Chapter 8: C Library Functions

Level 1 — Acquaintance

The standard library: what C provides out of the box and how Auto wraps it.

No C program works in isolation. The C standard library provides essential functions for I/O, string manipulation, mathematics, memory management, and system interaction. Auto wraps most of these in safer, more ergonomic interfaces while preserving the underlying C implementation.

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8.1 General Properties

The C standard library is organized into headers, each declaring a set of related functions, types, and macros:

Header	Purpose	Key functions
<stdio.h>	Input/output	printf, scanf, fopen
<stdlib.h>	General utilities	malloc, free, exit, abs
<string.h>	String handling	strlen, strcmp, strcpy
<math.h>	Mathematics	sqrt, sin, pow, ceil
<time.h>	Date and time	time, clock, strftime
<ctype.h>	Character classification	isdigit, toupper
<assert.h>	Diagnostics	assert
<errno.h>	Error indicators	errno, strerror
<stddef.h>	Common definitions	size_t, NULL, offsetof

To use a library function, you #include its header:

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

Auto implicitly includes the transpiled equivalents. You do not write #include directly — the a2c transpiler adds the necessary headers based on which functions your code uses.

Error handling with errno: Many C library functions report errors by setting the global variable errno and returning a sentinel value (often NULL or -1).

// C: checking errno
errno = 0;
double result = sqrt(-1.0);
if (errno != 0) {
    fprintf(stderr, "Error: %s\n", strerror(errno));
}

Auto wraps these patterns into return types that force error checking (such as optional types ?T or result types Result<T, E>), eliminating the risk of forgetting to check errno.

Takeaway: In C, always check return values and errno for library calls. In Auto, the type system forces you to handle errors.

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8.2 Integer Arithmetic

The <stdlib.h> header provides integer arithmetic utilities:

// C
int absolute = abs(-42);          // 42
div_t result = div(17, 5);        // result.quot = 3, result.rem = 2

Auto provides these through built-in operators and library functions:

let quotient int = 17 / 5        // 3
let remainder int = 17 % 5       // 2
let absolute int = abs(-42)      // 42