Opening multiple MySQL connections in one program (with and without threads).

[terry jones]

I'm trying to open multiple connections to my MySQL server from the
one executable, but I keep getting segmentation violations. Can
someone take a look at the following code and tell me if it works for
them, or what I am doing wrong? There must be something basic I am
missing (I guess). I am running mysql-3.22.21 on a 400MHz Pentium II
GNU/Linux Redhat 5.1 with 512MB of memory.

I built my version of mysql via:

  ./configure \
    --prefix=/usr/local/mysql \
    --with-unix-socket-path=/usr/local/mysql/.mysql-pipe


I have been compiling the following program with

  gcc -g -o x x.c -L/usr/local/mysql/lib/mysql -I/usr/local/mysql -lmysqlclient -lpthread

Which you will need to change if you do not have MySQL installed under
/usr/local/mysql.

Also, in the code below, change the strings for UNIVERSE, USER, and
PASSWORD so you can connect to your own mysqld.

The program takes a single numeric argument (0 to 5) telling it what
to do. I get core dumps for behaviors 2, 3, and 4.

Thanks for any help. If no-one can help, I'll make a -g version of the
client library and get details on exactly where the SEGV is happening.

Terry.

--------------------------------------------------------

#include <stdio.h>
#include <pthread.h>
#include "mysql/mysql.h"

void *conn(void *arg);

#define t_create                                                \
if (pthread_create(&thread_id, NULL, conn, (void *)1)) {        \
    fprintf(stderr, "error creating thread.\n");                \
    exit(1);                                                    \
}

#define t_join                                                  \
if (pthread_join(thread_id, NULL)){                             \
    fprintf(stderr, "error rejoining thread.\n");               \
    exit(1);                                                    \
}

int
main(argc, argv)
int argc;
char **argv;
{
    int behavior = 0;

    if (argc > 1){
        behavior = atoi(argv[1]);
    }

    fprintf(stderr, "BEHAVIOR %d: ", behavior);

    switch (behavior){

        case 0:{
            fprintf(stderr, "One connection, unthreaded.\n");
            conn((void *)0);
            break;
        }

        case 1:{
            pthread_t thread_id;
            fprintf(stderr, "One connection, threaded.\n");
            t_create;
            t_join;
            break;
        }

        case 2:{
            pthread_t thread_id;
            fprintf(stderr, "Two connections, one unthreaded, one threaded.\n");
            conn((void *)0);
            t_create;
            t_join;
            break;
        }

        case 3:{
            fprintf(stderr, "Two connections, unthreaded.\n");
            conn((void *)0);
            fprintf(stderr, "First done.\n");
            conn((void *)0);
            fprintf(stderr, "Second done.\n");
            break;
        }

        case 4:{
            pthread_t thread_id;
            fprintf(stderr, "Two connections, threaded.\n");
            t_create;
            t_join;
            fprintf(stderr, "First done.\n");
            t_create;
            t_join;
            fprintf(stderr, "Second done.\n");
            break;
        }

        case 5:{
            fprintf(stderr, "Two connections, forked.\n");
            if (fork()){
                conn((void *)0);
                fprintf(stderr, "First done.\n");
            }
            else {
                conn((void *)0);
                fprintf(stderr, "Second done.\n");
            }
            break;
        }

        default:{
            fprintf(stderr, "unknown behavior.\n");
            exit(1);
        }
    }

    return 0;
}


void *
conn(void *arg)
{
    int threaded = (int)arg;
    
    MYSQL *mysql = (MYSQL *) malloc(sizeof(MYSQL));
    
    if (!mysql){
        fprintf(stderr, "bad mysql malloc.\n");
        exit(1);
    }
    
    fprintf(stderr, "Attempting to connect (%sthreaded).\n", threaded ? "" : "un");

    mysql_init(mysql);
    
    if (!mysql_real_connect(mysql, "localhost", "USER", "PASSWORD", "UNIVERSE", 3306,
NULL, 0)){
        fprintf(stderr, "bad mysql real connect (%sthreaded).\n", threaded ? "" : "un");
        exit(1);
    }

    if (threaded){
        fprintf(stderr, "Connected in thread %ld.\n", pthread_self());
    }
    else {
        fprintf(stderr, "Connected (unthreaded).\n");
    }
    
    return;
}