dup() — Duplicate an open file descriptor

Standards

Standards / Extensions	C or C++	Dependencies
POSIX.1 XPG4 XPG4.2 Single UNIX Specification, Version 3	both

Format

#define _POSIX_SOURCE
#include <unistd.h>

int dup(int fildes);

General description

Returns a new file descriptor that is the lowest numbered available descriptor. The new file descriptor refers to the same open file as fildes and shares any locks that may be associated with fildes.

The following operations are equivalent:

fd = dup(fildes);
fd = fcntl(fildes,F_DUPFD,0);

For further information, see fcntl() — Control open file descriptors.

Note: When fildes is an XTI endpoint, the lowest numbered available file descriptor must not exceed 65535.

Returned value

If successful, dup() returns a new file descriptor.

If unsuccessful, dup() returns -1 and sets errno to one of the following values:

Error Code: Description
EBADF: fildes is not a valid open file descriptor.
EMFILE: The process has already reached its maximum number of open file descriptors.

Example

CELEBD05

/* CELEBD05

   This example duplicates an open file descriptor, using dup().

 */
#define _POSIX_SOURCE
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#undef _POSIX_SOURCE
#include <stdio.h>

void print_inode(int fd) {
 struct stat info;
 if (fstat(fd, &info) != 0)
   fprintf(stderr,"fstat() error for fd %d: %s\n",fd,strerror(errno));
 else
   printf("The inode of fd %d is %d\n", fd, (int) info.st_ino);
}

main() {
  int fd;
  if ((fd = dup(0)) < 0)
    perror("&dupf error");
  else {
    print_inode(0);
    print_inode(fd);
    puts("The file descriptors are different but");
    puts("they point to the same file.");
    close(fd);
  }
}

Output

The inode of fd 0 is 30
The inode of fd 3 is 30
The file descriptors are different but
they point to the same file.