c - Behavior of program changes depending on where fork is called -

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i working on little program getting familliar pipes , file descriptors , spent lot of time debugging problem not make sense.

i spent ton of time thinking misunderstood file descriptors when program behaving differently depending on used fork.

int main(void) { int fid; int p[2]; pipe(p); char buf[20];  fid = fork(); if (fid==0){     close(p[1]);     dup2(p[0],0);     close(p[0]);     execlp("cat","cat",(char *)null);  } else{     close(p[0]);     dup2(p[1],1);     close(p[1]);     execlp("ls","ls",(char *)null);  }  return 0; }

gives me expected output of whats in directory. ls out put gets piped cat.

if move fork line above pipe(p); wont output. don't understand why happens?

so you're wondering why works:

pipe(p); fid = fork();

and doesn't:

fid = fork(); pipe(p);

the reason pretty straightforward: in first case, process creates pipe, splits 2 (and both processes have access pipe).

in second case, process splits two, , each of 2 processes creates own pipe that's unrelated other one.

so in first case, 1 process writes pipe , other process reads it. in second case, 1 process writes pipe , other process reads a different pipe, hasn't received data because it's different pipe.


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