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   popen    ( 3 )

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)