dup (system call)
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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
functions is dup3(), which is the same as dup2() except that if oldfd equals newfd the system call fails with error EINVAL and the caller can force the close-on-exec flag to be set by specifying O_CLOEXEC in flags.[2] dup3() was formally added to Linux kernel
version 2.6.27 (glibc support is available on version 2.9 and above).
int dup (int oldfd);
int dup2 (int oldfd, int newfd);
The former allocates the first available descriptor, just like
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
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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
- ^ "dup, dup2". opengroup.org.
- ^ "Linux Man Pages".
- Advanced Programming in the UNIX Environment by W. Richard Stevens ISBN 81-7808-096-6