fileno() — Get the file descriptor from an open stream

Standards

Standards / Extensions C or C++ Dependencies

POSIX.1a
XPG4
XPG4.2
Single UNIX Specification, Version 3
Language Environment

 both  

Format

#define _POSIX_SOURCE
#include <stdio.h>

int fileno(const FILE *stream);

#define _OPEN_SYS_UNLOCKED_EXT 1
#include <stdio.h>

int fileno_unlocked(const FILE *stream);

General description

Returns the file descriptor number associated with a specified z/OS® XL C/C++ I/O stream. The argument stream points to a FILE structure controlling a z/OS XL C/C++ I/O stream.

The unistd.h header file defines the following macros, which are constants that map to the file descriptors of the standard streams:
STDIN_FILENO
Standard input, stdin (value 0)
STDOUT_FILENO
Standard output, stdout (value 1)
STDERR_FILENO
Standard error, stderr (value 2)

Note that stdin, stdout, and stderr are macros, not constants.

fileno_unlocked() is functionally equivalent to fileno() with the exception that it is not thread-safe. This function can safely be used in a multithreaded application if and only if it is called while the invoking thread owns the (FILE*) object, as is the case after a successful call to either the flockfile() or ftrylockfile() function.

Returned value

If successful, fileno() returns the file descriptor number associated with an open HFS stream (that is, one opened with fopen() or freopen()). MVS™ data sets are not supported, so fileno() of an MVS data set returns -1.

If unsuccessful, fileno() returns -1 and sets errno to one of the following values:
Error Code
Description
EBADF
One of the following error conditions exists:
  • stream points to a closed stream
  • stream is an incorrect stream pointer
  • stream points to a stream associated with an MVS data set.

Example

CELEBF21 
⁄* CELEBF21

   This example illustrates one use of fileno().

 *⁄
#define _POSIX_SOURCE
#include <errno.h>
#include <stdio.h>

main() {
  FILE *stream;
  char hfs_file[]=".⁄hfs_file", mvs_ds[]="⁄⁄mvs.ds";

  printf("fileno(stdin) = %d\n", fileno(stdin));

  if ((stream = fopen(hfs_file, "w")) == NULL)
    perror("fopen() error for HFS file");
  else {
    printf("fileno() of the HFS file is %d\n", fileno(stream));
    fclose(stream);
    remove(hfs_file);
  }

  if ((stream = fopen(mvs_ds, "w")) == NULL)
    perror("fopen() error for MVS data set");
  else {
    errno = 0;
    printf("fileno() returned %d for MVS data set,\n",fileno(stream));
    printf(" errno=%s\n", strerror(errno));
    fclose(stream);
    remove(mvs_ds);
  }
}
Output 
fileno(stdin) = 0
fileno() of the HFS file is 3
fileno() returned -1 for the MVS data set,
errno=Bad file descriptor

Related information

Parent topic: Library functions