dup()

Duplicate a file descriptor

Synopsis:

#include <unistd.h>

int dup( int filedes );

Arguments:

filedes
The file descriptor that you want to duplicate.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The dup() function duplicates the file descriptor specified by filedes. The new file descriptor refers to the same open file descriptor as the original, and shares any locks. The new file descriptor also:

  • 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).

Changing the file position with one descriptor results in a changed position for the other.

Calling:

dup_filedes = dup( filedes );

is the same as:

dup_filedes = fcntl( filedes, F_DUPFD, 0 );

Returns:

The new file descriptor for success, or -1 if an error occurs ( errno is set).

Errors:

EBADF
The file descriptor, filedes, isn't a valid.
EMFILE
There are already OPEN_MAX file descriptors in use.
ENOSYS
The dup() 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 = dup( filedes );
        if( dup_filedes != -1 ) {
            /* process file */
            /* ... */

            close( dup_filedes );
        }
        close( filedes );
        
        return EXIT_SUCCESS;
    }
    
    return EXIT_FAILURE;
}

Classification:

POSIX 1003.1

Safety:  
Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes

Last modified: 2013-12-23

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