# <getopt.h>

**[<getopt.h>](https://en.cppreference.com/w/c/header/getopt)** (POSIX/GNU) provides `getopt` for short options (`-v`, `-o file`) and `getopt_long` for long options (`--verbose`, `--output=file`). It handles argument parsing, `optarg` tracking, and stops at the first non-option argument or `--`.

Use it to parse command-line arguments portably instead of handling `argv` manually.

## Example

This example parses both short and long options, handling argument requirements.

```c
// compile: gcc -o getoptexample getoptexample.c
// run: ./getoptexample -v --output=out.txt file1.txt
// description: parse short and long options with arguments

#include <getopt.h>
#include <stdio.h>

int main(int argc, char* argv[]) {
    int verbose = 0;
    const char* outfile = "a.out";
    int c;
    
    struct option opts[] = {
        {"verbose", no_argument, NULL, 'v'},
        {"output", required_argument, NULL, 'o'},
        {0, 0, 0, 0}
    };
    
    while ((c = getopt_long(argc, argv, "vo:", opts, NULL)) != -1) {
        switch (c) {
        case 'v': verbose = 1; break;
        case 'o': outfile = optarg; break;
        case '?': return 1;
        }
    }
    
    printf("verbose=%d output=%s\n", verbose, outfile);
    for (int i = optind; i < argc; i++)
        printf("  input: %s\n", argv[i]);
    return 0;
}
```

## Common functions and variables

**Short option parsing**:
- `getopt(argc, argv, optstring)`: parse options (GNU extension, POSIX)
  - `optstring` format: `"ab:c::"` means `-a` (no arg), `-b` (required arg), `-c` (optional arg)
  - Returns option character, `?` on error, `:` on missing arg, or -1 when done
  - Resets on each call with new argv

**Long option parsing**:
- `getopt_long(argc, argv, optstring, longopts, &longindex)`: parse long options
- `struct option { const char *name; int has_arg; int *flag; int val; };`
  - Terminate array with `{0, 0, 0, 0}`
  - Long option matching: `--verbose=value` or `--verbose value`

**Argument flags** (for struct option.has_arg):
- `no_argument`: option takes no argument
- `required_argument`: option requires argument
- `optional_argument`: argument is optional

**Global variables** (set by getopt):
- `optarg`: pointer to argument for current option (NULL if none)
- `optind`: index in argv of next element to process (initialize to 1)
- `opterr`: if 1 (default), print error messages to stderr on unknown options
- `optopt`: option character that caused error (unknown option or missing arg)

**Usage pattern**:
```c
while ((opt = getopt_long(argc, argv, "abc:d::", longopts, &idx)) != -1) {
    switch (opt) {
        case 'a': /* handle -a */ break;
        case 'c': /* optarg has argument */ break;
        case 'd': /* optional arg in optarg */ break;
        case 0:   /* long option, check longopts[idx] */ break;
        case '?': /* unknown option or missing arg */ break;
    }
}

// argv[optind] is first non-option argument
```