While checking return status first WIFEXITED has to be checked to know if child
returned normally.
waitpid's man page:
WEXITSTATUS(status) returns the exit status of the child. (...) This macro
should be employed only if WIFEXITED returned true.
WIFEXITED(status) returns true if the child terminated normally, that is, by
calling exit(3) or _exit(2), or by returning from main().
Signed-off-by: Adam Wujek <adam.wujek@cern.ch>