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:
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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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abc
abc
def
ghi
ghi
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dup(2) System Calls Manual dup(2)
Name
dup, dup2 - duplicate an open file descriptor
Syntax
newd = dup(oldd)
int newd, oldd;
dup2(oldd, newd)
int oldd, newd;
Description
The system call duplicates an existing object descriptor. The argument oldd is a small non-negative integer index in the per-process
descriptor table. The value must be less than the size of the table, which is returned by The new descriptor, newd, returned by the call
is the lowest numbered descriptor that is not currently in use by the process.
The object referenced by the descriptor does not distinguish between references using oldd and newd in any way. Thus, if newd and oldd are
duplicate references to an open file, and calls all move a single pointer into the file. If a separate pointer into the file is desired, a
different object reference to the file must be obtained by issuing an additional call.
In the second form of the call, specify the value of newd needed. If this descriptor is already in use, the descriptor is first deallocated
as if a call had been done.
Return Values
The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error.
Diagnostics
The and system calls fail under the following conditions:
[EBADF] The oldd or newd is not a valid active descriptor.
[EMFILE] Too many descriptors are active.
[EINTR] The or function was terminated prematurely by a signal.
See Also
accept(2), close(2), getdtablesize(2), lseek(2), open(2), pipe(2), read(2), socket(2), socketpair(2), write(2)
dup(2)