Question

I’m writing some C code for an embedded linux system using an open_memstream and I don’t understand why I am getting a compile warning: assignment makes pointer from integer without a cast

To make things simple, rather than pasting all my code I reproduced the problem with the small example from here:

#include <stdio.h>
#include <stdlib.h>

int
main (void)
{
    FILE *stream;
    char *buf;
    size_t len;
    off_t eob;

    stream = open_memstream (&buf, &len);
    if (stream == NULL)
        /* handle error */ ;
    fprintf (stream, "hello my world");
    fflush (stream);
    printf ("buf=%s, len=%zu\n", buf, len);
    eob = ftello(stream);
    fseeko (stream, 0, SEEK_SET);
    fprintf (stream, "good-bye");
    fseeko (stream, eob, SEEK_SET);
    fclose (stream);
    printf ("buf=%s, len=%zu\n", buf, len);
    free (buf);
    return 0;
}

The code works, but the compiler complains about the line stream = open_memstream (&buf, &len);

What integer is it talking about? We’re passing in a pointer to a size_t as required by the function prototype.

FILE *open_memstream(char **bufp, size_t *sizep);

Is there a problem with this code, or do I need to take a look at my compiler? I want to get rid of this warning the right way.

---

UPDATE:

Using gcc 4.3.2, glibc 2.9

---

UPDATE 2:

Tried the following:

powerpc-860-linux-gnu-gcc -std=c99 -Wall -D_XOPEN_SOURCE=700 -c source.c

Result:

source.c: In function 'main':
source.c:12: warning: implicit declaration of function 'open_memstream'
source.c:12: warning: assignment makes pointer from integer without a cast

According to this, it seems that _XOPEN_SOURCE=700 is available since glibc 2.10.

Since I’m using glibc 2.9, what other alternatives do I have (other than upgrading glibc)?

---

UPDATE 3:

Adding the following got rid of the warning:

extern FILE *open_memstream(char **bufp, size_t *sizep);

Is there anything wrong with this solution?

---

UPDATE 4:

This worked instead of the extern:

powerpc-860-linux-gnu-gcc -std=c99 -Wall -D_GNU_SOURCE -c ops_cmds.c

So according to the manpage, need to use _GNU_SOURCE if glibc pre-2.10 (in my case) and _XOPEN_SOURCE=700 if 2.10+

Answer 1

Define:

#define _POSIX_C_SOURCE 200809L

or

#define _XOPEN_SOURCE 700

in your source code before including stdio.h. Or with gcc you can define and pass the macro value to the source file with -D option:

gcc -std=c99 -Wall -D_XOPEN_SOURCE=700 -c source.c 

open_memstream is a POSIX function and its declaration is not visible in your program without this define.