Why Does My Python Subprocess Hang When Reading Output from a C Program?

Python C Program Subprocess Hangs at "for line in iter"

Using subprocess to run a C program in Python can be tricky when it comes to reading the output from the C program. In this case, the Python script freezes at "for line in iter(process.stdout.readline, '')".

Understanding the Issue: Buffering

The issue here is buffering. By default, C programs use block buffering for their stdout when writing to a pipe (as is the case when running the program from Python). This means that data is not flushed until the buffer is full or the program terminates.

Fixing the Issue

There are several ways to address this issue:

1. Modify the C Program:

Add setvbuf(stdout, (char *) NULL, _IOLBF, 0); at the beginning of the C program to force line buffering. This will flush the buffer after each newline.

2. Use stdbuf Tool:

Redirect the C program's stdout through the stdbuf tool to control the buffering behavior. For example:

import subprocess

process = subprocess.Popen(["stdbuf", "-oL", "./main"], stdout=subprocess.PIPE, bufsize=1)
for line in iter(process.stdout.readline, ''):
    print(line)

3. Use Pseudo-TTY:

Use a pseudo-TTY to simulate an interactive terminal environment for the C program. This will force the program to use line buffering.

import pexpect

child = pexpect.spawn("./main")
for line in child:
    print(line)

4. Read from Both Ends:

Use the select function in Python to read from both the C program's stdout and the Python script's stdin simultaneously. This will unblock the Python script even if the C program is not producing any output.

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