Length of each string argument

Question

Recently I've started to get a little bit into the stdarg.h functions, because I want to have something similar to printf but instead of writing to the console I want to return it as a string.

Here's what I've come up with so far:

char *write(const char *format, ...)
{
    // init
    va_list arg;
    char *string;

    va_start (arg, format);
    vsprintf (string, format, arg);

    // done
    va_end (arg);
    return string;
}

Now the problem is, that string has not reserved memory and that's where I need help with a way to fix this function as I have not found any solution yet.

Thanks in advance

Answer 1

Use snprintf(NULL, 0 to check how long buffer you need. Then allocate memory. Then print to the string.

char *my_write(const char *format, ...) {
    va_list va;
    va_start(va, format);

    // remember to have a separate va_list for each v*print function
    va_list va2;
    va_copy(va2, va);
    const int len = vsnprintf(NULL, 0, format, va2);
    va_end(va2);

    char *string = malloc((len + 1) * sizeof(*string));
    if (string != NULL) {
       vsprintf(string, format, va);
    }
    va_end(va);

    return string;
}

As suggested by @IanAbbott in comments you can invoke va_start twice, which seems to nicely simplify the code:

char *my_write(const char *format, ...) {
    va_list va;

    va_start(va, format);
    const int len = vsnprintf(NULL, 0, format, va);
    va_end(va);

    char *string = malloc((len + 1) * sizeof(*string));
    if (string == NULL) {
       return NULL;
    }

    va_start(va, format);
    vsprintf(string, format, va);
    va_end(va);

    return string;
}

On platforms with with glibc you can also use vasprintf. Note that the name write is already used by posix write() function, I suggest to use a different name. With vasprintf GNU extension it becomes just:

#define _GNU_SOURCE
#include <stdio.h>
char *write2(const char *format, ...) {
    va_list va;
    va_start(va, format);
    char *string;
    const int err = vasprintf(&string, format, va);
    va_end(va);
    if (err == -1) {
          return NULL;
    }
    return string;
}