IEEE Std 1003.1-2017
fork() function shall create a new process. The new process (child process) shall be an exact copy of the calling
process (parent process) except as detailed below:
The child process shall have a unique process ID.
The child process ID also shall not match any active process group ID.
The child process shall have a different parent process ID, which shall be the process ID of the calling process.
The child process shall have its own copy of the parent's file descriptors. Each of the child's file descriptors shall refer to the same open file description with the corresponding file descriptor of the parent.
The child process shall have its own copy of the parent's open directory streams. Each open directory stream in the child process may share directory stream positioning with the corresponding directory stream of the parent.
The child process shall have its own copy of the parent's message catalog descriptors.
The child process values of
tms_cstimeshall be set to
The time left until an alarm clock signal shall be reset to zero, and the alarm, if any, shall be canceled. See alarm
All semaphore adjustment values shall be cleared.
File locks set by the parent process shall not be inherited by the child process.
The set of signals pending for the child process shall be initialized to the empty set.
Interval timers shall be reset in the child process.
Any semaphores that are open in the parent process shall also be open in the child process.
The child process shall not inherit any address space memory locks established by the parent process via calls to
Memory mappings created in the parent shall be retained in the child process.
MAP_PRIVATEmappings inherited from the parent shall also be
MAP_PRIVATEmappings in the child, and any modifications to the data in these mappings made by the parent prior to calling
fork()shall be visible to the child. Any modifications to the data in
MAP_PRIVATEmappings made by the parent after
fork()returns shall be visible only to the parent. Modifications to the data in
MAP_PRIVATEmappings made by the child shall be visible only to the child.
SCHED_RRscheduling policies, the child process shall inherit the policy and priority settings of the parent process during a
fork()function. For other scheduling policies, the policy and priority settings on
Per-process timers created by the parent shall not be inherited by the child process.
The child process shall have its own copy of the message queue descriptors of the parent. Each of the message descriptors of the child shall refer to the same open message queue description as the corresponding message descriptor of the parent.
No asynchronous input or asynchronous output operations shall be inherited by the child process. Any use of asynchronous control blocks created by the parent produces undefined behavior.
A process shall be created with a single thread. If a multi-threaded process calls
fork(), the new process shall contain a replica of the calling thread and its entire address space, possibly including the states of mutexes and other resources. Consequently, to avoid errors, the child process may only execute async-signal-safe operations until such time as one of the exec functions is called.
When the application calls
fork() from a signal handler and any of the fork handlers registered by
pthread_atfork() calls a function that is not async-signal-safe, the behavior is undefined.
- If the Trace option and the Trace Inherit option are both supported:
If the calling process was being traced in a trace stream that had its inheritance policy set to
POSIX_TRACE_INHERITED, the child process shall be traced into that trace stream, and the child process shall inherit the parent's mapping of trace event names to trace event type identifiers. If the trace stream in which the calling process was being traced had its inheritance policy set to
POSIX_TRACE_CLOSE_FOR_CHILD, the child process shall not be traced into that trace stream. The inheritance policy is set by a call to the
- If the Trace option is supported, but the Trace Inherit option is not supported:
The child process shall not be traced into any of the trace streams of its parent process.
If the Trace option is supported, the child process of a trace controller process shall not control the trace streams controlled by its parent process.
The initial value of the CPU-time clock of the child process shall be set to zero.
The initial value of the CPU-time clock of the single thread of the child process shall be set to zero.
All other process characteristics defined by
POSIX.1-2017 shall be the same in the parent and child processes. The inheritance
of process characteristics not defined by
POSIX.1-2017 is unspecified.
fork(), both the parent and the child processes shall be capable of executing independently before either one
Upon successful completion,
fork() shall return
0 to the child process and shall return the process ID of the child process to the parent process. Both processes shall continue to execute from the
fork() function. Otherwise,
-1 shall be returned to the parent process, no child process shall be created, and
errno shall be set to indicate the error.