diff options
Diffstat (limited to 'man-pages-posix-2013/man3p/fork.3p')
| -rw-r--r-- | man-pages-posix-2013/man3p/fork.3p | 435 |
1 files changed, 435 insertions, 0 deletions
diff --git a/man-pages-posix-2013/man3p/fork.3p b/man-pages-posix-2013/man3p/fork.3p new file mode 100644 index 0000000..c9dcf0e --- /dev/null +++ b/man-pages-posix-2013/man3p/fork.3p @@ -0,0 +1,435 @@ +'\" et +.TH FORK "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual" +.SH PROLOG +This manual page is part of the POSIX Programmer's Manual. +The Linux implementation of this interface may differ (consult +the corresponding Linux manual page for details of Linux behavior), +or the interface may not be implemented on Linux. + +.SH NAME +fork +\(em create a new process +.SH SYNOPSIS +.LP +.nf +#include <unistd.h> +.P +pid_t fork(void); +.fi +.SH DESCRIPTION +The +\fIfork\fR() +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: +.IP " *" 4 +The child process shall have a unique process ID. +.IP " *" 4 +The child process ID also shall not match any active process group ID. +.IP " *" 4 +The child process shall have a different parent process ID, which shall +be the process ID of the calling process. +.IP " *" 4 +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. +.IP " *" 4 +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. +.IP " *" 4 +The child process shall have its own copy of the parent's message +catalog descriptors. +.IP " *" 4 +The child process values of +.IR tms_utime , +.IR tms_stime , +.IR tms_cutime , +and +.IR tms_cstime +shall be set to 0. +.IP " *" 4 +The time left until an alarm clock signal shall be reset to zero, and +the alarm, if any, shall be canceled; see +.IR "\fIalarm\fR\^(\|)". +.IP " *" 4 +All +.IR semadj +values shall be cleared. +.IP " *" 4 +File locks set by the parent process shall not be inherited by the +child process. +.IP " *" 4 +The set of signals pending for the child process shall be initialized +to the empty set. +.IP " *" 4 +Interval timers shall be reset in the child process. +.IP " *" 4 +Any semaphores that are open in the parent process shall also be open +in the child process. +.IP " *" 4 +The child process shall not inherit any address space memory locks +established by the parent process via calls to +\fImlockall\fR() +or +\fImlock\fR(). +.IP " *" 4 +Memory mappings created in the parent shall be retained in the child +process. MAP_PRIVATE mappings inherited from the parent shall also be +MAP_PRIVATE mappings in the child, and any modifications to the data in +these mappings made by the parent prior to calling +\fIfork\fR() +shall be visible to the child. Any modifications to the data in +MAP_PRIVATE mappings made by the parent after +\fIfork\fR() +returns shall be visible only to the parent. Modifications to the data +in MAP_PRIVATE mappings made by the child shall be visible only to the +child. +.IP " *" 4 +For the SCHED_FIFO and SCHED_RR scheduling policies, +the child process shall inherit the policy and priority settings of the +parent process during a +\fIfork\fR() +function. For other scheduling policies, the policy and priority +settings on +\fIfork\fR() +are implementation-defined. +.IP " *" 4 +Per-process timers created by the parent shall not be inherited by +the child process. +.IP " *" 4 +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. +.IP " *" 4 +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. +.IP " *" 4 +A process shall be created with a single thread. If a multi-threaded +process calls +\fIfork\fR(), +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 +.IR exec +functions is called. Fork handlers may be established by means of the +\fIpthread_atfork\fR() +function in order to maintain application invariants across +\fIfork\fR() +calls. +.RS 4 +.P +When the application calls +\fIfork\fR() +from a signal handler and any of the fork handlers registered by +\fIpthread_atfork\fR() +calls a function that is not async-signal-safe, the behavior is +undefined. +.RE +.IP " *" 4 +If the Trace option and the Trace Inherit option are both supported: +.RS 4 +.P +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 +\fIposix_trace_attr_setinherited\fR() +function. +.RE +.IP " *" 4 +If the Trace option is supported, but the Trace Inherit option is not +supported: +.RS 4 +.P +The child process shall not be traced into any of the trace streams +of its parent process. +.RE +.IP " *" 4 +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. +.IP " *" 4 +The initial value of the CPU-time clock of the child process shall be +set to zero. +.IP " *" 4 +The initial value of the CPU-time clock of the single thread of the +child process shall be set to zero. +.P +All other process characteristics defined by POSIX.1\(hy2008 shall be the same in +the parent and child processes. The inheritance of process +characteristics not defined by POSIX.1\(hy2008 is unspecified by POSIX.1\(hy2008. +.P +After +\fIfork\fR(), +both the parent and the child processes shall be capable of executing +independently