课堂练习2

一、xxd的使用

二、

xxd的主要功能是将文件转化为十六进制显示，通过man xxd可以得到，需要使用unistd.h实现

三、伪代码

    openfile
    readbyte
    0x
    new text
    print 0x

四、

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

void print_hex_dump(FILE *file, long offset, int length) {
    fseek(file, offset, SEEK_SET);

    unsigned char buffer[length];
    size_t bytesRead = fread(buffer, 1, length, file);

    if (bytesRead == 0) {
        printf("Error reading file.\n");
        return;
    }

    printf("File Length: %ld bytes\n", bytesRead);

    for (size_t i = 0; i < bytesRead; i++) {
        printf("%02X ", buffer[i]);
    }
    printf("\n");
}

int main(int argc, char *argv[]) {
    if (argc != 4) {
        printf("Usage: %s <option> <n> <filename>\n", argv[0]);
        printf("Options:\n");
        printf("  -h n  Display the first n bytes of the file.\n");
        printf("  -t n  Display the last n bytes of the file.\n");
        return 1;
    }

    int n = atoi(argv[2]);
    if (n <= 0) {
        printf("Invalid value for n. Please provide a positive integer.\n");
        return 1;
    }

    char *filename = argv[3];
    FILE *file = fopen(filename, "rb");

    if (!file) {
        printf("Error opening file: %s\n", filename);
        return 1;
    }

    if (strcmp(argv[1], "-h") == 0) {
        print_hex_dump(file, 0, n);
    } else if (strcmp(argv[1], "-t") == 0) {
        fseek(file, 0, SEEK_END);
        long fileLength = ftell(file);
        long offset = (fileLength - n) >= 0 ? (fileLength - n) : 0;
        print_hex_dump(file, offset, n);
    } else {
        printf("Invalid option: %s\n", argv[1]);
        fclose(file);
        return 1;
    }

    fclose(file);
    return 0;
}