asprintf, aswprintf, vasprintf, vaswprintf

From cppreference.com
C
Technical Specifications
Dynamic memory extensions
Defined in header <stdio.h>
int asprintf ( char ** restrict strp, const char * restrict fmt, ... ) ;
(1) (dynamic memory TR)
int aswprintf ( wchar_t ** restrict strp, const wchar_t * restrict fmt, ... ) ;
(2) (dynamic memory TR)
int vasprintf ( char ** restrict strp, const char * restrict fmt,
va_list arg ) ;
(3) (dynamic memory TR)
int vaswprintf ( wchar_t ** restrict strp, const wchar_t * restrict fmt,
va_list arg ) ;
(4) (dynamic memory TR)
1) Analog of sprintf , except that it allocates a storage large enough to hold the output including the terminating null character, as if by a call to malloc , and returns a pointer to that storage via the first argument. This pointer should be passed to free to release the allocated storage when it is no longer needed.
2) Same as (1) , except that it works with wide characters wchar_t (by analogy with swprintf ).
3) Same as (1) , with the variable argument list replaced by arg , which shall be initialized by the va_start macro (and possibly subsequent va_arg calls).
4) Same as (3) , except that it works with wide characters wchar_t .

Parameters

strp - A pointer to a char * or wchar_t * which will contain the formatted output
fmt - A format string as with printf / wprintf and related functions
arg - Any extra arguments are used as with vsprintf and vswprintf

Return value

The number of characters written, just like sprintf (1) , swprintf (2) , vsprintf (3) , or vswprintf (4) , respectively. If memory allocation wasn't possible, or some other error occurs, these functions will return - 1 , and the contents of strp is undefined.

Notes

These functions are GNU extensions, not in C or POSIX. They are also available under *BSD. The FreeBSD implementation sets strp to NULL on error.

The vasprintf and vaswprintf functions do not invoke the va_end macro.

Example

Can be tested with clang (C11)

Run this code 
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
 
void test(const char *fmt, ...)
{
    char* dyn_buf;
 
    printf("Demo asprintf:\n");
    const int written_1 = asprintf(&dyn_buf, "%s", fmt);
    printf("dyn_buf: \"%s\"; %i chars were written\n", dyn_buf, written_1);
    free(dyn_buf);
 
    printf("Demo vasprintf:\n");
    va_list args;
    va_start(args, fmt);
    const int written_2 = vasprintf(&dyn_buf, fmt, args);
    va_end(args);
    printf("dyn_buf: \"%s\"; %i chars were written\n", dyn_buf, written_2);
    free(dyn_buf);
}
 
int main(void)
{
    test("Testing... %d, %d, %d", 1, 2, 3);
}

Output: 

Demo asprintf:
dyn_buf: "Testing... %d, %d, %d"; 21 chars were written
Demo vasprintf:
dyn_buf: "Testing... 1, 2, 3"; 18 chars were written
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