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fork() and dup()


 
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# 1  
Old 11-13-2002
fork() and dup()

I have met this code:

switch(fork()) {

case 0:
close(1);
dup(p[1]);
close(p[0]);
close(p[1]);
execvp(<whatever>);
perror("Exec failed");
}

Can anyone tell me what this piece of code does?
Thx alot..
# 2  
Old 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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