fork() 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 childs parent process ID is the same as the parents process ID.
The child does not inherit its parents memory locks
Process resource utilizations
(getrusage(2)) and CPU time counters
(times(2)) are reset to zero in the child.
The childs set of pending signals is initially empty
The child does not inherit semaphore adjustments from its parent
The child does not inherit record 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
PR_SET_PDEATHSIG 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 parents
current timer slack value.
See the description of
Memory mappings that have been marked with the
MADV_DONTFORK flag are not inherited across a
The termination signal of the child is always
Note the following further points:
The child process is created with a single threadthe
one that called
fork(). 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 parents 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 parents 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 parents set of open directory streams (see
POSIX.1-2001 says that the corresponding directory streams
in the parent and child
may 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
errno is set appropriately.
fork() cannot allocate sufficient memory to copy the parents page tables and
allocate a task structure for the child.
It was not possible to create a new process because the callers
RLIMIT_NPROC resource limit was encountered.
To exceed this limit, the process must have either the
CAP_SYS_ADMIN or the
fork() failed to allocate the necessary kernel structures because memory is tight.
fork() is not supported on this platform (for example,
hardware without a Memory-Management Unit).
fork() is implemented using copy-on-write pages, so the only penalty that it incurs
is the time and memory required to duplicate the parents page tables,
and to create a unique task structure for the child.
Since version 2.3.3,
rather than invoking the kernels
fork() system call,
fork() 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
fork() is equivalent to a call to
flags as just
SIGCHLD.) The glibc wrapper invokes any fork handlers that have been