c - sprintf() with automatic memory allocation?

Question

I'm searching for a sprintf()-like implementation of a function that automatically allocates required memory. So I want to say

char* my_str = dynamic_sprintf( "Hello %s, this is a %.*s nice %05d string", a, b, c, d );

and my_str retrieves the adress of an allocated memory that holds the result of this sprintf().

In another forum, I read that this can be solved like this:

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

int main()
{
    char*   ret;
    char*   a = "Hello";
    char*   b = "World";
    int     c = 123;

    int     numbytes;

    numbytes = sprintf( (char*)NULL, "%s %d %s!", a, c, b );
    printf( "numbytes = %d", numbytes );

    ret = (char*)malloc( ( numbytes + 1 ) * sizeof( char ) );
    sprintf( ret, "%s %d %s!", a, c, b );

    printf( "ret = >%s<
", ret );
    free( ret );

    return 0;
}

But this immediatelly results in a segfault when the sprintf() with the NULL-pointer is invoked.

So any idea, solution or tips? A small implementation of a sprintf()-like parser that is placed in the public domain would already be enought, then I could get it myself done.

Thanks a lot!

Answer 1

Here is the original answer from Stack Overflow. As others have mentioned, you need snprintf not sprintf. Make sure the second argument to snprintf is zero. That will prevent snprintf from writing to the NULL string that is the first argument.

The second argument is needed because it tells snprintf that enough space is not available to write to the output buffer. When enough space is not available snprintf returns the number of bytes it would have written, had enough space been available.

Reproducing the code from that link here ...

char* get_error_message(char const *msg) {
    size_t needed = snprintf(NULL, 0, "%s: %s (%d)", msg, strerror(errno), errno) + 1;
    char  *buffer = malloc(needed);
    sprintf(buffer, "%s: %s (%d)", msg, strerror(errno), errno);
    return buffer;
}