GNU Info
(libc.info)Creating a Process
Creating a Process
==================
The `fork' function is the primitive for creating a process. It is
declared in the header file `unistd.h'.
- Function: pid_t fork (void)
The `fork' function creates a new process.
If the operation is successful, there are then both parent and
child processes and both see `fork' return, but with different
values: it returns a value of `0' in the child process and returns
the child's process ID in the parent process.
If process creation failed, `fork' returns a value of `-1' in the
parent process. The following `errno' error conditions are
defined for `fork':
`EAGAIN'
There aren't enough system resources to create another
process, or the user already has too many processes running.
This means exceeding the `RLIMIT_NPROC' resource limit, which
can usually be increased; Note: Limits on Resources.
`ENOMEM'
The process requires more space than the system can supply.
The specific attributes of the child process that differ from the
parent process are:
* The child process has its own unique process ID.
* The parent process ID of the child process is the process ID of its
parent process.
* The child process gets its own copies of the parent process's open
file descriptors. Subsequently changing attributes of the file
descriptors in the parent process won't affect the file
descriptors in the child, and vice versa. Note: Control
Operations. However, the file position associated with each
descriptor is shared by both processes; Note: File Position.
* The elapsed processor times for the child process are set to zero;
see Note: Processor Time.
* The child doesn't inherit file locks set by the parent process.
Note: Control Operations.
* The child doesn't inherit alarms set by the parent process. Note:
Setting an Alarm.
* The set of pending signals (Note: Delivery of Signal) for the
child process is cleared. (The child process inherits its mask of
blocked signals and signal actions from the parent process.)
- Function: pid_t vfork (void)
The `vfork' function is similar to `fork' but on some systems it
is more efficient; however, there are restrictions you must follow
to use it safely.
While `fork' makes a complete copy of the calling process's address
space and allows both the parent and child to execute
independently, `vfork' does not make this copy. Instead, the
child process created with `vfork' shares its parent's address
space until it calls `_exit' or one of the `exec' functions. In
the meantime, the parent process suspends execution.
You must be very careful not to allow the child process created
with `vfork' to modify any global data or even local variables
shared with the parent. Furthermore, the child process cannot
return from (or do a long jump out of) the function that called
`vfork'! This would leave the parent process's control
information very confused. If in doubt, use `fork' instead.
Some operating systems don't really implement `vfork'. The GNU C
library permits you to use `vfork' on all systems, but actually
executes `fork' if `vfork' isn't available. If you follow the
proper precautions for using `vfork', your program will still work
even if the system uses `fork' instead.
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