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   posix_spawn.3p    ( 3 )

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Обоснование (Rationale)

The posix_spawn() function and its close relation posix_spawnp() have been introduced to overcome the following perceived difficulties with fork(): the fork() function is difficult or impossible to implement without swapping or dynamic address translation.

* Swapping is generally too slow for a realtime environment.

* Dynamic address translation is not available everywhere that POSIX might be useful.

* Processes are too useful to simply option out of POSIX whenever it must run without address translation or other MMU services.

Thus, POSIX needs process creation and file execution primitives that can be efficiently implemented without address translation or other MMU services.

The posix_spawn() function is implementable as a library routine, but both posix_spawn() and posix_spawnp() are designed as kernel operations. Also, although they may be an efficient replacement for many fork()/exec pairs, their goal is to provide useful process creation primitives for systems that have difficulty with fork(), not to provide drop-in replacements for fork()/exec.

This view of the role of posix_spawn() and posix_spawnp() influenced the design of their API. It does not attempt to provide the full functionality of fork()/exec in which arbitrary user-specified operations of any sort are permitted between the creation of the child process and the execution of the new process image; any attempt to reach that level would need to provide a programming language as parameters. Instead, posix_spawn() and posix_spawnp() are process creation primitives like the Start_Process and Start_Process_Search Ada language bindings package POSIX_Process_Primitives and also like those in many operating systems that are not UNIX systems, but with some POSIX-specific additions.

To achieve its coverage goals, posix_spawn() and posix_spawnp() have control of six types of inheritance: file descriptors, process group ID, user and group ID, signal mask, scheduling, and whether each signal ignored in the parent will remain ignored in the child, or be reset to its default action in the child.

Control of file descriptors is required to allow an independently written child process image to access data streams opened by and even generated or read by the parent process without being specifically coded to know which parent files and file descriptors are to be used. Control of the process group ID is required to control how the job control of the child process relates to that of the parent.

Control of the signal mask and signal defaulting is sufficient to support the implementation of system(). Although support for system() is not explicitly one of the goals for posix_spawn() and posix_spawnp(), it is covered under the ``at least 50%'' coverage goal.

The intention is that the normal file descriptor inheritance across fork(), the subsequent effect of the specified spawn file actions, and the normal file descriptor inheritance across one of the exec family of functions should fully specify open file inheritance. The implementation need make no decisions regarding the set of open file descriptors when the child process image begins execution, those decisions having already been made by the caller and expressed as the set of open file descriptors and their FD_CLOEXEC flags at the time of the call and the spawn file actions object specified in the call. We have been assured that in cases where the POSIX Start_Process Ada primitives have been implemented in a library, this method of controlling file descriptor inheritance may be implemented very easily.

We can identify several problems with posix_spawn() and posix_spawnp(), but there does not appear to be a solution that introduces fewer problems. Environment modification for child process attributes not specifiable via the attrp or file_actions arguments must be done in the parent process, and since the parent generally wants to save its context, it is more costly than similar functionality with fork()/exec. It is also complicated to modify the environment of a multi-threaded process temporarily, since all threads must agree when it is safe for the environment to be changed. However, this cost is only borne by those invocations of posix_spawn() and posix_spawnp() that use the additional functionality. Since extensive modifications are not the usual case, and are particularly unlikely in time- critical code, keeping much of the environment control out of posix_spawn() and posix_spawnp() is appropriate design.

The posix_spawn() and posix_spawnp() functions do not have all the power of fork()/exec. This is to be expected. The fork() function is a wonderfully powerful operation. We do not expect to duplicate its functionality in a simple, fast function with no special hardware requirements. It is worth noting that posix_spawn() and posix_spawnp() are very similar to the process creation operations on many operating systems that are not UNIX systems.

Requirements The requirements for posix_spawn() and posix_spawnp() are:

* They must be implementable without an MMU or unusual hardware.

* They must be compatible with existing POSIX standards.

Additional goals are:

* They should be efficiently implementable.

* They should be able to replace at least 50% of typical executions of fork().

* A system with posix_spawn() and posix_spawnp() and without fork() should be useful, at least for realtime applications.

* A system with fork() and the exec family should be able to implement posix_spawn() and posix_spawnp() as library routines.

Two-Syntax POSIX exec has several calling sequences with approximately the same functionality. These appear to be required for compatibility with existing practice. Since the existing practice for the posix_spawn*() functions is otherwise substantially unlike POSIX, we feel that simplicity outweighs compatibility. There are, therefore, only two names for the posix_spawn*() functions.

The parameter list does not differ between posix_spawn() and posix_spawnp(); posix_spawnp() interprets the second parameter more elaborately than posix_spawn().

