dup (system call)

In Unix-like operating systems, dup (short for "duplicate") and dup2 system calls create a copy of a given file descriptor. This new descriptor actually does not behave like a copy, but like an alias of the old one.

C library POSIX definition

The dup and dup2 calls are standardized by the POSIX specification.

int dup (int oldfd);
int dup2 (int oldfd, int newfd);

The former allocates the first available descriptor, just like open() behaves; an alternative way to duplicate a file descriptor to an unspecified place is the fcntl system call with F_DUPFD command.

The latter places the copy into newfd. If newfd is open, it is closed first.

dup2 for input/output redirection

Unix shells use dup2 for input/output redirection. Along with pipe(), it is a tool on which Unix pipes rely.

The following example uses pipe() and dup() in order to connect two separate processes (program1 and program2) using Unix pipes:

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

/* function prototypes */
void die(const char*);
 
int main(int argc, char **argv) {
	int pdes[2];
	pid_t child;
 
	if(pipe(pdes) == -1)
		die("pipe()");
 
	child = fork();
	if(child == (pid_t)(-1))
        	die("fork()"); /* fork failed */
 
	if(child == (pid_t)0) {
        	/* child process */

        	close(1);       /* close stdout */
        	
        	if(dup(pdes[1]) == -1)
        		die("dup()");
        	
        	/* now stdout and pdes[1] are equivalent (dup returns lowest free descriptor) */

        	if((execlp("program1", "program1", "arg1", NULL)) == -1)
        		die("execlp()");

		_exit(EXIT_SUCCESS);
	} else {
        	/* parent process */

        	close(0);       /* close stdin */
        	
        	if(dup(pdes[0]) == -1)
        		die("dup()");

        	/* now stdin and pdes[0] are equivalent (dup returns lowest free descriptor) */

        	if((execlp("program2", "program2", "arg1", NULL)) == -1)
        		die("execlp()");

		exit(EXIT_SUCCESS);
	}
 
	return 0;
}

void die(const char *msg) {
	perror(msg);
	exit(EXIT_FAILURE);
}

See also

• File descriptor – how it works and other functions related to open

References

• Advanced Programming in the UNIX Environment by W. Richard Stevens ISBN 81-7808-096-6