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The fork
function is the primitive for creating a process.
It is declared in the header file unistd.h.
Preliminary: | MT-Safe | AS-Unsafe plugin | AC-Unsafe lock | See POSIX Safety Concepts.
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;
see 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:
Preliminary: | MT-Safe | AS-Unsafe plugin | AC-Unsafe lock | See POSIX Safety Concepts.
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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