
[解决办法]
其实你安装的时候可以指定相应的安装目录
./congfigure --prefix =/usr/local/mysql
不过也没关系
你在编译的时候把库和头文件指定到你安装目录当中就可以了!
gcc -g connect_db.c-I/usr/include/mysql -L/usr/lib/mysql -lmysqlclient -lz -lm