how to correctly wait for execve to finish?

Question

A C source code (compiled and running Linux Centos 6.3) has the line:

execve(cmd, argv, envp);

execve does not return, but I w

Answer 1

Read carefully documentation of execve(2) and of fork(2) and of waitpid(2). Both execve & fork are tricky and fork is difficult to understand. I strongly suggest to read Advanced Linux Programming (freely available online, but you could buy the paper book) which has several chapters for these questions.

^{(Don't be afraid of spending a few days reading and understanding these system calls, they are tricky)}

Some important points.

• every system call can fail and you should always handle its failure, at least by showing some error message with perror(3) and immediately exit(3)-ing.

• the execve(2) syscall usually never returns, since it returns only on failure (when successful, it does not return, since the calling program has been replaced so wiped out!) hence most calls to it (and similar exec(3) functions) are often like:

  if (execve(cmd, argv, envp)) { perror (cmd); exit(127); };
  /* else branch cannot be reached! */
  

  it is customary to use a weird exit code like 127 (usually unused, except like above) on execve failure, and very often you could not do anything else. When used (almost always) with fork you'll often call execve in the child process.

• the fork(2) syscall returns twice on success (once in parent process, once in child process). This is tricky to understand, read the references I gave. It returns once only on failure. So you always keep the result of fork, so typical code would be:

   pid_t pid = fork ();
   if (pid<0) { // fork has failed 
      perror("fork"); exit(EXIT_FAILURE);
   }
   else if (pid==0) { // successful fork in the child process
     // very often you call execve in child, so you don't continue here.
     // example code:
     if (execve(cmd, argv, envp)) { perror (cmd); exit(127); };
     // not reached! 
  };
  // here pid is positive, we are in the parent and fork succeeded....
  /// do something sensible, at some point you need to call waitpid and use pid
  

Suggestion: use strace(1) on some programs, perhaps try strace -f bash -c 'date; pwd' and study the output. It mentions many syscalls(2)....

Your sample code might (sometimes) work by just adding some else like

// better code, but still wrong because of unhandled failures....
if ((child = fork())>0) { 
  waitpid(child, NULL, 0);
  /*now I know execve is finished*/
  exit(0);
}
/// missing handling of `fork`  failure!
else if (!child) {
   execve(cmd, argv, envp);
   /// missing handling of `execve` failure
}

but that code is still incorrect because failures are not handled.