POPEN(3) Linux Programmer's Manual POPEN(3)
NAME
 popen, pclose - pipe stream to or from a process
SYNOPSIS
 #include <stdio.h>
 FILE *popen(const char *command, const char *type);
 int pclose(FILE *stream);
 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
 popen(), pclose():
 _POSIX_C_SOURCE >= 2
 || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
DESCRIPTION
 The popen() function opens a process by creating a pipe, forking, and
 invoking the shell. Since a pipe is by definition unidirectional, the
 type argument may specify only reading or writing, not both; the re-
 sulting stream is correspondingly read-only or write-only.
 The command argument is a pointer to a null-terminated string contain-
 ing a shell command line. This command is passed to /bin/sh using the
 -c flag; interpretation, if any, is performed by the shell.
 The type argument is a pointer to a null-terminated string which must
 contain either the letter 'r' for reading or the letter 'w' for writ-
 ing. Since glibc 2.9, this argument can additionally include the let-
 ter 'e', which causes the close-on-exec flag (FD_CLOEXEC) to be set on
 the underlying file descriptor; see the description of the O_CLOEXEC
 flag in open(2) for reasons why this may be useful.
 The return value from popen() is a normal standard I/O stream in all
 respects save that it must be closed with pclose() rather than
 fclose(3). Writing to such a stream writes to the standard input of
 the command; the command's standard output is the same as that of the
 process that called popen(), unless this is altered by the command it-
 self. Conversely, reading from the stream reads the command's standard
 output, and the command's standard input is the same as that of the
 process that called popen().
 Note that output popen() streams are block buffered by default.
 The pclose() function waits for the associated process to terminate and
 returns the exit status of the command as returned by wait4(2).
RETURN VALUE
 popen(): on success, returns a pointer to an open stream that can be
 used to read or write to the pipe; if the fork(2) or pipe(2) calls
 fail, or if the function cannot allocate memory, NULL is returned.
 pclose(): on success, returns the exit status of the command; if
 wait4(2) returns an error, or some other error is detected, -1 is re-
 turned.
 Both functions set errno to an appropriate value in the case of an er-
 ror.
ERRORS
 The popen() function does not set errno if memory allocation fails. If
 the underlying fork(2) or pipe(2) fails, errno is set appropriately.
 If the type argument is invalid, and this condition is detected, errno
 is set to EINVAL.
 If pclose() cannot obtain the child status, errno is set to ECHILD.
ATTRIBUTES
 For an explanation of the terms used in this section, see at-
 tributes(7).
 +------------------+---------------+---------+
 |Interface | Attribute | Value |
 +------------------+---------------+---------+
 |popen(), pclose() | Thread safety | MT-Safe |
 +------------------+---------------+---------+
CONFORMING TO
 POSIX.1-2001, POSIX.1-2008.
 The 'e' value for type is a Linux extension.
NOTES
 Note: carefully read Caveats in system(3).
BUGS
 Since the standard input of a command opened for reading shares its
 seek offset with the process that called popen(), if the original
 process has done a buffered read, the command's input position may not
 be as expected. Similarly, the output from a command opened for writ-
 ing may become intermingled with that of the original process. The
 latter can be avoided by calling fflush(3) before popen().
 Failure to execute the shell is indistinguishable from the shell's
 failure to execute command, or an immediate exit of the command. The
 only hint is an exit status of 127.
SEE ALSO
 sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3),
 stdio(3), system(3)
COLOPHON
 This page is part of release 5.10 of the Linux man-pages project. A
 description of the project, information about reporting bugs, and the
 latest version of this page, can be found at
 https://www.kernel.org/doc/man-pages/.
GNU 2017年09月15日 POPEN(3)