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 || _XOPEN_SOURCE || _BSD_SOURCE ||
19 _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. The type
30 argument is a pointer to a null-terminated string which must contain
31 either the letter 'r' for reading or the letter 'w' for writing. Since
32 glibc 2.9, this argument can additionally include the letter 'e', which
33 causes the close-on-exec flag (FD_CLOEXEC) to be set on the underlying
34 file descriptor; see the description of the O_CLOEXEC flag in open(2)
35 for reasons why this may be useful.
36 The return value from popen() is a normal standard I/O stream in all
38 respects save that it must be closed with pclose() rather than
39 fclose(3). Writing to such a stream writes to the standard input of
40 the command; the command's standard output is the same as that of the
41 process that called popen(), unless this is altered by the command
42 itself. Conversely, reading from a "popened" stream reads the com‐
43 mand's standard output, and the command's standard input is the same as
44 that of the process that called popen().
45 Note that output popen() streams are fully buffered by default.
47 The pclose() function waits for the associated process to terminate and
49 returns the exit status of the command as returned by wait4(2).
50
52 The popen() function returns NULL if the fork(2) or pipe(2) calls fail,
53 or if it cannot allocate memory.
54 The pclose() function returns -1 if wait4(2) returns an error, or some
56 other error is detected. In the event of an error, these functions set
57 errnro to indicate the cause of the error.
58
60 The popen() function does not set errno if memory allocation fails. If
61 the underlying fork(2) or pipe(2) fails, errno is set appropriately.
62 If the type argument is invalid, and this condition is detected, errno
63 is set to EINVAL.
64 If pclose() cannot obtain the child status, errno is set to ECHILD.
66
68 POSIX.1-2001.
69 The 'e' value for type is a Linux extension.
71
73 Since the standard input of a command opened for reading shares its
74 seek offset with the process that called popen(), if the original
75 process has done a buffered read, the command's input position may not
76 be as expected. Similarly, the output from a command opened for writ‐
77 ing may become intermingled with that of the original process. The
78 latter can be avoided by calling fflush(3) before popen().
79 Failure to execute the shell is indistinguishable from the shell's
81 failure to execute command, or an immediate exit of the command. The
82 only hint is an exit status of 127.
83
85 sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3),
86 stdio(3), system(3)
87
89 This page is part of release 3.53 of the Linux man-pages project. A
90 description of the project, and information about reporting bugs, can
91 be found at http://www.kernel.org/doc/man-pages/.
92GNU 2013-04-19 POPEN(3)