apue_vprintf.h

#include <stdarg.h>
#include <stdio.h>

int vprintf(const char *restrict format, va_list arg);
int vfprintf(FILE *restrict fp, const char *restrict format,
             va_list arg);
int vdprintf(int fd, const char *restrict format, va_list arg);

/* All three return: number of characters output if OK, negative value if output error */

int vsprintf(char *restrict buf, const char *restrict format, va_list arg);

/* Returns: number of characters stored in array if OK, negative value if encoding error */

int vsnprintf(char *restrict buf, size_t n,
              const char *restrict format, va_list arg);
              
/* Returns: number of characters that would have been stored in array if buffer was
   large enough, negative value if encoding error */