C basics, string to pointer

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.

Brian Roach · Accepted Answer · 2012-02-02 20:56:50Z

2

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.

answered Feb 2, 2012 at 20:56

Brian Roach

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1 Comment

ugoren Over a year ago

Changing mysqlerror to be static is much simpler, and usually works. Don't do it with multi-threading, and not unless you're going to immediately print the string and forget about it. But under these conditions, it saves you the bother of allocation (and, more importantly, the risk of a leak).

perreal · Accepted Answer · 2012-02-02 20:48:16Z

1

You are returning a pointer to a local variable (char mysqlerror[255]). You should define mysqlerror in your main file and call your function like:

 if (!conn_check(mysqlerror)==0)

and change the prototype:

int conn_check(char *mysqlerror)

and remove lines:

 char mysqlerror[255] = {0};
 *m_error = mysqlerror;

answered Feb 2, 2012 at 20:48

perreal

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3 Comments

Wine Too Over a year ago

Here comes segmentation fault on sprintf.

Wine Too Over a year ago

On header which is included in main and function file.

Wine Too Over a year ago

Yes, your solution also work. I am declared mysqlerror as pointer not char array, that causes error. Now is OK. Thanks for helping. Now I don't need to free mysqlerror after usage?

Richard J. Ross III · Accepted Answer · 2012-02-02 20:53:31Z

1

Here would be my solution:

char m_error[255];
if (!conn_check(&m_error)==0)
{
 printf("%s\n", m_error);
}

int conn_check(char **m_error)
{
    int retval = 0;
    char mysqlerror[255];
    MYSQL *conn;
    ...
    sprintf(mysqlerror, "%u: %s", mysql_errno(conn), mysql_error(conn));
    strcopy(*m_error, mysqlerror);
    retval = -1;
    ...
}

answered Feb 2, 2012 at 20:53

Richard J. Ross III

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Comments

Nick Shaw · Accepted Answer · 2012-02-02 20:59:54Z

Upon return from conn_check(), if you do the *m_error = mysqlerror; line, you'll end up with a quite possibly invalid pointer by then, as the local char array mysqlerror is not valid outside of the local function.

You'll need to pass in a pointer to a buffer and copy the string in, or duplicate the string using strdup to allocate some global memory to give you a valid pointer for returning (but if you do this, don't forget to free the memory in main() using free afterwards).

EDIT: If you choose to pass in a buffer, it's also good practice to pass in the max buffer size, so when you copy your string in, you don't overflow the buffer.

EDIT 2: A very hacky way of fixing your existing code with minimum code is of course to declare mysqlerror as static, so it is valid outside of the function. I wouldn't ever recommend doing that though, as it means the function wouldn't be thread-safe.

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