Why Does My Python `subprocess` Hang When Reading Output from a C Program Using `for line in iter(process.stdout.readline, \'\')`?

Python subprocess hangs with "for line in iter"

In this scenario, the Python script is intended to execute a C program and receive its output. However, the script hangs at the line "for line in iter(process.stdout.readline, '')".

The Buffering Issue

The problem arises from a buffering issue. In C programs, stdout is typically block buffered when output is piped. This means that the output is buffered until a certain amount is written or a newline is encountered.

In the C program, each iteration of the while loop prints "2000" and sleeps for a second. Since no newline is printed, the output is buffered.

For Line in Iter Issue

In Python, subprocess.stdout.readline() reads the stdout of the child process and returns a line. The "for line in iter()" loop iterates over the lines returned by readline().

However, since the C program's output is not being flushed due to buffering, there are no lines to read at the point where the Python script enters the loop. Hence, the script waits indefinitely at this line.

Solutions

Fix Buffering in C Program

• setvbuf(): The C program can use setvbuf() to force line buffering for stdout. This will ensure that each line is flushed immediately after printing.

setvbuf(stdout, (char *) NULL, _IOLBF, 0);

• stdbuf Utility: The stdbuf utility can be used to wrap the C program and change its buffering behavior.

stdbuf -oL ./main

Pseudo-TTY

• Pty Module: The pty module can be used to create a pseudo-TTY that tricks the C program into thinking it's running interactively. In this mode, output will be line buffered by default.

import pty
master_fd, slave_fd = pty.openpty()
process = Popen("./main", stdin=slave_fd, stdout=slave_fd, stderr=STDOUT,
                bufsize=0, close_fds=True)

• pexpect Library: The pexpect library provides a higher-level interface for working with ptys.

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

By applying one of these solutions, the buffering issue will be resolved, allowing the Python script to read the output of the C program as expected.

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