Python C program subprocess hangs at "for line in iter"

It is a block buffering issue.

What follows is an extended for your case version of my answer to Python: read streaming input from subprocess.communicate() question.

Fix stdout buffer in C program directly

stdio-based programs as a rule are line buffered if they are running interactively in a terminal and block buffered when their stdout is redirected to a pipe. In the latter case, you won't see new lines until the buffer overflows or flushed.

To avoid calling fflush() after each printf() call, you could force line buffered output by calling in a C program at the very beginning:

setvbuf(stdout, (char *) NULL, _IOLBF, 0); /* make line buffered stdout */

As soon as a newline is printed the buffer is flushed in this case.

Or fix it without modifying the source of C program

There is stdbuf utility that allows you to change buffering type without modifying the source code e.g.:

from subprocess import Popen, PIPEprocess = Popen(["stdbuf", "-oL", "./main"], stdout=PIPE, bufsize=1)for line in iter(process.stdout.readline, b''):    print line,process.communicate() # close process' stream, wait for it to exit

There are also other utilities available, see Turn off buffering in pipe.

Or use pseudo-TTY

To trick the subprocess into thinking that it is running interactively, you could use pexpect module or its analogs, for code examples that use pexpect and pty modules, see Python subprocess readlines() hangs. Here's a variation on the pty example provided there (it should work on Linux):

#!/usr/bin/env pythonimport osimport ptyimport sysfrom select import selectfrom subprocess import Popen, STDOUTmaster_fd, slave_fd = pty.openpty()  # provide tty to enable line bufferingprocess = Popen("./main", stdin=slave_fd, stdout=slave_fd, stderr=STDOUT,                bufsize=0, close_fds=True)timeout = .1 # ugly but otherwise `select` blocks on process' exit# code is similar to _copy() from pty.pywith os.fdopen(master_fd, 'r+b', 0) as master:    input_fds = [master, sys.stdin]    while True:        fds = select(input_fds, [], [], timeout)[0]        if master in fds: # subprocess' output is ready            data = os.read(master_fd, 512) # <-- doesn't block, may return less            if not data: # EOF                input_fds.remove(master)            else:                os.write(sys.stdout.fileno(), data) # copy to our stdout        if sys.stdin in fds: # got user input            data = os.read(sys.stdin.fileno(), 512)            if not data:                input_fds.remove(sys.stdin)            else:                master.write(data) # copy it to subprocess' stdin        if not fds: # timeout in select()            if process.poll() is not None: # subprocess ended                # and no output is buffered <-- timeout + dead subprocess                assert not select([master], [], [], 0)[0] # race is possible                os.close(slave_fd) # subproces don't need it anymore                breakrc = process.wait()print("subprocess exited with status %d" % rc)

Or use pty via pexpect

pexpect wraps pty handling into higher level interface:

#!/usr/bin/env pythonimport pexpectchild = pexpect.spawn("/.main")for line in child:    print line,child.close()

Q: Why not just use a pipe (popen())? explains why pseudo-TTY is useful.

Your program isn't hung, it just runs very slowly. Your program is using buffered output; the "2000\n" data is not being written to stdout immediately, but will eventually make it. In your case, it might take BUFSIZ/strlen("2000\n") seconds (probably 1638 seconds) to complete.

After this line:

printf("2000\n");

add

fflush(stdout);

See readline docs.

Your code:

process.stdout.readline

Is waiting for EOF or a newline.

I cannot tell what you are ultimately trying to do, but adding a newline to your printf, e.g., printf("2000\n");, should at least get you started.