Multi-platform asprintf, vasprintf and vscprintf implementation, C (gcc)

Language: Layout:

#if defined(__GNUC__) && ! defined(_GNU_SOURCE)
#define _GNU_SOURCE /* needed for (v)asprintf, affects '#include <stdio.h>' */
#endif
#include <stdio.h>  /* needed for vsnprintf    */
#include <stdlib.h> /* needed for malloc, free */
#include <stdarg.h> /* needed for va_*         */

/*
 * vscprintf:
 * MSVC implements this as _vscprintf, thus we just 'symlink' it here
 * GNU-C-compatible compilers do not implement this, thus we implement it here
 */
#ifdef _MSC_VER
#define VSCPRINTF "native (symlinked)"
#define vscprintf _vscprintf
#endif

#ifdef __GNUC__
#define VSCPRINTF "implemented"
int vscprintf(const char *format, va_list ap)
{
    va_list ap_copy;
    va_copy(ap_copy, ap);
    int retval = vsnprintf(NULL, 0, format, ap_copy);
    va_end(ap_copy);
    return retval;
}
#endif

/*
 * asprintf, vasprintf:
 * MSVC does not implement these, thus we implement them here
 * GNU-C-compatible compilers implement these with the same names, thus we
 * don't have to do anything
 */
#ifdef _MSC_VER
#define VASPRINTF "implemented"
#define ASPRINTF "implemented"
int vasprintf(char **strp, const char *format, va_list ap)
{
    int len = vscprintf(format, ap);
    if (len == -1)
        return -1;
    char *str = (char*)malloc((size_t) len + 1);
    if (!str)
        return -1;
    int retval = vsnprintf(str, len + 1, format, ap);
    if (retval == -1) {
        free(str);
        return -1;
    }
    *strp = str;
    return retval;
}

int asprintf(char **strp, const char *format, ...)
{
    va_list ap;
    va_start(ap, format);
    int retval = vasprintf(strp, format, ap);
    va_end(ap);
    return retval;
}
#endif

#ifdef __GNUC__
#define VASPRINTF "native"
#define ASPRINTF "native"
#endif

int main(void)
{
    printf("vscprintf: %s, vasprintf: %s, asprintf: %s\n", VSCPRINTF, VASPRINTF, ASPRINTF);
    char *str = NULL;
    int len = asprintf(&str, "The answer to %s is %d", "life, the universe and everything", 42);
    if (str != NULL) {
        printf("String: %s\n", str);
        printf("Length: %d\n", len);
        free(str);
    }
    return 0;
}

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Compilation time: 0.15 sec, absolute running time: 0.18 sec, cpu time: 0.09 sec, memory peak: 3 Mb, absolute service time: 0,34 sec