before either one terminates. +.SH "RETURN VALUE" +Upon successful completion, +\fIfork\fR() +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 +\fIfork\fR() +function. Otherwise, \(mi1 shall be +returned to the parent process, no child process shall be created, and +.IR errno +shall be set to indicate the error. +.SH ERRORS +The +\fIfork\fR() +function shall fail if: +.TP +.BR EAGAIN +The system lacked the necessary resources to create another process, or +the system-imposed limit on the total number of processes under +execution system-wide or by a single user +{CHILD_MAX} +would be exceeded. +.br +.P +The +\fIfork\fR() +function may fail if: +.TP +.BR ENOMEM +Insufficient storage space is available. +.LP +.IR "The following sections are informative." +.SH EXAMPLES +None. +.SH "APPLICATION USAGE" +None. +.SH RATIONALE +Many historical implementations have timing windows where a signal sent +to a process group (for example, an interactive SIGINT) +just prior to or during execution of +\fIfork\fR() +is delivered to the parent following the +\fIfork\fR() +but not to the child because the +\fIfork\fR() +code clears the child's set of pending signals. This volume of POSIX.1\(hy2008 does not require, +or even permit, this behavior. However, it is pragmatic to expect that +problems of this nature may continue to exist in implementations that +appear to conform to this volume of POSIX.1\(hy2008 and pass available verification suites. This +behavior is only a consequence of the implementation failing to make +the interval between signal generation and delivery totally invisible. +From the application's perspective, a +\fIfork\fR() +call should appear atomic. A signal that is generated prior to the +\fIfork\fR() +should be delivered prior to the +\fIfork\fR(). +A signal sent to the process group after the +\fIfork\fR() +should be delivered to both parent and child. The implementation may +actually initialize internal data structures corresponding to the +child's set of pending signals to include signals sent to the process +group during the +\fIfork\fR(). +Since the +\fIfork\fR() +call can be considered as atomic from the application's perspective, +the set would be initialized as empty and such signals would have +arrived after the +\fIfork\fR(); +see also +.IR <signal.h> . +.P +One approach that has been suggested to address the problem of signal +inheritance across +\fIfork\fR() +is to add an +.BR [EINTR] +error, which would be returned when a signal is detected during the +call. While this is preferable to losing signals, it was not +considered an optimal solution. Although it is not recommended for +this purpose, such an error would be an allowable extension for an +implementation. +.P +The +.BR [ENOMEM] +error value is reserved for those implementations that detect and +distinguish such a condition. This condition occurs when an +implementation detects that there is not enough memory to create the +process. This is intended to be returned when +.BR [EAGAIN] +is inappropriate because there can never be enough memory (either +primary or secondary storage) to perform the operation. Since +\fIfork\fR() +duplicates an existing process, this must be a condition where there is +sufficient memory for one such process, but not for two. Many +historical implementations actually return +.BR [ENOMEM] +due to temporary lack of memory, a case that is not generally distinct +from +.BR [EAGAIN] +from the perspective of a conforming application. +.P +Part of the reason for including the optional error +.BR [ENOMEM] +is because the SVID specifies it and it should be reserved for the +error condition specified there. The condition is not applicable on +many implementations. +.P +IEEE\ Std\ 1003.1\(hy1988 neglected to require concurrent execution of the parent and child +of +\fIfork\fR(). +A system that single-threads processes was clearly not intended and is +considered an unacceptable ``toy implementation'' of this volume of POSIX.1\(hy2008. +The only objection anticipated to the phrase ``executing +independently'' is testability, but this assertion should be testable. +Such tests require that both the parent and child can block on a +detectable action of the other, such as a write to a pipe or a signal. +An interactive exchange of such actions should be possible for the +system to conform to the intent of this volume of POSIX.1\(hy2008. +.P +The +.BR [EAGAIN] +error exists to warn applications that such a condition might occur. +Whether it occurs or not is not in any practical sense under the +control of the application because the condition is usually a +consequence of the user's use of the system, not of the application's +code. Thus, no application can or should rely upon its occurrence +under any circumstances, nor should the exact semantics of what concept +of ``user'' is used be of concern to the application developer. +Validation writers should be cognizant of this limitation. +.P +There are two reasons why POSIX programmers call +\fIfork\fR(). +One reason is to create a new thread of control within the same program +(which was originally only possible in POSIX by creating a new +process); the other is to create a new process running a different +program. In the latter case, the call to +\fIfork\fR() +is soon followed by a call to one of the +.IR exec +functions. +.P +The general problem with making +\fIfork\fR() +work in a multi-threaded world is what to do with all of the threads. +There are two alternatives. One is to copy all of the threads into the +new process. This causes the programmer or implementation to deal with +threads that are suspended on system calls or that might be about to +execute system calls that should not be executed in the new process. +The other alternative is to copy only the thread that calls +\fIfork\fR(). +This creates the difficulty that the state of process-local resources +is usually held in process memory. If a thread that is not calling +\fIfork\fR() +holds a resource, that resource is never released in the child process +because the thread whose job it is to release the resource does not +exist in the child process. +.P +When a programmer is writing a multi-threaded program, the first +described use of +\fIfork\fR(), +creating new threads in the same program, is provided by the +\fIpthread_create\fR() +function. The +\fIfork\fR() +function is thus used only to run new programs, and the effects of +calling functions that require certain resources between the call to +\fIfork\fR() +and the call to an +.IR exec +function are undefined. +.P +The addition of the +\fIforkall\fR() +function to the standard was considered and rejected. The +\fIforkall\fR() +function lets all the threads in the parent be duplicated in the +child. This essentially duplicates the state of the parent in the +child. This allows threads in the child to continue processing and +allows locks and the state to be preserved without explicit +\fIpthread_atfork\fR() +code. The calling process has to ensure that the threads processing +state that is shared between the parent and child (that is, file +descriptors or MAP_SHARED +memory) behaves properly after +\fIforkall\fR(). +For example, if a thread is reading a file descriptor in the parent +when +\fIforkall\fR() +is called, then two threads (one in the parent and one in the child) +are reading the file descriptor after the +\fIforkall\fR(). +If this is not desired behavior, the parent process has to synchronize +with such threads before calling +\fIforkall\fR(). +.P +While the +\fIfork\fR() +function is async-signal-safe, there is no way for an implementation to +determine whether the fork handlers established by +\fIpthread_atfork\fR() +are async-signal-safe. The fork handlers may attempt to execute +portions of the implementation that are not async-signal-safe, such as +those that are protected by mutexes, leading to a deadlock condition. +It is therefore undefined for the fork handlers to execute functions +that are not async-signal-safe when +\fIfork\fR() +is called from a signal handler. +.P +When +\fIforkall\fR() +is called, threads, other than the calling thread, that are in +functions that can return with an +.BR [EINTR] +error may have those functions return +.BR [EINTR] +if the implementation cannot ensure that the function behaves correctly +in the parent and child. In particular, +\fIpthread_cond_wait\fR() +and +\fIpthread_cond_timedwait\fR() +need to return in order to ensure that the condition has not changed. +These functions can be awakened by a spurious condition wakeup rather +than returning +.BR [EINTR] . +.SH "FUTURE DIRECTIONS" +None. +.SH "SEE ALSO" +.IR "\fIalarm\fR\^(\|)", +.IR "\fIexec\fR\^", +.IR "\fIfcntl\fR\^(\|)", +.IR "\fIposix_trace_attr_getinherited\fR\^(\|)", +.IR "\fIposix_trace_eventid_equal\fR\^(\|)", +.IR "\fIpthread_atfork\fR\^(\|)", +.IR "\fIsemop\fR\^(\|)", +.IR "\fIsignal\fR\^(\|)", +.IR "\fItimes\fR\^(\|)" +.P +The Base Definitions volume of POSIX.1\(hy2008, +.IR "Section 4.11" ", " "Memory Synchronization", +.IR "\fB<sys_types.h>\fP", +.IR "\fB<unistd.h>\fP" +.SH COPYRIGHT +Portions of this text are reprinted and reproduced in electronic form +from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology +-- Portable Operating System Interface (POSIX), The Open Group Base +Specifications Issue 7, Copyright (C) 2013 by the Institute of +Electrical and Electronics Engineers, Inc and The Open Group. +(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the +event of any discrepancy between this version and the original IEEE and +The Open Group Standard, the original IEEE and The Open Group Standard +is the referee document. The original Standard can be obtained online at +http://www.unix.org/online.html . + +Any typographical or formatting errors that appear +in this page are most likely +to have been introduced during the conversion of the source files to +man page format. To report such errors, see +https://www.kernel.org/doc/man-pages/reporting_bugs.html . |