dup function

C

Function

Reference manual

@see man dup

The dup function creates a new file descriptor that refers to the same open file as the given file descriptor. The new file descriptor number is guaranteed to be the lowest-numbered file descriptor that was unused in the calling process.

Syntax

#include <unistd.h>

int dup(int oldfd);

Parameters

• oldfd: The file descriptor to be duplicated.

Return value

• If the duplication is successful, the function returns a new file descriptor that refers to the same open file description as oldfd.

• If an error occurs, -1 is returned, and errno is set to indicate the error.

Usage example

In the following example we use the dup function to duplicate the fd file descriptor into the new_fd file descriptor meaning we now have two different file descriptors that point to the same file.

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int main(void)
{
    int fd;
    int new_fd;

    fd = open("file.txt", O_RDONLY);
    if (fd == -1)
    {
        perror("open");
        return (EXIT_FAILURE);
    }
    new_fd = dup(fd);
    if (new_fd == -1)
    {
        perror("File descriptor duplication failed");
        close(fd);
        return (EXIT_FAILURE);
    }
    printf("File descriptor duplicated successfully\n");
    close(fd);
    close(new_fd);
    return (EXIT_SUCCESS);
}