Pipe, Fork, and Exec - Two Way Communication Between Parent and Child Process

Question

An assignment in my Operating Systems class requires me to build a binary process tree by recursively calling exec on the same program. The goal is to split some arbitrary task

Answer 1

As Jonathon Reinhart says you should change this lines:

char *argv2[] = {"some random arg to make sure that argc == 2 in the child", NULL};
execvp("two_way_pipes", argv2);

to:

char *argv2[] = {argv[0], "some random arg...", NULL};
execvp(argv[0], argv2);

then it works as expected:

echo test | ./two_way_pipes
in parent | message received: test

In your program you wrote "two_way_pipes" but it is not in your PATH so you really need the extra ./ so argv[0] then is ("./two_way_pipes").

Answer 2

Besides the issue mentioned by Jonathon Reinhart, most probably the call to execv() fails.

To test this modify these lines

execvp("two_way_pipes", argv2);
_exit(0);

to be

...
#include <errno.h>
...


execvp("two_way_pipes", argv2); /* On sucess exec*() functions never return. */
perror("execvp() failed); /* Getting here means execvp() failed. */
_exit(errno);

Expect to receive

execvp() failed: No such file or directory

To fix this change

execvp("two_way_pipes", argv2);

to be

execvp("./two_way_pipes", argv2);

---

Also if the child was not exec*()ed then this line

read(PARENT_READ, buff, 4); // should read "test" which was written by the child to stdout

fails and in turn buff is not initialised and therefore this line

fprintf(stderr, "in parent | message received: %s\n", buff);  

provokes undefined behaviour.

To fix this at least properly initialise buff by changing

char buff[5];

to be

char buff[5] = "";

Answer 3

The first entry in your argv2 should be the name of the executable (just like your incoming argv[0].

char *argv2[] = {"two_way_pipes", "some random arg...", NULL};
execvp("two_way_pipes", argv2);

From the execvp man page:

The execv(), execvp(), and execvpe() functions provide an array of pointers to null-terminated strings that represent the argument list available to the new program. The first argument, by convention, should point to the filename associated with the file being executed. The array of pointers must be terminated by a NULL pointer.