According to https://linux.die.net/man/3/pclose:

The pclose() function waits for the associated process to terminate and returns the exit status of the command as returned by wait4(2).

And from https://linux.die.net/man/2/wait4:

Return Value
As for waitpid(2).

And from https://linux.die.net/man/2/waitpid:

WIFEXITED(status)
____ returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by returning from main().
WEXITSTATUS(status)
____ returns the exit status of the child. This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main(). This macro should only be employed if WIFEXITED returned true.
WIFSIGNALED(status)
____ returns true if the child process was terminated by a signal.
WTERMSIG(status)
____ returns the number of the signal that caused the child process to terminate. This macro should only be employed if WIFSIGNALED returned true.
WCOREDUMP(status)
____ returns true if the child produced a core dump. This macro should only be employed if WIFSIGNALED returned true. This macro is not specified in POSIX.1-2001 and is not available on some UNIX implementations (e.g., AIX, SunOS). Only use this enclosed in #ifdef WCOREDUMP ... #endif.
WIFSTOPPED(status)
____ returns true if the child process was stopped by delivery of a signal; this is only possible if the call was done using WUNTRACED or when the child is being traced (see ptrace(2)).
WSTOPSIG(status)
____ returns the number of the signal which caused the child to stop. This macro should only be employed if WIFSTOPPED returned true.