File Descriptors & ulimit

Also Known As

TL;DR

Explanation

Every open file, socket, pipe, or device is a file descriptor (FD). FD 0 = stdin, 1 = stdout, 2 = stderr. The soft limit (ulimit -n) is the default per-process maximum (typically 1024); the hard limit is the kernel maximum. Symptoms of FD exhaustion: 'Too many open files' errors, inability to accept new connections, failed file opens. For PHP web servers and queue workers: increase limits in /etc/security/limits.conf (system) or in the systemd service file (LimitNOFILE). Monitoring: /proc/PID/fd shows open FDs for a process; lsof -p PID lists them with paths.

Common Misconception

Why It Matters

Common Mistakes

• Not closing file handles in PHP after use — FDs accumulate over time in long-running scripts.
• Low ulimit for high-concurrency services — Nginx default (1024) is insufficient for busy servers.
• Not setting LimitNOFILE in systemd service files — systemd ignores /etc/security/limits.conf for services.
• No monitoring of FD usage — exhaustion typically happens gradually, not suddenly.

Code Examples

// PHP FD leak — file handle not closed:
function processLogs(array $paths): void {
    foreach ($paths as $path) {
        $handle = fopen($path, 'r'); // Opens FD
        while (!feof($handle)) processLine(fgets($handle));
        // Missing: fclose($handle) — FD leaked!
    }
    // 1000 log files = 1000 leaked FDs — ulimit hit
}

// Close handles explicitly:
function processLogs(array $paths): void {
    foreach ($paths as $path) {
        $handle = fopen($path, 'r');
        try {
            while (!feof($handle)) processLine(fgets($handle));
        } finally {
            fclose($handle); // Always closed
        }
    }
}

# Increase FD limit for PHP-FPM in systemd:
# /etc/systemd/system/php8.3-fpm.service.d/override.conf:
[Service]
LimitNOFILE=65536

# Check current FD usage:
lsof -p $(pgrep php-fpm | head -1) | wc -l

References

• ↗ https://www.kernel.org/doc/Documentation/sysctl/fs.txt

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