to system() or fork()/exec()?

Question

There appear to be two common ways of running an external executable from C in unix, the

system()

call and

pid = fork()
switch(pid)
//switch statement based on return value of pid, 
//one branch of which will include and exec() command

Is there any reason to prefer a fork/exec over system in the case where they are functionally equivalent (parent process waits for child to finish, no complex information is returned from child)?.

Answer 1

system executes a command-interpreter, i.e. a shell, which (a) is slower than a direct fork/exec, (b) may behave differently on different systems and (c) is a potential security hazard if you pass it a string from an untrusted source. Also, system waits for the child process to exit, while you might want it to run concurrently with the parent process.

More in general, the low-level fork/exec gives you additional control: before or in between the two operations, you might want to chdir, open pipes, close file descriptors, set up shared memory, etc.

(By different systems, I don't mean Windows vs. Unix (as Windows doesn't even have fork): I'm talking Red Hat Linux vs. Ubuntu. The former uses Bash to execute what is passed to system, the latter a lightweight POSIX-compatible shell.)

Answer 2

fork() creates a new process. If you don't need to do that, just use system() (or popen()). You might want a second process to achieve parallelism, or for finer-grained control over the job, but often you just don't care for that if the job is meant to be synchronous.

On the other hand, I find that 95% of uses of system() are unnecessary or would somehow be better off done another way (e.g. using zlib instead of system("gzip")). So maybe the best answer is to use neither!