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
(setpgid(2)).
*
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
(mlock(2),
mlockall(2)).
*
Process resource utilisations
(getrusage(2)) and CPU time counters
(times(2)) are reset to zero in the child.
*
The child set of pending signals is initially empty
(sigpending(2)).
*
The child does not inherit semaphore adjustments from its parent
(semop(2)).
*
The child does not inherit record locks from its parent
(fcntl(2)).
*
The parent does not inherit timers from its parent
(setitimer(2)
alarm(3),
timer_create(3)).
*
The child does not inherit outstanding asynchronous I/O operations
from its parent
(aio_read(3),
aio_write(3)).
The process attributes in the preceding list are all specified
in POSIX.1-2001.
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
F_NOTIFY in
fcntl(2)).
*
The
prctl(2)
PR_SET_PDEATHSIG setting is reset so that the child does not receive a signal
when its parent terminates.
*
Memory mappings that have been marked with the
madvise(2)
MADV_DONTFORK flag are not inherited across a
fork(2).
*
The termination signal of the child is always SIGCHLD
(see
clone(2)).
Note the following further points:
*
The child process is created with a single thread — the
one that called
fork(2).
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
pthread_atfork(3)
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
open(2))
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
F_SETOWN and
F_SETSIG in
fcntl(2)).
*
The child inherits copies of the parent’s set of open message
queue descriptors (see
mq_overview(7)).
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
(mq_flags).
On success, the PID of the child process is returned in the parent’s thread
of execution, and a 0 is returned in the child’s thread of execution. On
failure, a -1 will be returned in the parent’s context, no child process
will be created, and
errno will be set appropriately.
fork() cannot allocate sufficient memory to copy the parent’s page tables and
allocate a task structure for the child.
EAGAIN
It was not possible to create a new process because the caller’s
RLIMIT_NPROC resource limit was encountered.
To exceed this limit, the process must have either the
CAP_SYS_ADMIN or the
CAP_SYS_RESOURCE capability.
ENOMEM
fork() failed to allocate the necessary kernel structures because memory is tight.
Under Linux,
fork() 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.
