# <stdlib.h>

**[<stdlib.h>](https://en.cppreference.com/w/c/header/stdlib)** covers memory allocation (`malloc`, `calloc`, `free`), type conversion (`atoi`, `strtol`), sorting/searching (`qsort`, `bsearch`), and process control (`exit`, `abort`). It's the "junk drawer" of the C standard library.

Always check `malloc` for NULL; always check `qsort` comparators carefully for overflow.

## Example

This example sorts integers with qsort and binary-searches the result.

```c
// compile: gcc -o stdlibexample stdlibexample.c
// run: ./stdlibexample
// description: sort array with qsort, then search it with bsearch

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

int cmp_int(const void* a, const void* b) {
    return *(int*)a - *(int*)b;
}

int main() {
    int arr[] = {5, 2, 8, 1, 9, 3};
    int n = sizeof(arr) / sizeof(arr[0]);
    
    qsort(arr, n, sizeof(int), cmp_int);
    
    printf("sorted: ");
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");
    
    int key = 8;
    int* found = bsearch(&key, arr, n, sizeof(int), cmp_int);
    printf("search for 8: %s\n", found ? "found" : "not found");
    
    return 0;
}
```

## Common functions

**Memory allocation**:
- `malloc(size)`: allocate memory (returns NULL on failure)
- `calloc(count, size)`: allocate zeroed memory
- `realloc(ptr, size)`: resize allocated memory (can move pointer)
- `free(ptr)`: deallocate memory (safe to free NULL)

**String to number conversion**:
- `atoi(str)`: convert string to int (returns 0 on error, no error reporting)
- `atol(str)`: convert string to long
- `atoll(str)`: convert string to long long (C99)
- `strtol(str, &endptr, base)`: convert to long (safer: reports errors, sets endptr)
- `strtoll(str, &endptr, base)`: convert to long long
- `strtoul(str, &endptr, base)`: convert to unsigned long
- `strtoull(str, &endptr, base)`: convert to unsigned long long
- `strtof(str, &endptr)`, `strtod(str, &endptr)`, `strtold(str, &endptr)`: floating-point

**Sorting and searching**:
- `qsort(arr, count, size, cmp)`: quicksort in-place
- `bsearch(key, arr, count, size, cmp)`: binary search (requires sorted array)

**Pseudo-random numbers**:
- `rand()`: pseudo-random integer [0, RAND_MAX]
- `srand(seed)`: seed generator (call once before first rand call)

**Environment and exit**:
- `exit(code)`: terminate program (flushes streams, calls atexit functions)
- `abort()`: terminate program (no cleanup, status ABORT)
- `atexit(func)`: register function to call at exit
- `getenv(name)`: get environment variable
- `system(cmd)`: execute shell command

**Math**:
- `abs(x)`: absolute value of int
- `labs(x)`: absolute value of long
- `llabs(x)`: absolute value of long long
- `div(a, b)`: quotient and remainder (returns div_t struct)
- `abs(x)`: absolute value
- `exit(code)`: terminate program
- `abort()`: abnormal termination