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(). 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 does not set errno if memory allocation fails.
If the underlying fork() or pipe() 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
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.