popen, pclose - process I/O
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 defini-
tion 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 mode 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(). 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
The pclose function waits for the associated process to ter-
minate and returns the exit status of the command as
returned by wait4.
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 returns an error, or
some other error is detected.
The popen function does not set errno if memory allocation
fails. If the underlying fork() or pipe() fails, errno is
set appropriately. If the mode argument is invalid, and
this condition is detected, errno is set to EINVAL.
If pclose() cannot obtain the child status, errno is set to
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.
A popen() and a pclose() function appeared in Version 7 AT&T
fork(2), sh(1), pipe(2), wait4(2), fflush(3), fclose(3),
fopen(3), stdio(3), system(3).