Section: Linux Programmer's Manual (2)
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fork - create a child process
creates a new process by duplicating the calling process.
The new process, referred to as the child
is an exact duplicate of the calling process,
referred to as the parent
, except for the following points:
The child has its own unique process ID,
and this PID does not match the ID of any existing process group
The child's parent process ID is the same as the parent's process ID.
The child does not inherit its parent's memory locks
Process resource utilizations
and CPU time counters
are reset to zero in the child.
The child's set of pending signals is initially empty
The child does not inherit semaphore adjustments from its parent
The child does not inherit process-associated record locks from its parent
(On the other hand, it does inherit
open file description locks and
locks from its parent.)
The child does not inherit timers from its parent
The child does not inherit outstanding asynchronous I/O operations
from its parent
nor does it inherit any asynchronous I/O contexts from its parent (see
The process attributes in the preceding list are all specified
The parent and child also differ with respect to the following
Linux-specific process attributes:
The child does not inherit directory change notifications (dnotify)
from its parent
(see the description of
setting is reset so that the child does not receive a signal
when its parent terminates.
The default timer slack value is set to the parent's
current timer slack value.
See the description of
Memory mappings that have been marked with the
flag are not inherited across a
The termination signal of the child is always
The port access permission bits set by
are not inherited by the child;
the child must turn on any bits that it requires using
Note the following further points:
The child process is created with a single thread---the
one that called
The entire virtual address space of the parent is replicated in the child,
including the states of mutexes, condition variables,
and other pthreads objects; the use of
may be helpful for dealing with problems that this can cause.
The child inherits copies of the parent's set of open file descriptors.
Each file descriptor in the child refers to the same
open file description (see
as the corresponding file descriptor in the parent.
This means that the two descriptors share open file status flags,
current file offset,
and signal-driven I/O attributes (see the description of
The child inherits copies of the parent's set of open message
queue descriptors (see
Each descriptor in the child refers to the same
open message queue description
as the corresponding descriptor in the parent.
This means that the two descriptors share the same flags
The child inherits copies of the parent's set of open directory streams (see
POSIX.1-2001 says that the corresponding directory streams
in the parent and child
share the directory stream positioning;
on Linux/glibc they do not.
On success, the PID of the child process is returned in the parent,
and 0 is returned in the child.
On failure, -1 is returned in the parent,
no child process is created, and
is set appropriately.
cannot allocate sufficient memory to copy the parent's page tables and
allocate a task structure for the child.
A system-imposed limit on the number of threads was encountered.
There are a number of limits that may trigger this error: the
soft resource limit (set via
which limits the number of processes and threads for a real user ID,
the kernel's system-wide limit on the number of processes and threads,
was reached (see
or the maximum number of PIDs,
was reached (see
The caller is operating under the
scheduling policy and does not have the reset-on-fork flag set.
failed to allocate the necessary kernel structures because memory is tight.
is not supported on this platform (for example,
hardware without a Memory-Management Unit).
SVr4, 4.3BSD, POSIX.1-2001.
is implemented using copy-on-write pages, so the only penalty that it incurs
is the time and memory required to duplicate the parent's page tables,
and to create a unique task structure for the child.
Since version 2.3.3,
rather than invoking the kernel's
wrapper that is provided as part of the
NPTL threading implementation invokes
with flags that provide the same effect as the traditional system call.
(A call to
is equivalent to a call to
The glibc wrapper invokes any fork handlers that have been
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
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