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.

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

dup