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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 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 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 default.
The pclose function waits for the associated process to terminate 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 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.
A popen() and a pclose() function appeared in Version 7 AT&T UNIX.
fork(2), sh(1), pipe(2), wait4(2), fflush(3), fclose(3), fopen(3),