fork(), exec(), wait()

ostep/code/cpu-api

p1.c - after fork parent or child could be scheduled next:
----
% ./p1
hello world (pid:21918)
hello, I am parent of 21919 (pid:21918)
hello, I am child (pid:21919)
% ./p1
hello world (pid:21920)
hello, I am child (pid:21921)
hello, I am parent of 21921 (pid:21920)
%

- perror() if fork() fails

p2.c - parent waits for child to go first
----
% ./p2
hello world (pid:21937)
hello, I am child (pid:21938)
hello, I am parent of 21938 (wc:21938) (pid:21937)
% ./p2
hello world (pid:21939)
hello, I am child (pid:21940)
hello, I am parent of 21940 (wc:21940) (pid:21939)
%

check: man 2 wait and/or man 3 wait
pid_t wait(int *wstatus);
wait returns pid of child
-> try checking the child exit status
% ./p2
hello world (pid:22202)
hello, I am child (pid:22203)
hello, I am parent of 22203 (wc:22203) (pid:22202)
s = 0x1200
WIFEXITED = 1
WEXITSTATUS = 18
%

- perror() if fork() fails

p3.c - child exec's wc
----
% ./p3
hello world (pid:22206)
hello, I am child (pid:22207)
 32 123 966 p3.c
hello, I am parent of 22207 (wc:22207) (pid:22206)
%

- perror() if fork() fails
- strdup not needed
- perror() and exit() after execvp(), if it returns it failed

p4.c - child redirects stdout to file p4.output then runs wc
----
% ./p4
% cat p4.output
 34 114 884 p4.c
%

- perror() if fork() fails
- strdup not needed
- perror() and exit() after execvp(), if it returns it failed