1POSIX_SPAWN_FILE_ACTIONS_ADDPCOLSOISXE(P3rPo)grammePrO'SsIXM_aSnPuAaWlN_FILE_ACTIONS_ADDCLOSE(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 posix_spawn_file_actions_addclose, posix_spawn_file_actions_addopen -
13 add close or open action to spawn file actions object (ADVANCED REAL‐
14 TIME)
15
17 #include <spawn.h>
18 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t *
20 file_actions, int fildes);
21 int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *
22 restrict file_actions, int fildes,
23 const char *restrict path, int oflag, mode_t mode);
24
27 These functions shall add or delete a close or open action to a spawn
28 file actions object.
29 A spawn file actions object is of type posix_spawn_file_actions_t
31 (defined in <spawn.h>) and is used to specify a series of actions to be
32 performed by a posix_spawn() or posix_spawnp() operation in order to
33 arrive at the set of open file descriptors for the child process given
34 the set of open file descriptors of the parent. IEEE Std 1003.1-2001
35 does not define comparison or assignment operators for the type
36 posix_spawn_file_actions_t.
37 A spawn file actions object, when passed to posix_spawn() or
39 posix_spawnp(), shall specify how the set of open file descriptors in
40 the calling process is transformed into a set of potentially open file
41 descriptors for the spawned process. This transformation shall be as if
42 the specified sequence of actions was performed exactly once, in the
43 context of the spawned process (prior to execution of the new process
44 image), in the order in which the actions were added to the object;
45 additionally, when the new process image is executed, any file descrip‐
46 tor (from this new set) which has its FD_CLOEXEC flag set shall be
47 closed (see posix_spawn()).
48 The posix_spawn_file_actions_addclose() function shall add a close
50 action to the object referenced by file_actions that shall cause the
51 file descriptor fildes to be closed (as if close( fildes) had been
52 called) when a new process is spawned using this file actions object.
53 The posix_spawn_file_actions_addopen() function shall add an open
55 action to the object referenced by file_actions that shall cause the
56 file named by path to be opened (as if open( path, oflag, mode) had
57 been called, and the returned file descriptor, if not fildes, had been
58 changed to fildes) when a new process is spawned using this file
59 actions object. If fildes was already an open file descriptor, it shall
60 be closed before the new file is opened.
61 The string described by path shall be copied by the
63 posix_spawn_file_actions_addopen() function.
64
66 Upon successful completion, these functions shall return zero; other‐
67 wise, an error number shall be returned to indicate the error.
68
70 These functions shall fail if:
71 EBADF The value specified by fildes is negative or greater than or
73 equal to {OPEN_MAX}.
74 These functions may fail if:
77 EINVAL The value specified by file_actions is invalid.
79 ENOMEM Insufficient memory exists to add to the spawn file actions
81 object.
82 It shall not be considered an error for the fildes argument passed to
85 these functions to specify a file descriptor for which the specified
86 operation could not be performed at the time of the call. Any such
87 error will be detected when the associated file actions object is later
88 used during a posix_spawn() or posix_spawnp() operation.
89 The following sections are informative.
91
93 None.
94
96 These functions are part of the Spawn option and need not be provided
97 on all implementations.
98
100 A spawn file actions object may be initialized to contain an ordered
101 sequence of close(), dup2(), and open() operations to be used by
102 posix_spawn() or posix_spawnp() to arrive at the set of open file
103 descriptors inherited by the spawned process from the set of open file
104 descriptors in the parent at the time of the posix_spawn() or
105 posix_spawnp() call. It had been suggested that the close() and dup2()
106 operations alone are sufficient to rearrange file descriptors, and that
107 files which need to be opened for use by the spawned process can be
108 handled either by having the calling process open them before the
109 posix_spawn() or posix_spawnp() call (and close them after), or by
110 passing filenames to the spawned process (in argv) so that it may open
111 them itself. The standard developers recommend that applications use
112 one of these two methods when practical, since detailed error status on
113 a failed open operation is always available to the application this
114 way. However, the standard developers feel that allowing a spawn file
115 actions object to specify open operations is still appropriate because:
116 1. It is consistent with equivalent POSIX.5 (Ada) functionality.
118 2. It supports the I/O redirection paradigm commonly employed by POSIX
120 programs designed to be invoked from a shell. When such a program
121 is the child process, it may not be designed to open files on its
122 own.
123 3. It allows file opens that might otherwise fail or violate file own‐
125 ership/access rights if executed by the parent process.
126 Regarding 2. above, note that the spawn open file action provides to
128 posix_spawn() and posix_spawnp() the same capability that the shell re‐
129 direction operators provide to system(), only without the intervening
130 execution of a shell; for example:
131 system ("myprog <file1 3<file2");
134 Regarding 3. above, note that if the calling process needs to open one
136 or more files for access by the spawned process, but has insufficient
137 spare file descriptors, then the open action is necessary to allow the
138 open() to occur in the context of the child process after other file
139 descriptors have been closed (that must remain open in the parent).
140 Additionally, if a parent is executed from a file having a "set-user-
142 id" mode bit set and the POSIX_SPAWN_RESETIDS flag is set in the spawn
143 attributes, a file created within the parent process will (possibly
144 incorrectly) have the parent's effective user ID as its owner, whereas
145 a file created via an open() action during posix_spawn() or
146 posix_spawnp() will have the parent's real ID as its owner; and an open
147 by the parent process may successfully open a file to which the real
148 user should not have access or fail to open a file to which the real
149 user should have access.
150 File Descriptor Mapping
152 The standard developers had originally proposed using an array which
153 specified the mapping of child file descriptors back to those of the
154 parent. It was pointed out by the ballot group that it is not possible
155 to reshuffle file descriptors arbitrarily in a library implementation
156 of posix_spawn() or posix_spawnp() without provision for one or more
157 spare file descriptor entries (which simply may not be available). Such
158 an array requires that an implementation develop a complex strategy to
159 achieve the desired mapping without inadvertently closing the wrong
160 file descriptor at the wrong time.
161 It was noted by a member of the Ada Language Bindings working group
163 that the approved Ada Language Start_Process family of POSIX process
164 primitives use a caller-specified set of file actions to alter the nor‐
165 mal fork()/ exec semantics for inheritance of file descriptors in a
166 very flexible way, yet no such problems exist because the burden of
167 determining how to achieve the final file descriptor mapping is com‐
168 pletely on the application. Furthermore, although the file actions
169 interface appears frightening at first glance, it is actually quite
170 simple to implement in either a library or the kernel.
171
173 None.
174
176 close(), dup(), open(), posix_spawn(),
177 posix_spawn_file_actions_adddup2(), posix_spawn_file_actions_destroy(),
178 posix_spawnp(), the Base Definitions volume of IEEE Std 1003.1-2001,
179 <spawn.h>
180
182 Portions of this text are reprinted and reproduced in electronic form
183 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
184 -- Portable Operating System Interface (POSIX), The Open Group Base
185 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
186 Electrical and Electronics Engineers, Inc and The Open Group. In the
187 event of any discrepancy between this version and the original IEEE and
188 The Open Group Standard, the original IEEE and The Open Group Standard
189 is the referee document. The original Standard can be obtained online
190 at http://www.opengroup.org/unix/online.html .
191IEEE/The Open Group 2003POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE(3P)