This is a C program designed to process binary data...

July 1, 2025 at 05:57 PM

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <stdint.h>
#include <string.h>

#ifdef _WIN32
#include <io.h>
#include <fcntl.h>
#endif

const size_t BUFFER_SIZE = 1024 * 1024;

int main(int argc, const char **argv) {
    if (argc < 2) {
        printf("usage: %s [padding amount]", argv[0]);
        exit(EXIT_FAILURE);
    }

    setvbuf(stdout, NULL, _IONBF, 0);

    #ifdef _WIN32
    _setmode(_fileno(stdin), _O_BINARY);
    _setmode(_fileno(stdout), _O_BINARY);
    #endif

    char *buffer = malloc(BUFFER_SIZE);
    if (buffer == NULL) {
        exit(EXIT_FAILURE);
    }

    size_t total_bytes_read = 0;
    
    unsigned long long padding_amount = strtoull(argv[1], NULL, 10);
    if (padding_amount > SIZE_MAX || padding_amount < 1) {
        exit(EXIT_FAILURE);
    }

    while (1) {
        size_t bytes_read = fread(buffer, 1, BUFFER_SIZE, stdin);
        total_bytes_read += bytes_read;

        if (bytes_read < BUFFER_SIZE && ferror(stdin)) {
            exit(EXIT_FAILURE);
        }


        if (fwrite(buffer, 1, bytes_read, stdout) < bytes_read) {
            exit(EXIT_FAILURE);
        }

        if (bytes_read < BUFFER_SIZE) {
            size_t remainder = total_bytes_read % padding_amount;
            if (remainder != 0) {
                size_t padding_bytes_count = padding_amount - (total_bytes_read % padding_amount);
                char *padding_bytes = malloc(padding_bytes_count);
                if (padding_bytes == NULL) {
                    exit(EXIT_FAILURE);
                }
                memset(padding_bytes, 0, padding_bytes_count);
                if (fwrite(padding_bytes, 1, padding_bytes_count, stdout) < padding_bytes_count) {
                    exit(EXIT_FAILURE);
                }
            }

            exit(EXIT_SUCCESS);
        }
    }
}

This is a C program designed to process binary data from stdin and write it to stdout, ensuring that the total number of bytes written to stdout is aligned to a specific byte size specified by the user (padding amount). Here's what the code does in detail:

Key Functionality:

Usage & Arguments:
- The program expects a single command-line argument: the padding amount, which is the number of bytes to align the output data to.
- If no argument is provided, it prints a usage message and exits.
Buffer Setup:
- The program uses a constant-sized buffer (BUFFER_SIZE = 1 MB) for reading and writing chunks of data from stdin to stdout.
Binary Mode (for Windows):
- On Windows platforms, the code sets the stdin and stdout streams to binary mode (_O_BINARY) to avoid newline translation issues.
Padding Calculation:
- The program ensures that the total amount of data written to stdout is a multiple of the padding amount specified as the command-line argument.
- If the total bytes read from stdin are not divisible by padding_amount, additional 0x00 bytes (padding bytes) are appended to the output until alignment is achieved.
Detailed Steps:
- Continuously read data from stdin into the buffer (fread).
- Write the read data to stdout (fwrite).
- Maintain a total count of all bytes read (total_bytes_read).
- When the end of the input is reached (fread returns fewer bytes than requested and doesn't set an error), compute the number of padding bytes needed to make total_bytes_read a multiple of padding_amount.
- Allocate a temporary buffer for padding bytes, initialize them to 0 using memset, and write them to stdout.
- Exit the program after successfully reading, writing, and applying padding.
Error Handling:
- Exits if memory allocation fails (malloc for the buffer or padding bytes).
- Exits if reading from stdin encounters an error (checked via ferror).
- Exits if writing to stdout fails.
Cross-Platform Behavior:
- On non-Windows systems, the program assumes no need for binary mode configuration and skips the _setmode calls.

Primary Use Case:

This program is likely intended for use in binary file processing, where the data must be padded to ensure block alignment. For example:

Writing binary files for systems requiring fixed-size blocks.
Preparing data for transmission or storage where padding to specific boundaries is necessary.

Example Usage:

Command: program_name 512 < input.bin > output.bin
- Reads from input.bin, writes to output.bin, ensuring the total size is a multiple of 512 bytes.
- If input.bin contains 1000 bytes, the program pads 24 additional 0x00 bytes to make the total 1024 (nearest multiple of 512).

Edge Cases and Considerations:

The program ensures padding_amount is greater than 0 and less than or equal to SIZE_MAX.
It allocates memory for both the data buffer and padding buffer, which could fail on very constrained systems or if padding_amount is large.

This program is a robust utility tool for managing binary data with byte alignment requirements.

Generate your own explanations

Download our vscode extension

Read other generated explanations

Built by @thebuilderjr