Introduction

Reading a file line by line efficiently is less about finding one magical API and more about choosing a streaming API that matches the platform. The fastest reliable approach usually avoids loading the whole file into memory, keeps buffering in place, and works with the runtime's native text-decoding model instead of fighting it.

General Rule: Stream, Do Not Slurp

For large files, the first performance decision is simple: avoid readAll, readlines, or similar whole-file helpers unless the file is genuinely small and you need random access to all lines.

Streaming matters because it keeps memory usage flat. It also lets the runtime buffer reads from disk efficiently instead of allocating a giant string or array up front.

The best line-by-line API is usually the one the platform already optimizes internally.

POSIX C: Use getline

On POSIX systems, getline is a strong default because it manages buffer growth for you and reads from a buffered FILE* stream.

1#include <stdio.h>
2#include <stdlib.h>
3
4int main(void) {
5    FILE *fp = fopen("app.log", "r");
6    if (!fp) {
7        perror("fopen");
8        return 1;
9    }
10
11    char *line = NULL;
12    size_t cap = 0;
13    ssize_t len;
14
15    while ((len = getline(&line, &cap, fp)) != -1) {
16        printf("Read %zd bytes: %s", len, line);
17    }
18
19    free(line);
20    fclose(fp);
21    return 0;
22}

This is usually better than manually growing a buffer with repeated fgets calls when line lengths are unpredictable. The standard I/O layer already buffers disk reads, so you get good baseline performance without writing custom low-level code.

Python: Iterate the File Object

In Python, the built-in file iterator is already buffered and is the idiomatic solution:

with open("app.log", "r", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip("\n"))

This is both readable and efficient. Under the hood, Python reads buffered chunks and yields lines one at a time.

If you need raw bytes because decoding is expensive or the file is not valid text, open in binary mode instead:

with open("app.log", "rb") as f:
    for line in f:
        process_bytes(line)

That avoids text decoding entirely and can be the correct choice for protocol dumps or mixed-encoding data.

.NET: File.ReadLines or StreamReader

In .NET, File.ReadLines is often the best high-level choice because it streams lazily instead of loading the whole file:

1using System;
2using System.IO;
3
4foreach (var line in File.ReadLines("app.log"))
5{
6    Console.WriteLine(line);
7}

If you need more control over encoding or reader lifetime, use StreamReader directly:

1using var reader = new StreamReader("app.log");
2string? line;
3
4while ((line = reader.ReadLine()) != null)
5{
6    Console.WriteLine(line);
7}

File.ReadAllLines is easy to write, but it is the wrong choice for large files because it materializes the full content in memory.

Efficiency Is Also About Encoding and Work Per Line

The line-reading API is only part of the performance story. If you parse JSON on every line, split multiple times, or allocate many temporary strings, that work may dominate I/O costs.

A few practical rules help:

• choose the correct encoding up front
• avoid unnecessary copies of each line
• do the minimum parsing needed inside the read loop
• measure before replacing buffered APIs with lower-level code

In many programs, the bottleneck is not "reading lines" but the processing done after the line arrives.

Common Pitfalls

The biggest mistake is using whole-file APIs for large inputs and then describing the problem as a line-reading performance issue. Memory pressure is often the real cause.

Another common issue is ignoring encoding. A fast reader configured with the wrong encoding will still produce bad data or extra recovery work.

It is also easy to overengineer. Platform-provided buffered line readers are usually the right default, and custom byte-level parsers only pay off when profiling proves they are necessary.

Summary

• The most efficient line-by-line approach is usually a buffered streaming API native to the platform.
• On POSIX C, getline is a strong default for variable-length lines.
• In Python, iterate the file object directly.
• In .NET, prefer File.ReadLines or StreamReader over whole-file helpers.
• Measure the full pipeline, because per-line processing often costs more than the I/O itself.