popen, pclose — pipe stream to or from a process
||const char *command,|
|const char *type
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.
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.
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
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
Note that output
streams are block buffered by default.
pclose() function waits
for the associated process to terminate and returns the exit
status of the command as returned by wait4(2).
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.
Both functions set
an appropriate value in the case of an error.
popen() function does
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.
pclose() cannot obtain
the child status,
errno is set
For an explanation of the terms used in this section, see attributes(7).
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
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.
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(#)popen.3 6.4 (Berkeley) 4/30/91
Converted for Linux, Mon Nov 29 14:45:38 1993, faithcs.unc.edu
Modified Sat May 18 20:37:44 1996 by Martin Schulze (joeylinux.de)
Modified 7 May 1998 by Joseph S. Myers (jsm28cam.ac.uk)