Luke Shumaker » blog » fd_printf

dprintf: print formatted text directly to a file descriptor

This already existed as dprintf(3). I now feel stupid for having Implemented fd_printf.

The original post is as follows:

I wrote this while debugging some code, and thought it might be useful to others:

#define _GNU_SOURCE     /* vasprintf() */
#include <stdarg.h>     /* va_start()/va_end() */
#include <stdio.h>      /* vasprintf() */
#include <stdlib.h>     /* free() */
#include <unistd.h>     /* write() */

fd_printf(int fd, const char *format, ...)
    va_list arg;
    int len;
    char *str;

    va_start(arg, format);
    len = vasprintf(&str, format, arg);

    write(fd, str, len);

    return len;

It is a version of printf that prints to a file descriptor—where fprintf prints to a FILE* data structure.

The appeal of this is that FILE* I/O is buffered—which means mixing it with raw file descriptor I/O is going to produce weird results.