Linux – How does copy-on-write in fork() handle multiple fork

The new process will be created within the fork() call, and will start by returning from it just like the parent. The return value (which you stored in retval) from fork() will be:

• 0 in the child process
• The PID of the child in the parent process
• -1 in the parent if there was a failure (there is no child, naturally)

Your testing code works correctly; it stores the return value from fork() in child_pid and uses if to check if it's 0 or not (although it doesn't check for an error)

It's to simplify the interface. The alternative to fork and exec would be something like Windows' CreateProcess function. Notice how many parameters CreateProcess has, and many of them are structs with even more parameters. This is because everything you might want to control about the new process has to be passed to CreateProcess. In fact, CreateProcess doesn't have enough parameters, so Microsoft had to add CreateProcessAsUser and CreateProcessWithLogonW.

With the fork/exec model, you don't need all those parameters. Instead, certain attributes of the process are preserved across exec. This allows you to fork, then change whatever process attributes you want (using the same functions you'd use normally), and then exec. In Linux, fork has no parameters, and execve has only 3: the program to run, the command line to give it, and its environment. (There are other exec functions, but they're just wrappers around execve provided by the C library to simplify common use cases.)

If you want to start a process with a different current directory: fork, chdir, exec.

If you want to redirect stdin/stdout: fork, close/open files, exec.

If you want to switch users: fork, setuid, exec.

All these things can be combined as needed. If somebody comes up with a new kind of process attribute, you don't have to change fork and exec.

As larsks mentioned, most modern Unixes use copy-on-write, so fork doesn't involve significant overhead.