C Header Files: Complete Guide to stdio.h, stdlib.h, string.h, math.h & More

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "w");
    if (!fp) { perror("fopen"); return 1; }

    fprintf(fp, "Line 1: score = %d\n", 95);
    fprintf(fp, "Line 2: name = %s\n", "Alice");
    fclose(fp);

    printf("File written successfully.\n");
    return 0;
}

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

int main() {
    srand((unsigned)time(NULL));   // seed once

    int *arr = malloc(5 * sizeof(int));
    if (!arr) { perror("malloc"); return 1; }

    for (int i = 0; i < 5; i++) {
        arr[i] = rand() % 100;       // 0–99
        printf("%d ", arr[i]);
    }
    printf("\n");
    free(arr);
    return 0;
}

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

int main() {
    char s1[50] = "Hello";
    const char *s2 = "World";

    printf("Length of s1: %zu\n", strlen(s1));   // 5
    strncat(s1, ", ", sizeof(s1) - strlen(s1) - 1);
    strncat(s1, s2, sizeof(s1) - strlen(s1) - 1);
    printf("%s\n", s1);                          // Hello, World

    if (strcmp(s1, "Hello, World") == 0) {
        printf("Strings are equal\n");
    }

    char *pos = strstr(s1, "World");
    if (pos) printf("Found at index: %td\n", pos - s1);
    return 0;
}

#include <stdio.h>
#include <math.h>

#define PI  3.14159265358979

int main() {
    double r = 5.0;
    printf("Area of circle:   %.4f\n", PI * pow(r, 2));
    printf("Hypotenuse (3,4): %.4f\n", hypot(3.0, 4.0));
    printf("log2(1024):       %.1f\n",  log2(1024.0));
    return 0;
}
/* Compile: gcc program.c -o program -lm */

#include <stdio.h>
#include <time.h>

int main() {
    /* Print current date/time */
    time_t now = time(NULL);
    struct tm *t = localtime(&now);
    char buf[64];
    strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", t);
    printf("Now: %s\n", buf);

    /* Time a computation */
    clock_t start = clock();
    long sum = 0;
    for (long i = 0; i < 100000000L; i++) sum += i;
    clock_t end = clock();

    printf("Elapsed: %.3f s\n",
           (double)(end - start) / CLOCKS_PER_SEC);
    return 0;
}

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

int main() {
    const char *s = "Hello, World! 42";
    int letters = 0, digits = 0, spaces = 0;

    for (int i = 0; s[i]; i++) {
        if (isalpha((unsigned char)s[i])) letters++;
        else if (isdigit((unsigned char)s[i])) digits++;
        else if (isspace((unsigned char)s[i])) spaces++;
    }
    printf("Letters: %d  Digits: %d  Spaces: %d\n",
           letters, digits, spaces);
    return 0;
}