What are the differences between fork
and exec
?
The use of fork
and exec
exemplifies the spirit of UNIX in that it provides a very simple way to start new processes.
The fork
call basically makes a duplicate of the current process, identical in almost every way. Not everything is copied over (for example, resource limits in some implementations) but the idea is to create as close a copy as possible.
The new process (child) gets a different process ID (PID) and has the PID of the old process (parent) as its parent PID (PPID). Because the two processes are now running exactly the same code, they can tell which is which by the return code of fork
- the child gets 0, the parent gets the PID of the child. This is all, of course, assuming the fork
call works - if not, no child is created and the parent gets an error code.
The exec
call is a way to basically replace the entire current process with a new program. It loads the program into the current process space and runs it from the entry point.
So, fork
and exec
are often used in sequence to get a new program running as a child of a current process. Shells typically do this whenever you try to run a program like find
- the shell forks, then the child loads the find
program into memory, setting up all command line arguments, standard I/O and so forth.
But they're not required to be used together. It's perfectly acceptable for a program to fork
itself without exec
ing if, for example, the program contains both parent and child code (you need to be careful what you do, each implementation may have restrictions). This was used quite a lot (and still is) for daemons which simply listen on a TCP port and fork
a copy of themselves to process a specific request while the parent goes back to listening.
Similarly, programs that know they're finished and just want to run another program don't need to fork
, exec
and then wait
for the child. They can just load the child directly into their process space.
Some UNIX implementations have an optimized fork
which uses what they call copy-on-write. This is a trick to delay the copying of the process space in fork
until the program attempts to change something in that space. This is useful for those programs using only fork
and not exec
in that they don't have to copy an entire process space.
If the exec
is called following fork
(and this is what happens mostly), that causes a write to the process space and it is then copied for the child process.
Note that there is a whole family of exec
calls (execl
, execle
, execve
and so on) but exec
in context here means any of them.
The following diagram illustrates the typical fork/exec
operation where the bash
shell is used to list a directory with the ls
command:
+--------+
| pid=7 |
| ppid=4 |
| bash |
+--------+
|
| calls fork
V
+--------+ +--------+
| pid=7 | forks | pid=22 |
| ppid=4 | ----------> | ppid=7 |
| bash | | bash |
+--------+ +--------+
| |
| waits for pid 22 | calls exec to run ls
| V
| +--------+
| | pid=22 |
| | ppid=7 |
| | ls |
V +--------+
+--------+ |
| pid=7 | | exits
| ppid=4 | <---------------+
| bash |
+--------+
|
| continues
V