01_cli.c

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

int main(int argc, char* argv[]) {
    printf("argc: %d\n", argc);
    for (int i = 0; i < argc; i++) {
        printf("argv[%d]: %s\n", i, argv[i]);
    }
    printf("\n");
    
    if (argc < 3) {
        printf("At least two arguments must be provided\n");
        return 2;
    } else if (strcmp(argv[1], "print") == 0) {
        printf("%s\n", argv[2]);
    } else if (strcmp(argv[1], "sum") == 0) {
        if (argc < 4) {
            printf("sum: at least two arguments must be provided\n");
            return 2;
        }
        int x, y;
        if (sscanf(argv[2], "%d", &x) != 1) {
            printf("sum: %s: not a number\n", argv[2]);
            return 2;
        }
        if (sscanf(argv[3], "%d", &y) != 1) {
            printf("sum: %s: not a number\n", argv[3]);
            return 2;
        }
        printf("%d\n", x + y);
    } else {
        printf("Unknown command: %s\n", argv[1]);
        return 2;
    }
    
    return 0;
}

02_fptr.c

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

const void* min(const void* a, const void* b, int (*compare)(const void*, const void*)) {
    if (compare(a, b) < 0) {
        return a;
    } else {
        return b;
    }
}

int int_compare(const void* x, const void* y) {
    if (*(const int*)x < *(const int*)y)
        return -1;
    else if (*(const int*)x == *(const int*)y)
        return 0;
    return 1;
}

int string_compare(const void* x, const void* y) {
    return strcmp((const char*)x, (const char*)y);
}

int main() {
    int x = 239, y = 179;
    printf("%d\n", *(const int*)min(&x, &y, int_compare));
    
    const char* a = "abac";
    const char* b = "abacaba";
    printf("%s\n", (const char*)min(a, b, string_compare));
    
    return 0;
}

03_file.c

#include <stdio.h>
#include <stddef.h>

int main() {
    int retval = 0;
    
    FILE* in = fopen("input.txt", "r");
    
    if (in == NULL || ferror(in)) {
        printf("Failed to open input file\n");
        retval =  1;
        goto ret;
    }
    
    FILE* out = fopen("output.txt", "w");
    
    if (out == NULL || ferror(out)) {
        printf("Failed to open output file\n");
        retval = 1;
        goto close_in;
    }
    
    int x;
    while (fscanf(in, "%d", &x) == 1) {
        fprintf(out, "%d\n", x + 1);
    }
    if (!feof(in)) {
        printf("File format error\n");
        retval = 1;
        goto close_out;
    }
    
  close_out:
    fclose(out);
  close_in:
    fclose(in);
  ret:
    return retval;
}

04_file.c

#include <stdio.h>
#include <stddef.h>

int main() {
    int retval = 0;
    
    FILE* in = fopen("input.bin", "rb");
    
    if (in == NULL || ferror(in)) {
        printf("Failed to open input file\n");
        retval =  1;
        goto ret;
    }
    
    FILE* out = fopen("output.bin", "wb");
    
    if (out == NULL || ferror(out)) {
        printf("Failed to open output file\n");
        retval = 1;
        goto close_in;
    }
    
    unsigned char buf[5];
    size_t read;
    while ((read = fread(buf, 1, 5, in)) > 0) {
        for (int i = 0; i < 5; i++)
            buf[i]++;
        if (fwrite(buf, 1, read, out) != read) {
            printf("File write error\n");
            retval = 1;
            goto close_out;
        }
    }
    if (ferror(in)) {
        printf("File read error\n");
        retval = 1;
        goto close_out;
    }
    
  close_out:
    fclose(out);
  close_in:
    fclose(in);
  ret:
    return retval;
}