C Standard Library

What You’ll Learn

The C standard library is a toolbox of headers shipped with every C compiler—<stdio.h> for printing, <string.h> for text, <stdlib.h> for memory, and more. This page is your central hub: understand how #include works, browse all 19 header tutorials, and try examples that combine the most common libraries.

Definition and Usage

A header file (.h) declares functions, types, and macros. The library implementation (linked automatically on most systems) provides the machine code. You write #include <stdio.h> so the compiler knows printf exists; the linker attaches the real printf from the C runtime.

The C standard defines many headers. This site documents 19 of the most-used ones—each with its own tutorial (syntax, five examples, FAQs). Start here for the big picture, then dive into individual headers from the index below.

📝 Syntax

Include standard headers with angle brackets at the top of your source file:

#include <stdio.h>   /* printf, scanf, fopen */
#include <stdlib.h>  /* malloc, exit, atoi */
#include <string.h>  /* strlen, strcpy, strcmp */

Most common headers for beginners

• <stdio.h> — input and output (printf, fgets, files)
• <stdlib.h> — memory, conversion, exit, rand
• <string.h> — C strings (strlen, strcmp, strcpy)
• <ctype.h> — test characters (isdigit, tolower)
• <math.h> — sqrt, sin, pow (often needs -lm)

Compile and link

gcc main.c -std=c11 -o main
gcc main.c -std=c11 -o main -lm    /* if you use math.h */

Examples Gallery

Five small programs that combine the most common standard library headers. Compile with gcc file.c -std=c11 -o out.

#include <stdio.h>

int main(void) {
    printf("Hello from the C standard library!\n");
    printf("stdio.h provides printf, scanf, and file I/O.\n");
    return 0;
}

Hello from the C standard library!
stdio.h provides printf, scanf, and file I/O.

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

int main(void) {
    char part1[32] = "codetofun";
    char part2[] = ".com";
    char domain[48];

    strcpy(domain, part1);

    if (strcmp(part1, part2) == 0) {
        printf("Strings are equal.\n");
    } else {
        printf("'%s' and '%s' differ.\n", part1, part2);
    }

    strcat(domain, part2);
    printf("Full domain: %s\n", domain);
    printf("Length: %zu\n", strlen(domain));

    return 0;
}

'codetofun' and '.com' differ.
Full domain: codetofun.com
Length: 13

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

int main(void) {
    size_t n = 5;
    size_t i;
    int *nums = malloc(n * sizeof *nums);

    if (nums == NULL) {
        fprintf(stderr, "malloc failed\n");
        return 1;
    }

    for (i = 0; i < n; i++) {
        nums[i] = (int)(i * 10);
    }

    printf("Dynamic array: ");
    for (i = 0; i < n; i++) {
        printf("%d ", nums[i]);
    }
    printf("\n");

    free(nums);
    return 0;
}

Dynamic array: 0 10 20 30 40

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

int main(void) {
    const char *text = "Room 42, Building 7";
    size_t i;
    size_t digits = 0;

    for (i = 0; i < strlen(text); i++) {
        if (isdigit((unsigned char)text[i])) {
            digits++;
        }
    }

    printf("Text: %s\n", text);
    printf("Digit characters: %zu\n", digits);

    return 0;
}

Text: Room 42, Building 7
Digit characters: 3

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

int main(void) {
    char buffer[64];
    char message[] = "User logged in";
    time_t now = time(NULL);
    struct tm *t = localtime(&now);

    if (t == NULL) {
        return 1;
    }

    strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", t);
    printf("[%s] %s (len %zu)\n", buffer, message, strlen(message));

    return 0;
}

[2026-07-05 17:00:00] User logged in (len 14)

🚀 Common Use Cases

• Console apps — stdio.h for user interaction.
• File tools — fopen, fread, fprintf.
• Text processing — string.h for parsing and building strings.
• Dynamic data — stdlib.h malloc/realloc.
• Math & games — math.h, stdlib.h rand.
• International apps — wchar.h, wctype.h, locale.h.

📝 Notes

• Angle brackets <> for standard headers; quotes for your own .h files.
• Not every function lives where you expect—abs is in stdlib.h, not math.h.
• Check return values: malloc → NULL, fopen → NULL, scanf → item count.
• errno from errno.h after failed system/library calls; use perror from stdio.h.
• C99+ features (stdbool.h, stdint.h) need -std=c99 or later.
• POSIX adds extra headers (not covered here)—stick to standard C for portability.

🚀 Usage Tips

• Include headers explicitly — do not rely on transitive includes.
• Read each tutorial — every header page has five examples and FAQs.
• Handle errors — check return values; use errno and perror.
• Link math — gcc ... -lm when using math.h.
• Learn the sidebar order — tutorials chain from assert.h to wctype.h.

💡 Best Practices