11-13-2002
Since it tested for 0 after the fork(), this code will be executed by the the child process. First it closes fd 1 which is stdout. Next it dups the fd p[1]. The array p was created by pipe() or socketpair(). We write to p[1] and we read from p[0]. The dup call make a copy of a file descriptor. It will always use the lowest fd available. Since we just closed 1, 1 is available. So after the dup, the child process will have set its stdout to the "write" end of a pipe. Then it just closes the uneeded fd's and it runs a program.
Meanwhile, no doubt, the parent will be reading from p[0] to read the output of the child process.
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dup(2) System Calls Manual dup(2)
NAME
dup - duplicate an open file descriptor
SYNOPSIS
DESCRIPTION
fildes is a file descriptor obtained from a or system call. returns a new file descriptor having the following in common with the origi-
nal:
o Same open file (or pipe).
o Same file pointer (i.e., both file descriptors share one file pointer).
o Same access mode (read, write or read/write).
o Same file status flags (see fcntl(2),
The new file descriptor is set to remain open across system calls. See fcntl(2).
The file descriptor returned is the lowest one available.
RETURN VALUE
Upon successful completion, the file descriptor is returned as a non-negative integer. Otherwise, a value of -1 is returned and is set to
indicate the error.
ERRORS
fails if one or more of the following is true:
fildes is not a valid open file descriptor.
Request violates the maximum number of open file descriptors.
AUTHOR
was developed by AT&T and HP.
SEE ALSO
close(2), creat(2), dup2(2), exec(2), fcntl(2), open(2), pipe(2), privileges(5).
STANDARDS CONFORMANCE
dup(2)