get_next_line

A C function that reads one line at a time from any file descriptor. Built without the standard library, using only a static variable to maintain state between calls.

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Overview

get_next_line implements a line-by-line reader for file descriptors in C. Each successive call returns the next line from the given fd, including the terminating \n character (except at end of file). It works on regular files, standard input, or any valid file descriptor.

The function is designed to be minimal and efficient: it reads only as much data as necessary on each call, never loading the entire file into memory.

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Key Constraints & Design Highlights

• No standard library — only read, malloc, and free are used
• No global variables — state is maintained through a single static variable
• No lseek() — fully sequential, forward-only reading
• Configurable buffer size — set at compile time via -D BUFFER_SIZE=n, works correctly with any value from 1 to 10000000+
• Bonus: multiple fd support — simultaneous reading from several file descriptors without losing state on any of them

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Prototype

char *get_next_line(int fd);

Returns the next line read from fd, or NULL if there is nothing left to read or an error occurred.

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Compilation

# With a custom buffer size
cc -Wall -Wextra -Werror -D BUFFER_SIZE=42 get_next_line.c get_next_line_utils.c

# With default buffer size (defined in the header)
cc -Wall -Wextra -Werror get_next_line.c get_next_line_utils.c

# Bonus — multiple fd support
cc -Wall -Wextra -Werror -D BUFFER_SIZE=42 get_next_line_bonus.c get_next_line_utils_bonus.c

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Usage

#include "get_next_line.h"
#include <fcntl.h>
#include <stdio.h>

int main(void)
{
    int     fd;
    char    *line;

    fd = open("file.txt", O_RDONLY);
    while ((line = get_next_line(fd)) != NULL)
    {
        printf("%s", line);
        free(line);
    }
    close(fd);
    return 0;
}

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Project Structure

get_next_line/
├── get_next_line.h             # Header — prototype + BUFFER_SIZE fallback
├── get_next_line.c             # Core function
├── get_next_line_utils.c       # Helper functions (strjoin, strchr, strlen...)
├── get_next_line_bonus.h       # Bonus header
├── get_next_line_bonus.c       # Bonus — multiple fd support
├── get_next_line_utils_bonus.c # Bonus helpers
└── README.md

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Algorithm & Design Choices

The implementation relies on a static buffer (leftover) to persist unprocessed data between calls. Here is how a single call unfolds:

1. Check the leftover : if a \n is already present in the data saved from the previous call, extract and return the line immediately without calling read() again.

2. Read in chunks : read() fills a temporary buffer of BUFFER_SIZE bytes. Each chunk is appended to the leftover via a custom strjoin. This continues until a \n is found or EOF is reached.

3. Extract the line : once a newline is found (or EOF), the portion up to and including \n is returned as the line. The remainder is saved back into the static variable for the next call.

4. Cleanup on EOF : when read() returns 0, any remaining content in the leftover is returned as the final line (without \n), then the static is freed and set to NULL.

This approach avoids reading the whole file upfront and works correctly regardless of BUFFER_SIZE, even with pathological values like 1 or 10000000.

Bonus — multiple file descriptors: instead of a single char * static, the bonus uses a char * array indexed by fd (static char *leftover[OPEN_MAX]). Each fd gets its own independent leftover buffer, allowing interleaved reads across multiple descriptors without interference.

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Makefile Rules

Rule	Description
make / make all	Compile mandatory part
make bonus	Compile bonus (multiple fd support)
make clean	Remove object files
make fclean	Remove object files and binary
make re	Full rebuild

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Resources

• read(2) — Linux man page
• Static variables in C — GeeksforGeeks
• File descriptors explained

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This project was created as part of the 42 curriculum.