C cheat sheet

#include <stdio.h>
#include <stdlib.h>
 
typedef struct {
    int *items;
    size_t len;
    size_t cap;
} IntList;
 
// Returns false when memory runs out, and leaves the list as it was.
bool list_push(IntList *list, int value)
{
    if (list->len == list->cap) {
        size_t new_cap = list->cap ? list->cap * 2 : 8;
        int *grown = realloc(list->items, new_cap * sizeof *grown);
        if (grown == nullptr) {
            return false;
        }
        list->items = grown;
        list->cap = new_cap;
    }
    list->items[list->len++] = value;
    return true;
}
 
void list_free(IntList *list)
{
    free(list->items);
    *list = (IntList){};
}
 
int compare_ints(const void *a, const void *b)
{
    int x = *(const int *)a;
    int y = *(const int *)b;
    return (x > y) - (x < y);  // not x - y, which can overflow
}
 
int main(void)
{
    IntList scores = {};  // C23: empty braces set every member to zero
 
    int input[] = {42, 7, 19, 88, 3};
    for (size_t i = 0; i < sizeof input / sizeof input[0]; i++) {
        if (!list_push(&scores, input[i])) {
            fprintf(stderr, "out of memory\n");
            list_free(&scores);
            return EXIT_FAILURE;
        }
    }
 
    qsort(scores.items, scores.len, sizeof scores.items[0], compare_ints);
 
    for (size_t i = 0; i < scores.len; i++) {
        printf("%d ", scores.items[i]);
    }
    printf("\n");  // 3 7 19 42 88
 
    list_free(&scores);
    return EXIT_SUCCESS;
}

gcc -std=c23 -Wall -Wextra -g -fsanitize=address,undefined scores.c -o scores
./scores

#include <stdio.h>
 
void swap(int *a, int *b)
{
    int tmp = *a;
    *a = *b;
    *b = tmp;
}
 
int sum(const int *nums, size_t count)
{
    int total = 0;
    for (size_t i = 0; i < count; i++) {
        total += nums[i];  // same as *(nums + i)
    }
    return total;
}
 
int main(void)
{
    int x = 1;
    int y = 2;
    swap(&x, &y);
    printf("%d %d\n", x, y);  // 2 1
 
    int nums[] = {10, 20, 30};
    size_t count = sizeof nums / sizeof nums[0];  // 3, only works on the array itself
    printf("%d\n", sum(nums, count));  // 60
    return 0;
}

// point.h
#ifndef POINT_H
#define POINT_H
 
typedef struct {
    double x;
    double y;
} Point;
 
double point_distance(Point a, Point b);
 
#endif

// point.c
#include <math.h>
#include "point.h"
 
double point_distance(Point a, Point b)
{
    return hypot(b.x - a.x, b.y - a.y);
}

// main.c
#include <stdio.h>
#include "point.h"
 
int main(void)
{
    Point a = {.x = 0, .y = 0};
    Point b = {.x = 3, .y = 4};
 
    printf("%.1f\n", point_distance(a, b));  // 5.0
    return 0;
}

gcc -Wall -Wextra -c point.c
gcc -Wall -Wextra -c main.c
gcc main.o point.o -o app -lm
./app

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
int main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "usage: %s FILE\n", argv[0]);
        return EXIT_FAILURE;
    }
 
    FILE *f = fopen(argv[1], "r");
    if (f == nullptr) {
        fprintf(stderr, "%s: %s\n", argv[1], strerror(errno));
        return EXIT_FAILURE;
    }
 
    char line[256];
    int number = 0;
    while (fgets(line, sizeof line, f) != nullptr) {
        line[strcspn(line, "\n")] = '\0';  // fgets keeps the newline
        printf("%4d  %s\n", ++number, line);
    }
 
    if (ferror(f)) {
        perror(argv[1]);
    }
    fclose(f);
    return EXIT_SUCCESS;
}