# <errno.h>

**[<errno.h>](https://en.cppreference.com/w/c/header/errno)** provides `errno`, a thread-local variable that functions set to indicate why they failed. Common codes: `ENOENT` (no file), `EACCES` (permission denied), `EINVAL` (invalid argument).

Only check `errno` immediately after a function that failed; successful calls may overwrite it, giving you meaningless noise.

## Example

This example checks errno to distinguish different error conditions after failed file operations.

```c
// compile: gcc -o errnoexample errnoexample.c
// run: ./errnoexample
// description: interpret errno after system call failures

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

int main() {
    FILE* f = fopen("/nonexistent/file.txt", "r");
    if (!f) {
        printf("fopen failed: %s (errno %d)\n", strerror(errno), errno);
    }
    
    if (access("/etc/shadow", R_OK) < 0) {
        printf("access denied: %s\n", strerror(errno));
    }
    
    return 0;
}
```

## Common macros and functions

**Error reporting**:
- `errno`: thread-local variable holding last error code (set by library functions on error)
- `strerror(code)`: convert errno value to human-readable string
- `perror(prefix)`: print `prefix: error message` to stderr (from `<stdio.h>`)

**Common error codes** (POSIX):
- `ENOENT`: no such file or directory
- `EACCES`: permission denied
- `EINVAL`: invalid argument
- `ENOMEM`: out of memory (malloc, etc.)
- `EEXIST`: file exists
- `EISDIR`: is a directory (expected file)
- `ENOTDIR`: not a directory (expected directory)
- `EPERM`: operation not permitted
- `EAGAIN`, `EWOULDBLOCK`: resource temporarily unavailable
- `EINTR`: interrupted system call
- `EBADF`: bad file descriptor
- `ERANGE`: result out of range
- `EIO`: input/output error
- `ENOSPC`: no space left on device

**Usage notes**:
- Check errno **immediately** after suspect function (other functions may overwrite it)
- errno is meaningful only when function signals error (check return value first)
- Always `#include <errno.h>` before using errno
- errno is never set to 0 by functions (only by your code)