трубопроводный поток к или от процесса (pipe stream to or from a process)
Имя (Name)
popen, pclose - pipe stream to or from a process
Синопсис (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 <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
Описание (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 resulting stream is correspondingly read-
only or write-only.
The command argument is a pointer to a null-terminated string
containing 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 writing. Since glibc 2.9, this argument can additionally
include the letter '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 itself. 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).
Возвращаемое значение (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
returned.
On failure, both functions set errno to indicate the error.
Ошибки (Error)
The popen() function does not set errno if memory allocation
fails. If the underlying fork(2) or pipe(2) fails, errno is set
to indicate the error. 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.
Атрибуты (Attributes)
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│popen(), pclose() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
Стандарты (Conforming to)
POSIX.1-2001, POSIX.1-2008.
The 'e' value for type is a Linux extension.
Примечание (Note)
Note: carefully read Caveats in system(3).
Ошибки (баги) (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 writing 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.
Смотри также (See also)
sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3),
fopen(3), stdio(3), system(3)
