dup2()

Duplicate a file descriptor

Synopsis:

#include <unistd.h>

int dup2( int filedes, 
          int filedes2 );

Library:

libc

Description:

The dup2() function duplicates the file descriptor specified by filedes. The number of the new file descriptor will be filedes2. If a file already is opened with this descriptor, the file is closed before the duplication is attempted.

The new file descriptor:

• References the same file or device
• Has the same open mode (read and/or write)
• Has an identical file position to the original (changing the position with one descriptor results in a changed position in the other).

Calling:

dup_filedes = dup2( filedes, filedes2 );

Is the same as:

close( filedes2 );
dup_filedes = fcntl( filedes , F_DUPFD, filedes2 );

Returns:

The value of filedes2 for success, or -1 if an error occurs (errno is set).

Errors:

EBADF

The file descriptor, filedes isn't a valid open file descriptor, or filedes2 is out of range.

EMFILE

There are already OPEN_MAX file descriptors in use.

ENOSYS

The dup2() function isn't implemented for the filesystem specified by filedes.

Examples:

#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdlib.h>

int main( void )
{
    int filedes , dup_filedes ;

    filedes = open( "file",
        O_WRONLY | O_CREAT | O_TRUNC,
        S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP );

    if( filedes != -1 ) {
        dup_filedes = 4;
        if( dup2( filedes, dup_filedes ) != -1 ) {
            /* process file */
            /* ... */
        
            close( dup_filedes );
        }
        close( filedes );
        
        return EXIT_SUCCESS;
    }
    
    return EXIT_FAILURE;
}

Classification:

POSIX 1003.1

See also:

chsize(), close(), creat(), dup(), eof(), errno, execl(), execle(), execlp(), execlpe(), execv(), execve(), execvp(), execvpe(), fcntl(), fileno(), fstat(), isatty(), lseek(), open(), read(), sopen(), stat(), tell(), umask(), write()