asprintf.c

/* Compile with:
CFLAGS="-g -Wall -std=gnu11 -O3 -DTest_asprintf" make asprintf
*/
#ifndef HAVE_ASPRINTF
#include <stdio.h>  //vsnprintf
#include <stdlib.h> //malloc
#include <stdarg.h> //va_start et al

/* The declaration, to put into a .h file.    The __attribute__ tells the compiler to check printf-style type-compliance.
   It's not C standard, but a lot of compilers support it. Just remove it if yours doesn't. */

int asprintf(char **str, char* fmt, ...) __attribute__ ((format (printf,2,3)));


int asprintf(char **str, char* fmt, ...){
    va_list argp;
    va_start(argp, fmt);
    char one_char[1];
    int len = vsnprintf(one_char, 1, fmt, argp);
    if (len < 1){
        fprintf(stderr, "An encoding error occurred. Setting the input pointer to NULL.\n");
        *str = NULL;
        va_end(argp);
        return len;
    }
    va_end(argp);

    *str = malloc(len+1);
    if (!str) {
        fprintf(stderr, "Couldn't allocate %i bytes.\n", len+1);
        return -1;
    }
    va_start(argp, fmt);
    vsnprintf(*str, len+1, fmt, argp);
    va_end(argp);
    return len;
}
#endif

#ifdef Test_asprintf
int main(){
    char *s;
    asprintf(&s, "hello, %s.", "—Reader—");
    printf("%s\n", s);
    free(s);

    asprintf(&s, "%c", '\0');
    printf("blank string: [%s]\n", s);
    free(s);

    int i = 0;
    asprintf(&s, "%i", i++);
    printf("Zero: %s\n", s);
    free(s);
}
#endif