SNPRINTF

Section: Linux Programmer's Manual (3)
Updated: 16 September 1995

NAME

snprintf, vsnprintf - formatted output conversion

SYNOPSIS

#define _GNU_SOURCE
#include <stdio.h>

int snprintf ( char *str, size_t n,
const char *format, ... );

#include <stdarg.h>

int vsnprintf ( char *str, size_t n,
const char *format, va_list ap );

DESCRIPTION

snprintf writes output to the string str, under control of the format string that specifies how subsequent arguments are converted for output. It is similar to sprintf(3), except that n specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least n characters for the string str.

vsnprintf is the equivalent of snprintf with the variable argument list specified directly as for vprintf.

RETURN VALUE

If the output was truncated, the return value is -1, otherwise it is the number of characters stored, not including the terminating null. (Thus until glibc 2.0.6. Since glibc 2.1 these functions return the number of characters (excluding the trailing null) which would have been written to the final string if enough space had been available.)

EXAMPLE

Here is an example program which dynamically enlarges its output buffer, and works correctly under both glibc 2.0 and glibc 2.1.

    /* Construct a message describing the value of a 
       variable whose name is NAME and whose value is 
       VALUE. */
    char *
    make_message (char *name, char *value)
    {
      /* Guess we need no more than 100 chars of space. */
      int size = 100;
      char *buffer = (char *) xmalloc (size);
      while (1)
        {
          /* Try to print in the allocated space. */
          int nchars = snprintf (buffer, size,
                        "value of %s is %s", name, value);
          /* If that worked, return the string. */
          if (nchars > -1 && nchars < size)
            return buffer;
          /* Else try again with more space. */
          if (nchars > -1)
            size = nchars+1;  /* precisely what is needed */
          else
            size *= 2;        /* twice the old size */
          buffer = (char *) xrealloc (buffer, size);
        }
    }

CONFORMING TO

These are GNU extensions, expected to become part of ISO C9x.