How I Built a Simple Shell in C – A Beginner's Guide to System Programming (1/3)

Part 1: Reading User Input in a Custom Shell (C Programming) In this part of my custom shell project, I will explain how to read user input dynamically in C. Handling user input properly is crucial in a shell environment, as commands can vary in length. Instead of using fixed-size buffers, I implemented a dynamic memory allocation approach for better flexibility. Understanding the read_command Function The function read_command() is responsible for: ✅ Reading input from the user dynamically ✅ Handling memory allocation & reallocation to avoid buffer overflow ✅ Ensuring proper termination of the input string Breaking Down the Code #ifndef READ_COMMAND_H #define READ_COMMAND_H #include #include #include #define INT_BUFFER_SIZE 32 // Initial buffer size char read_command(); #endif Header Guard (#ifndef READ_COMMAND_H): Prevents multiple inclusions. Constant Definition (INT_BUFFER_SIZE): Sets an initial buffer size for input storage. Implementation of read_command() char read_command() { char command = malloc(INT_BUFFER_SIZE sizeof(char)); if (!command) { perror("Memory allocation failed"); exit(EXIT_FAILURE); } Memory Allocation (malloc): Initially allocates INT_BUFFER_SIZE bytes for storing user input. Error Handling: If malloc fails, the program prints an error and exits. int size = INT_BUFFER_SIZE; int length = 0; int c; size: Keeps track of the current buffer size. length: Tracks the actual number of characters read. c: Stores the input character retrieved from getchar(). Handling Dynamic Input Growth while ((c = getchar()) != '\n' && c != EOF) { if (length >= (size - 1)) { size = 2; // Double the buffer size when needed char new_command = realloc(command, (size + 1)); if (!new_command) { free(command); perror("Memory reallocation failed"); exit(EXIT_FAILURE); } command = new_command; } command[length++] = c; } Expanding Memory Dynamically (realloc) If input exceeds the allocated size, the buffer is doubled (size *= 2). realloc attempts to resize the buffer; if it fails, memory is freed, and an error is displayed. Storing Characters Each character from getchar() is stored sequentially in command[length++] Finalizing the Input command[length] = '\0'; // Null-terminate the string return command; Null-Termination (\0): Ensures that the string is properly terminated so it can be processed as a valid C string. Returning the Input: The function returns the dynamically allocated string for further use. Why This Approach?

Feb 15, 2025 - 00:15

How I Built a Simple Shell in C – A Beginner's Guide to System Programming (1/3)

Part 1: Reading User Input in a Custom Shell (C Programming)

In this part of my custom shell project, I will explain how to read user input dynamically in C. Handling user input properly is crucial in a shell environment, as commands can vary in length. Instead of using fixed-size buffers, I implemented a dynamic memory allocation approach for better flexibility.

Understanding the read_command Function

The function read_command() is responsible for:
✅ Reading input from the user dynamically
✅ Handling memory allocation & reallocation to avoid buffer overflow
✅ Ensuring proper termination of the input string

Breaking Down the Code

#ifndef READ_COMMAND_H
#define READ_COMMAND_H

#include 
#include 
#include 

#define INT_BUFFER_SIZE 32  // Initial buffer size

char *read_command();

#endif

Header Guard (#ifndef READ_COMMAND_H): Prevents multiple inclusions.
Constant Definition (INT_BUFFER_SIZE): Sets an initial buffer size for input storage.

Implementation of read_command()

char *read_command()
{
    char *command = malloc(INT_BUFFER_SIZE * sizeof(char));
    if (!command)
    {
        perror("Memory allocation failed");
        exit(EXIT_FAILURE);
    }

Memory Allocation (malloc): Initially allocates INT_BUFFER_SIZE bytes for storing user input.
Error Handling: If malloc fails, the program prints an error and exits.

int size = INT_BUFFER_SIZE;
    int length = 0;
    int c;

size: Keeps track of the current buffer size.
length: Tracks the actual number of characters read.
c: Stores the input character retrieved from getchar().

Handling Dynamic Input Growth

while ((c = getchar()) != '\n' && c != EOF)
{
    if (length >= (size - 1))
    {
        size *= 2;  // Double the buffer size when needed
        char *new_command = realloc(command, (size + 1));
        if (!new_command)
        {
            free(command);
            perror("Memory reallocation failed");
            exit(EXIT_FAILURE);
        }
        command = new_command;
    }
    command[length++] = c;
}

Expanding Memory Dynamically (realloc)
- If input exceeds the allocated size, the buffer is doubled (size *= 2).
- realloc attempts to resize the buffer; if it fails, memory is freed, and an error is displayed.
Storing Characters
- Each character from getchar() is stored sequentially in command[length++]

Finalizing the Input

command[length] = '\0';  // Null-terminate the string
return command;

Null-Termination (\0): Ensures that the string is properly terminated so it can be processed as a valid C string.
**Returning the Input: **The function returns the dynamically allocated string for further use.