1POPEN(3) Linux Programmer's Manual POPEN(3)
2
6 popen, pclose - pipe stream to or from a process
7
9 #include <stdio.h>
10 FILE *popen(const char *command, const char *type);
12 int pclose(FILE *stream);
14 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
16 popen(), pclose():
18 _POSIX_C_SOURCE >= 2
19 || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
20
22 The popen() function opens a process by creating a pipe, forking, and
23 invoking the shell. Since a pipe is by definition unidirectional, the
24 type argument may specify only reading or writing, not both; the
25 resulting stream is correspondingly read-only or write-only.
26 The command argument is a pointer to a null-terminated string contain‐
28 ing a shell command line. This command is passed to /bin/sh using the
29 -c flag; interpretation, if any, is performed by the shell.
30 The type argument is a pointer to a null-terminated string which must
32 contain either the letter 'r' for reading or the letter 'w' for writ‐
33 ing. Since glibc 2.9, this argument can additionally include the let‐
34 ter 'e', which causes the close-on-exec flag (FD_CLOEXEC) to be set on
35 the underlying file descriptor; see the description of the O_CLOEXEC
36 flag in open(2) for reasons why this may be useful.
37 The return value from popen() is a normal standard I/O stream in all
39 respects save that it must be closed with pclose() rather than
40 fclose(3). Writing to such a stream writes to the standard input of
41 the command; the command's standard output is the same as that of the
42 process that called popen(), unless this is altered by the command
43 itself. Conversely, reading from the stream reads the command's stan‐
44 dard output, and the command's standard input is the same as that of
45 the process that called popen().
46 Note that output popen() streams are block buffered by default.
48 The pclose() function waits for the associated process to terminate and
50 returns the exit status of the command as returned by wait4(2).
51
53 popen(): on success, returns a pointer to an open stream that can be
54 used to read or write to the pipe; if the fork(2) or pipe(2) calls
55 fail, or if the function cannot allocate memory, NULL is returned.
56 pclose(): on success, returns the exit status of the command; if
58 wait4(2) returns an error, or some other error is detected, -1 is
59 returned.
60 Both functions set errno to an appropriate value in the case of an
62 error.
63
65 The popen() function does not set errno if memory allocation fails. If
66 the underlying fork(2) or pipe(2) fails, errno is set appropriately.
67 If the type argument is invalid, and this condition is detected, errno
68 is set to EINVAL.
69 If pclose() cannot obtain the child status, errno is set to ECHILD.
71
73 For an explanation of the terms used in this section, see
74 attributes(7).
75 ┌──────────────────┬───────────────┬─────────┐
77 │Interface │ Attribute │ Value │
78 ├──────────────────┼───────────────┼─────────┤
79 │popen(), pclose() │ Thread safety │ MT-Safe │
80 └──────────────────┴───────────────┴─────────┘
81
83 POSIX.1-2001, POSIX.1-2008.
84 The 'e' value for type is a Linux extension.
86
88 Note: carefully read Caveats in system(3).
89
91 Since the standard input of a command opened for reading shares its
92 seek offset with the process that called popen(), if the original
93 process has done a buffered read, the command's input position may not
94 be as expected. Similarly, the output from a command opened for writ‐
95 ing may become intermingled with that of the original process. The
96 latter can be avoided by calling fflush(3) before popen().
97 Failure to execute the shell is indistinguishable from the shell's
99 failure to execute command, or an immediate exit of the command. The
100 only hint is an exit status of 127.
101
103 sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3),
104 stdio(3), system(3)
105
107 This page is part of release 5.02 of the Linux man-pages project. A
108 description of the project, information about reporting bugs, and the
109 latest version of this page, can be found at
110 https://www.kernel.org/doc/man-pages/.
111GNU 2017-09-15 POPEN(3)