popen, pclose — process I/O
#include <stdio.h>
FILE
*popen( |
const char * | command, |
const char * | type) ; |
int
pclose( |
FILE * | stream) ; |
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 be either `r' for reading
or `w' for writing.
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 a ``popened'' 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 fully 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).
The popen
() function returns
NULL if the fork(2) or pipe(2) calls fail, or if
it cannot allocate memory.
The pclose
() function
returns −1 if wait4(2) returns an error,
or some other error is detected.
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.
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.
sh(1), fork(2), pipe(2), wait4(2), fclose(3), fflush(3), fopen(3), stdio(3), system(3)
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