10-30-2001
For example, think about inetd. It has dozens of fd's open. It gets a connection for telnet on one them. So it forks a child. The child must exec telnetd, but first that socket must be duped onto fd's 0 1 and 2.
And lots of times you want to open a file on fd 1 and dup it to fd 2. Opening the file twice would result in two file table entries and then fd 1 and fd 2 would be stepping on each other. Anytime you see "2>&1" in a shell script you're asking the shell to dup an fd.
Anyway, few people actually use dup() anymore. dup2() or fnctl() can dup an fd with more control over the target fd.
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I have met this code:
switch(fork()) {
case 0:
close(1);
dup(p);
close(p);
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execvp(<whatever>);
perror("Exec failed");
}
Can anyone tell me what this piece of code does?
Thx alot.. (1 Reply)
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I'm having difficulty understanding the purposes of using dup/dup2 when involving forks.
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Hi,
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Hi Gurus,
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abc
abc
def
ghi
ghi
jkl
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abc ,2
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dup(2) System Calls dup(2)
NAME
dup - duplicate an open file descriptor
SYNOPSIS
#include <unistd.h>
int dup(int fildes);
DESCRIPTION
The dup() function returns a new file descriptor having the following in common with the original open file descriptor fildes:
o same open file (or pipe)
o same file pointer (that is, both file descriptors share one file pointer)
o same access mode (read, write or read/write).
The new file descriptor is set to remain open across exec functions (see fcntl(2)).
The file descriptor returned is the lowest one available.
The dup(fildes) function call is equivalent to:
fcntl(fildes, F_DUPFD, 0)
RETURN VALUES
Upon successful completion, a non-negative integer representing the file descriptor is returned. Otherwise, -1 is returned and errno is set
to indicate the error.
ERRORS
The dup() function will fail if:
EBADF The fildes argument is not a valid open file descriptor.
EINTR A signal was caught during the execution of the dup() function.
EMFILE The process has too many open files (see getrlimit(2)).
ENOLINK The fildes argument is on a remote machine and the link to that machine is no longer active.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
|MT-Level |Async-Signal-Safe |
+-----------------------------+-----------------------------+
SEE ALSO
close(2), creat(2), exec(2), fcntl(2), getrlimit(2), open(2), pipe(2), dup2(3C), lockf(3C), attributes(5), standards(5)
SunOS 5.10 28 Dec 1996 dup(2)