1PCLOSE(3P) POSIX Programmer's Manual PCLOSE(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 pclose — close a pipe stream to or from a process
13
15 #include <stdio.h>
16 int pclose(FILE *stream);
18
20 The pclose() function shall close a stream that was opened by popen(),
21 wait for the command to terminate, and return the termination status of
22 the process that was running the command language interpreter. How‐
23 ever, if a call caused the termination status to be unavailable to
24 pclose(), then pclose() shall return -1 with errno set to [ECHILD] to
25 report this situation. This can happen if the application calls one of
26 the following functions:
27 * wait()
29 * waitpid() with a pid argument less than or equal to 0 or equal to
31 the process ID of the command line interpreter
32 * Any other function not defined in this volume of POSIX.1‐2017 that
34 could do one of the above
35 In any case, pclose() shall not return before the child process created
37 by popen() has terminated.
38 If the command language interpreter cannot be executed, the child ter‐
40 mination status returned by pclose() shall be as if the command lan‐
41 guage interpreter terminated using exit(127) or _exit(127).
42 The pclose() function shall not affect the termination status of any
44 child of the calling process other than the one created by popen() for
45 the associated stream.
46 If the argument stream to pclose() is not a pointer to a stream created
48 by popen(), the result of pclose() is undefined.
49 If a thread is canceled during execution of pclose(), the behavior is
51 undefined.
52
54 Upon successful return, pclose() shall return the termination status of
55 the command language interpreter. Otherwise, pclose() shall return -1
56 and set errno to indicate the error.
57
59 The pclose() function shall fail if:
60 ECHILD The status of the child process could not be obtained, as
62 described above.
63 The following sections are informative.
65
67 None.
68
70 None.
71
73 There is a requirement that pclose() not return before the child
74 process terminates. This is intended to disallow implementations that
75 return [EINTR] if a signal is received while waiting. If pclose()
76 returned before the child terminated, there would be no way for the
77 application to discover which child used to be associated with the
78 stream, and it could not do the cleanup itself.
79 If the stream pointed to by stream was not created by popen(), histori‐
81 cal implementations of pclose() return -1 without setting errno. To
82 avoid requiring pclose() to set errno in this case, POSIX.1‐2008 makes
83 the behavior unspecified. An application should not use pclose() to
84 close any stream that was not created by popen().
85 Some historical implementations of pclose() either block or ignore the
87 signals SIGINT, SIGQUIT, and SIGHUP while waiting for the child process
88 to terminate. Since this behavior is not described for the pclose()
89 function in POSIX.1‐2008, such implementations are not conforming.
90 Also, some historical implementations return [EINTR] if a signal is
91 received, even though the child process has not terminated. Such imple‐
92 mentations are also considered non-conforming.
93 Consider, for example, an application that uses:
95 popen("command", "r")
98 to start command, which is part of the same application. The parent
100 writes a prompt to its standard output (presumably the terminal) and
101 then reads from the popen()ed stream. The child reads the response from
102 the user, does some transformation on the response (pathname expansion,
103 perhaps) and writes the result to its standard output. The parent
104 process reads the result from the pipe, does something with it, and
105 prints another prompt. The cycle repeats. Assuming that both processes
106 do appropriate buffer flushing, this would be expected to work.
107 To conform to POSIX.1‐2008, pclose() must use waitpid(), or some simi‐
109 lar function, instead of wait().
110 The code sample below illustrates how the pclose() function might be
112 implemented on a system conforming to POSIX.1‐2008.
113 int pclose(FILE *stream)
116 {
117 int stat;
118 pid_t pid;
119 pid = <pid for process created for stream by popen()>
121 (void) fclose(stream);
122 while (waitpid(pid, &stat, 0) == -1) {
123 if (errno != EINTR){
124 stat = -1;
125 break;
126 }
127 }
128 return(stat);
129 }
130
132 None.
133
135 fork(), popen(), wait()
136 The Base Definitions volume of POSIX.1‐2017, <stdio.h>
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140 Portions of this text are reprinted and reproduced in electronic form
141 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
142 table Operating System Interface (POSIX), The Open Group Base Specifi‐
143 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
144 Electrical and Electronics Engineers, Inc and The Open Group. In the
145 event of any discrepancy between this version and the original IEEE and
146 The Open Group Standard, the original IEEE and The Open Group Standard
147 is the referee document. The original Standard can be obtained online
148 at http://www.opengroup.org/unix/online.html .
149 Any typographical or formatting errors that appear in this page are
151 most likely to have been introduced during the conversion of the source
152 files to man page format. To report such errors, see https://www.ker‐
153 nel.org/doc/man-pages/reporting_bugs.html .
154IEEE/The Open Group 2017 PCLOSE(3P)