Compatibility with POSIX.5 (Ada) The Start_Process and Start_Process_Search procedures from the POSIX_Process_Primitives package from the Ada language binding to POSIX.1 encapsulate fork() and exec functionality in a manner similar to that of posix_spawn() and posix_spawnp(). Originally, in keeping with our simplicity goal, the standard developers had limited the capabilities of posix_spawn() and posix_spawnp() to a subset of the capabilities of Start_Process and Start_Process_Search; certain non-default capabilities were not supported. However, based on suggestions by the ballot group to improve file descriptor mapping or drop it, and on the advice of an Ada Language Bindings working group member, the standard developers decided that posix_spawn() and posix_spawnp() should be sufficiently powerful to implement Start_Process and Start_Process_Search. The rationale is that if the Ada language binding to such a primitive had already been approved as an IEEE standard, there can be little justification for not approving the functionally-equivalent parts of a C binding. The only three capabilities provided by posix_spawn() and posix_spawnp() that are not provided by Start_Process and Start_Process_Search are optionally specifying the child's process group ID, the set of signals to be reset to default signal handling in the child process, and the child's scheduling policy and parameters.

For the Ada language binding for Start_Process to be implemented with posix_spawn(), that binding would need to explicitly pass an empty signal mask and the parent's environment to posix_spawn() whenever the caller of Start_Process allowed these arguments to default, since posix_spawn() does not provide such defaults. The ability of Start_Process to mask user-specified signals during its execution is functionally unique to the Ada language binding and must be dealt with in the binding separately from the call to posix_spawn().

Process Group The process group inheritance field can be used to join the child process with an existing process group. By assigning a value of zero to the spawn-pgroup attribute of the object referenced by attrp, the setpgid() mechanism will place the child process in a new process group.

Threads Without the posix_spawn() and posix_spawnp() functions, systems without address translation can still use threads to give an abstraction of concurrency. In many cases, thread creation suffices, but it is not always a good substitute. The posix_spawn() and posix_spawnp() functions are considerably ``heavier'' than thread creation. Processes have several important attributes that threads do not. Even without address translation, a process may have base-and-bound memory protection. Each process has a process environment including security attributes and file capabilities, and powerful scheduling attributes. Processes abstract the behavior of non-uniform- memory-architecture multi-processors better than threads, and they are more convenient to use for activities that are not closely linked.

The posix_spawn() and posix_spawnp() functions may not bring support for multiple processes to every configuration. Process creation is not the only piece of operating system support required to support multiple processes. The total cost of support for multiple processes may be quite high in some circumstances. Existing practice shows that support for multiple processes is uncommon and threads are common among ``tiny kernels''. There should, therefore, probably continue to be AEPs for operating systems with only one process.

Asynchronous Error Notification A library implementation of posix_spawn() or posix_spawnp() may not be able to detect all possible errors before it forks the child process. POSIX.1‐2008 provides for an error indication returned from a child process which could not successfully complete the spawn operation via a special exit status which may be detected using the status value returned by wait(), waitid(), and waitpid().

The stat_val interface and the macros used to interpret it are not well suited to the purpose of returning API errors, but they are the only path available to a library implementation. Thus, an implementation may cause the child process to exit with exit status 127 for any error detected during the spawn process after the posix_spawn() or posix_spawnp() function has successfully returned.

The standard developers had proposed using two additional macros to interpret stat_val. The first, WIFSPAWNFAIL, would have detected a status that indicated that the child exited because of an error detected during the posix_spawn() or posix_spawnp() operations rather than during actual execution of the child process image; the second, WSPAWNERRNO, would have extracted the error value if WIFSPAWNFAIL indicated a failure. Unfortunately, the ballot group strongly opposed this because it would make a library implementation of posix_spawn() or posix_spawnp() dependent on kernel modifications to waitpid() to be able to embed special information in stat_val to indicate a spawn failure.

The 8 bits of child process exit status that are guaranteed by POSIX.1‐2008 to be accessible to the waiting parent process are insufficient to disambiguate a spawn error from any other kind of error that may be returned by an arbitrary process image. No other bits of the exit status are required to be visible in stat_val, so these macros could not be strictly implemented at the library level. Reserving an exit status of 127 for such spawn errors is consistent with the use of this value by system() and popen() to signal failures in these operations that occur after the function has returned but before a shell is able to execute. The exit status of 127 does not uniquely identify this class of error, nor does it provide any detailed information on the nature of the failure. Note that a kernel implementation of posix_spawn() or posix_spawnp() is permitted (and encouraged) to return any possible error as the function value, thus providing more detailed failure information to the parent process.

Thus, no special macros are available to isolate asynchronous posix_spawn() or posix_spawnp() errors. Instead, errors detected by the posix_spawn() or posix_spawnp() operations in the context of the child process before the new process image executes are reported by setting the child's exit status to 127. The calling process may use the WIFEXITED and WEXITSTATUS macros on the stat_val stored by the wait() or waitpid() functions to detect spawn failures to the extent that other status values with which the child process image may exit (before the parent can conclusively determine that the child process image has begun execution) are distinct from exit status 127.