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
13 posix_spawn_file_actions_addclose, posix_spawn_file_actions_addopen —
14 add close or open action to spawn file actions object (ADVANCED REAL‐
15 TIME)
16
18 #include <spawn.h>
19 int posix_spawn_file_actions_addclose(posix_spawn_file_actions_t
21 *file_actions, int fildes);
22 int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t
23 *restrict file_actions, int fildes,
24 const char *restrict path, int oflag, mode_t mode);
25
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. POSIX.1‐2008 does not
35 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 been
57 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 The posix_spawn_file_actions_addopen() function shall fail if:
71 EBADF The value specified by fildes is negative or greater than or
73 equal to {OPEN_MAX}.
74 The posix_spawn_file_actions_addclose() function shall fail if:
76 EBADF The value specified by fildes is negative.
78 These functions may fail if:
80 EINVAL The value specified by file_actions is invalid.
82 ENOMEM Insufficient memory exists to add to the spawn file actions
84 object.
85 It shall not be considered an error for the fildes argument passed to
87 these functions to specify a file descriptor for which the specified
88 operation could not be performed at the time of the call. Any such
89 error will be detected when the associated file actions object is later
90 used during a posix_spawn() or posix_spawnp() operation.
91 The following sections are informative.
93
95 None.
96
98 These functions are part of the Spawn option and need not be provided
99 on all implementations.
100 Implementations may use file descriptors that must be inherited into
102 child processes for the child process to remain conforming, such as for
103 message catalog or tracing purposes. Therefore, an application that
104 calls posix_spawn_file_actions_addclose() with an arbitrary integer
105 risks non-conforming behavior, and this function can only portably be
106 used to close file descriptor values that the application has obtained
107 through explicit actions, or for the three file descriptors correspond‐
108 ing to the standard file streams. In order to avoid a race condition of
109 leaking an unintended file descriptor into a child process, an applica‐
110 tion should consider opening all file descriptors with the FD_CLOEXEC
111 bit set unless the file descriptor is intended to be inherited across
112 exec.
113
115 A spawn file actions object may be initialized to contain an ordered
116 sequence of close(), dup2(), and open() operations to be used by
117 posix_spawn() or posix_spawnp() to arrive at the set of open file
118 descriptors inherited by the spawned process from the set of open file
119 descriptors in the parent at the time of the posix_spawn() or
120 posix_spawnp() call. It had been suggested that the close() and dup2()
121 operations alone are sufficient to rearrange file descriptors, and that
122 files which need to be opened for use by the spawned process can be
123 handled either by having the calling process open them before the
124 posix_spawn() or posix_spawnp() call (and close them after), or by
125 passing pathnames to the spawned process (in argv) so that it may open
126 them itself. The standard developers recommend that applications use
127 one of these two methods when practical, since detailed error status on
128 a failed open operation is always available to the application this
129 way. However, the standard developers feel that allowing a spawn file
130 actions object to specify open operations is still appropriate because:
131 1. It is consistent with equivalent POSIX.5 (Ada) functionality.
133 2. It supports the I/O redirection paradigm commonly employed by POSIX
135 programs designed to be invoked from a shell. When such a program
136 is the child process, it may not be designed to open files on its
137 own.
138 3. It allows file opens that might otherwise fail or violate file own‐
140 ership/access rights if executed by the parent process.
141 Regarding 2. above, note that the spawn open file action provides to
143 posix_spawn() and posix_spawnp() the same capability that the shell re‐
144 direction operators provide to system(), only without the intervening
145 execution of a shell; for example:
146 system ("myprog <file1 3<file2");
148 Regarding 3. above, note that if the calling process needs to open one
150 or more files for access by the spawned process, but has insufficient
151 spare file descriptors, then the open action is necessary to allow the
152 open() to occur in the context of the child process after other file
153 descriptors have been closed (that must remain open in the parent).
154 Additionally, if a parent is executed from a file having a ``set-user-
156 id'' mode bit set and the POSIX_SPAWN_RESETIDS flag is set in the spawn
157 attributes, a file created within the parent process will (possibly
158 incorrectly) have the parent's effective user ID as its owner, whereas
159 a file created via an open() action during posix_spawn() or
160 posix_spawnp() will have the parent's real ID as its owner; and an open
161 by the parent process may successfully open a file to which the real
162 user should not have access or fail to open a file to which the real
163 user should have access.
164 File Descriptor Mapping
166 The standard developers had originally proposed using an array which
167 specified the mapping of child file descriptors back to those of the
168 parent. It was pointed out by the ballot group that it is not possible
169 to reshuffle file descriptors arbitrarily in a library implementation
170 of posix_spawn() or posix_spawnp() without provision for one or more
171 spare file descriptor entries (which simply may not be available). Such
172 an array requires that an implementation develop a complex strategy to
173 achieve the desired mapping without inadvertently closing the wrong
174 file descriptor at the wrong time.
175 It was noted by a member of the Ada Language Bindings working group
177 that the approved Ada Language Start_Process family of POSIX process
178 primitives use a caller-specified set of file actions to alter the nor‐
179 mal fork()/exec semantics for inheritance of file descriptors in a very
180 flexible way, yet no such problems exist because the burden of deter‐
181 mining how to achieve the final file descriptor mapping is completely
182 on the application. Furthermore, although the file actions interface
183 appears frightening at first glance, it is actually quite simple to
184 implement in either a library or the kernel.
185 The posix_spawn_file_actions_addclose() function is not required to
187 check whether the file descriptor is less than {OPEN_MAX} because on
188 some implementations {OPEN_MAX} reflects the RLIMIT_NOFILE soft limit
189 and therefore calling setrlimit() to reduce this limit can result in an
190 {OPEN_MAX} value less than or equal to an already open file descriptor.
191 Applications need to be able to close such file descriptors on spawn.
192 On implementations where {OPEN_MAX} does not change, it is recommended
193 that posix_spawn_file_actions_addclose() should return [EBADF] if
194 fildes is greater than or equal to {OPEN_MAX}.
195
197 None.
198
200 close(), dup(), open(), posix_spawn(),
201 posix_spawn_file_actions_adddup2(), posix_spawn_file_actions_destroy()
202 The Base Definitions volume of POSIX.1‐2008, <spawn.h>
204
206 Portions of this text are reprinted and reproduced in electronic form
207 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
208 -- Portable Operating System Interface (POSIX), The Open Group Base
209 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
210 cal and Electronics Engineers, Inc and The Open Group. (This is
211 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
212 event of any discrepancy between this version and the original IEEE and
213 The Open Group Standard, the original IEEE and The Open Group Standard
214 is the referee document. The original Standard can be obtained online
215 at http://www.unix.org/online.html .
216 Any typographical or formatting errors that appear in this page are
218 most likely to have been introduced during the conversion of the source
219 files to man page format. To report such errors, see https://www.ker‐
220 nel.org/doc/man-pages/reporting_bugs.html .
221IEEE/The Open Group 2013POSIX_SPAWN_FILE_ACTIONS_ADDCLOSE(3P)