Chapter 18: Standard Library Reference

The C Standard Library is a collection of headers and functions that provide essential services like memory management, mathematical calculations, and string manipulation. Mastering these headers is key to writing idiomatic and efficient C code.

I. Core Runtime Headers

1. <stdio.h> (Input and Output)

The most used header. Provides functions for reading from and writing to files and standard streams.

• printf, scanf, fopen, fclose, fgets, fputs.

2. <stdlib.h> (General Utilities)

Provides memory management, process control, and conversions.

• malloc, free, exit, rand, srand, abs, atoi (string to int).

3. <string.h> (String Manipulation)

Essential for handling C-style (null-terminated) strings.

• strlen, strcpy, strcat, strcmp, memset (set memory), memcpy (copy memory).

II. System and Type Headers

4. <stdint.h> (Fixed-width Integers)

Crucial for portable code. Defines types with exact bit widths.

• int8_t, uint32_t, intptr_t.

5. <stddef.h> (Standard Definitions)

Defines common types and macros used throughout the language.

• size_t (unsigned integer for sizes), NULL, offsetof (offset of a member in a struct).

6. <stdbool.h> (Boolean Type)

Introduced in C99. Defines bool, true (1), and false (0).

III. Mathematical and Diagnostic Headers

7. <math.h> (Common Mathematics)

Advanced math operations. Needs to be linked with -lm.

• sqrt, pow, log, sin, cos.

8. <errno.h> (Error Numbers)

Defines the errno global variable and various error codes (e.g., EACCES, ENOMEM) set by system calls.

9. <assert.h> (Diagnostics)

Provides the assert() macro for verifying assumptions during debugging.

• assert(x > 0); (Aborts the program if x is not greater than 0).

IV. The time.h Library

Used for handling date and time.

• time(): Returns the number of seconds since the Unix Epoch (Jan 1, 1970).
• localtime(): Converts a time_t to a struct containing hours, minutes, etc.

V. Advanced: <setjmp.h> and <signal.h>

• setjmp.h: Provides a way to perform "non-local jumps"—effectively an early version of exception handling.
• signal.h: Allows your program to handle OS-level signals (like SIGINT when the user presses Ctrl+C).

VI. Implementation Variations (glibc vs. musl)

While the headers are standard, their underlying implementation can vary. glibc (GNU C Library) is the standard for most Linux distributions, while musl is a lightweight alternative often used in Docker images and embedded systems.