C Standard Library math.h, stdlib.h, time.h: Tutorial, Examples, FAQs & Interview Tips

What is the C Standard Library?

The C Standard Library is a collection of header files and functions defined by the C standard (C89/C99/C11). It provides ready-to-use implementations for I/O, string manipulation, math, memory management, date/time, and more - so you don't have to write them from scratch.

math.h - Mathematical Functions

Provides common math functions. Compile with -lm flag: gcc program.c -lm

math.h Functions

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

int main() {
    printf("sqrt(16)      = %.2f\n", sqrt(16));      // 4.00
    printf("pow(2, 10)    = %.0f\n", pow(2, 10));    // 1024
    printf("fabs(-3.7)    = %.2f\n", fabs(-3.7));    // 3.70
    printf("ceil(4.1)     = %.0f\n", ceil(4.1));     // 5
    printf("floor(4.9)    = %.0f\n", floor(4.9));    // 4
    printf("round(4.5)    = %.0f\n", round(4.5));    // 5
    printf("log(M_E)      = %.2f\n", log(M_E));      // 1.00 (natural log)
    printf("log10(1000)   = %.2f\n", log10(1000));   // 3.00
    printf("sin(M_PI/2)   = %.2f\n", sin(M_PI/2));  // 1.00
    printf("cos(0)        = %.2f\n", cos(0));        // 1.00
    printf("fmod(10, 3)   = %.2f\n", fmod(10, 3));  // 1.00 (float modulo)

    return 0;
}
// Compile: gcc math_demo.c -lm -o math_demo

stdlib.h - General Utilities

stdlib.h - rand, qsort, bsearch, exit

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

// Comparator for qsort (ascending)
int compare(const void *a, const void *b) {
    return (*(int*)a - *(int*)b);
}

int main() {
    // Random numbers
    srand(time(NULL));  // seed with current time
    printf("Random numbers (1-100):\n");
    for (int i = 0; i < 5; i++) {
        printf("  %d\n", rand() % 100 + 1);
    }

    // qsort - sort any array with a comparator
    int arr[] = {64, 25, 12, 22, 11};
    int n = 5;
    qsort(arr, n, sizeof(int), compare);
    printf("\nSorted: ");
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");  // 11 12 22 25 64

    // bsearch - binary search (array must be sorted)
    int key = 22;
    int *found = (int*)bsearch(&key, arr, n, sizeof(int), compare);
    if (found) printf("Found %d at index %ld\n", key, found - arr);
    else       printf("%d not found\n", key);

    // abs, labs, llabs
    printf("\nabs(-42)  = %d\n",  abs(-42));
    printf("labs(-1M) = %ld\n", labs(-1000000L));

    // exit codes
    // exit(EXIT_SUCCESS);  // 0 - normal termination
    // exit(EXIT_FAILURE);  // 1 - error termination

    return 0;
}

time.h - Date and Time

time.h - Current Time and Benchmarking

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

int main() {
    // Current time as timestamp
    time_t now = time(NULL);
    printf("Unix timestamp: %ld\n", (long)now);

    // Convert to human-readable string
    printf("Current time:   %s", ctime(&now));  // includes \n

    // Structured time
    struct tm *t = localtime(&now);
    printf("Year:  %d\n", t->tm_year + 1900);
    printf("Month: %d\n", t->tm_mon + 1);
    printf("Day:   %d\n", t->tm_mday);
    printf("Hour:  %d\n", t->tm_hour);
    printf("Min:   %d\n", t->tm_min);

    // Format date string
    char buffer[80];
    strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", t);
    printf("Formatted: %s\n", buffer);

    // Benchmark: measure execution time
    clock_t start = clock();
    long sum = 0;
    for (long i = 0; i < 100000000L; i++) sum += i;
    clock_t end = clock();

    double elapsed = (double)(end - start) / CLOCKS_PER_SEC;
    printf("\nSum: %ld\n", sum);
    printf("Time: %.4f seconds\n", elapsed);

    return 0;
}

ctype.h - Character Classification

ctype.h - Character Functions

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

int main() {
    char ch = 'A';
    printf("isalpha('%c') = %d\n", ch, isalpha(ch));   // 1 (true)
    printf("isdigit('5') = %d\n",  isdigit('5'));       // 1
    printf("isspace(' ') = %d\n",  isspace(' '));       // 1
    printf("isupper('A') = %d\n",  isupper('A'));       // 1
    printf("islower('a') = %d\n",  islower('a'));       // 1
    printf("isalnum('3') = %d\n",  isalnum('3'));       // 1
    printf("ispunct('!') = %d\n",  ispunct('!'));       // 1

    printf("toupper('a') = %c\n",  toupper('a'));       // A
    printf("tolower('Z') = %c\n",  tolower('Z'));       // z

    // Convert string to uppercase
    char str[] = "Hello, World!";
    for (int i = 0; str[i]; i++) str[i] = toupper(str[i]);
    printf("Uppercase: %s\n", str);  // HELLO, WORLD!

    return 0;
}

Quick Reference - Standard Library Headers

Header	Key Functions
`<stdio.h>`	`printf, scanf, fopen, fclose, fread, fwrite, fgets, fputs, sprintf, sscanf`
`<stdlib.h>`	`malloc, calloc, realloc, free, atoi, atof, rand, srand, qsort, bsearch, exit, abs`
`<string.h>`	`strlen, strcpy, strncpy, strcat, strcmp, strstr, strchr, memcpy, memset, memmove`
`<math.h>`	`sqrt, pow, fabs, ceil, floor, round, sin, cos, tan, log, log10, exp, fmod`
`<time.h>`	`time, clock, difftime, localtime, gmtime, mktime, strftime, ctime`
`<ctype.h>`	`isalpha, isdigit, isspace, isupper, islower, toupper, tolower, isalnum, ispunct`
`<errno.h>`	`errno, perror, strerror`
`<limits.h>`	`INT_MAX, INT_MIN, CHAR_MAX, LONG_MAX, UINT_MAX, LLONG_MAX`
`<stdbool.h>`	`bool, true, false` (C99)
`<stdint.h>`	`int8_t, int16_t, int32_t, int64_t, uint8_t, uint32_t, SIZE_MAX`
`<assert.h>`	`assert(condition)` - aborts if condition is false
`<signal.h>`	`signal, raise, SIGINT, SIGTERM, SIGSEGV`

Previous Next