The C standard library supplies portable building blocks for mathematics, allocation, conversion, character handling, time, sorting, and searching. Learn which header owns each function, what contract the function expects, and which return value or error state must be checked.
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.
Provides common math functions. Compile with -lm flag: gcc program.c -lm
#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#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;
}#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;
}#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;
}| 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 |
The compiler may know the function declaration from math.h, while the linker still needs the math library implementation. On toolchains such as GCC, link with -lm and place it after the object or source inputs when order matters.
atoi cannot report conversion failure or overflow reliably. strtol provides an end pointer and uses errno for range errors, allowing the program to distinguish "123", "123abc", empty input, and values outside the target range. Validate before narrowing the result to int.
rand is a predictable pseudo-random generator intended for simulations and ordinary randomized behavior, not cryptography. Seeding with time does not make it secure. Use the operating system's cryptographic random source or a vetted security library for tokens, keys, reset links, and nonces.
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