Tuesday, August 5, 2008

What does fork() do?


pid_t fork(void);

The `fork()' function is used to create a new process from an existing
process. The new process is called the child process, and the existing
process is called the parent. You can tell which is which by checking the
return value from `fork()'. The parent gets the child's pid returned to
him, but the child gets 0 returned to him. Thus this simple code
illustrate's the basics of it.

pid_t pid;

switch (pid = fork())
case -1:
/* Here pid is -1, the fork failed */
/* Some possible reasons are that you're */
/* out of process slots or virtual memory */
perror("The fork failed!");

case 0:
/* pid of zero is the child */
/* Here we're the child...what should we do? */
/* ... */
/* but after doing it, we should do something like: */

/* pid greater than zero is parent getting the child's pid */
printf("Child's pid is %d\n",pid);

Of course, one can use `if()... else...' instead of `switch()', but the
above form is a useful idiom.

Of help when doing this is knowing just what is and is not inherited by the
child. This list can vary depending on Unix implementation, so take it
with a grain of salt. Note that the child gets *copies* of these things,
not the real thing.

Inherited by the child from the parent:

* process credentials (real/effective/saved UIDs and GIDs)

* environment

* stack

* memory

* open file descriptors (note that the underlying file positions are
shared between the parent and child, which can be confusing)

* close-on-exec flags

* signal handling settings

* nice value

* scheduler class

* process group ID

* session ID

* current working directory

* root directory

* file mode creation mask (umask)

* resource limits

* controlling terminal

Unique to the child:

* process ID

* different parent process ID

* Own copy of file descriptors and directory streams.

* process, text, data and other memory locks are NOT inherited.

* process times, in the tms struct

* resource utilizations are set to 0

* pending signals initialized to the empty set

* timers created by timer_create not inherited

* asynchronous input or output operations not inherited

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