Why Does My Python `subprocess` Hang When Reading from a C Program\'s `stdout`?

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

When attempting to run a C program from a Python script, users may encounter a hanging issue at the "for line in iter" statement in their Python code. This occurs when using subprocess to read the output of the C program, which in this case is a test program continuously printing "2000."

Understanding the Problem

The root cause of this hanging behavior lies in stdout buffer management. By default, C programs produce block-buffered output, meaning data is written to stdout in large chunks. When stdout is redirected to a pipe, as is the case with subprocess, this buffering can lead to delays in reading the output.

Solutions

1. Modify C Program Buffering:

To resolve this issue, one can modify the C program to use line-buffered stdout. This ensures that every new line is flushed immediately, eliminating buffer-related delays.

2. Use Pseudo-TTY:

An alternative solution is to use a pseudo-terminal (pty). PTYs provide a pseudo-tty interface to the C program, tricking it into thinking it's running interactively. This naturally enables line buffering of stdout.

3. Use Special Utilities:

Utilities like stdbuf can be used to modify the buffering behavior of subprocesses without modifying the source code. The following Python code demonstrates how to use stdbuf:

import subprocess

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

4. Use pexpect:

Pexpect is a module that simplifies the handling of ptys. It can be used as follows:

import pexpect

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

By implementing these solutions, the Python script will be able to read the output of the C program promptly. This resolves the hanging issue at the "for line in iter" statement.

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