Question

I have project with 3 files.
Common header contain function declaration for check mysql connection:

int conn_check(char **m_error);

Main file calls function and expect some message in m_error in case of error:

if (!conn_check(&m_error)==0)
{
 printf("%s\n", m_error);
}

And now function in which I have problem because weak knowing of pointers:

int conn_check(char **m_error)
{
int retval = 0;
char mysqlerror[255] = {0};
MYSQL *conn;
conn = mysql_init(NULL);
if (conn)
{
    if (mysql_real_connect(conn, mysql_server, mysql_user_name, mysql_password, "", (ulong)mysql_serverport, mysql_socket, 0)==NULL)
    {
        sprintf(mysqlerror, "%u: %s", mysql_errno(conn), mysql_error(conn));
        *m_error = mysqlerror;  // Problem here
        retval = -1;
    }
} else retval = -2;
mysql_close(conn);
return retval;
}

Question is how to properly assign string mysqlerror to char pointer m_error so error message can be printed through printf in main.

Answer 1

char **m_error means you’re passing a pointer to a pointer. Presumably this is because the function already returns an int and you want to also have the text of the error. As it is, you’re assigning the address of a stack variable to the pointer which you can not do.

You would need to allocate memory, assign it to the pointer, then write to it:

*m_error = calloc(255, sizeof(char));
snprintf(*m_error, 255, "%u: %s", mysql_errno(conn), mysql_error(conn));

vasprintf() will do all of it for you:

vasprintf(m_error, "%u: %s", mysql_errno(conn), mysql_error(conn));

Note that you would then need to free() this back in the calling function.