OPTIND

optind is an external integer used by the getopt family of functions to track parsing progress of command-line arguments. It is typically declared in POSIX headers such as unistd.h or getopt.h in GNU environments. The variable is initialized to 1, which corresponds to the first argument after the program name. Each call to getopt advances optind to point to the next argv element to be scanned.

During option parsing, optind serves as the index into the argv array. After all options have been processed, optind points to the first non-option argument, or to argc if there are no more arguments to process. This makes optind useful for separating option arguments from positional arguments in argv.

optarg and optopt are related: optarg points to the argument associated with the current option, and optopt holds the option character that caused an error if an issue is encountered during parsing.

Usage notes and portability: If a program needs to parse argv more than once, optind can be reset to 1 to re-scan from the beginning. This variable is not inherently thread-safe; in multi-threaded programs a reentrant parsing variant such as getopt_r should be used to avoid data races, with per-thread state maintained separately.

In summary, optind provides the index of the next command-line argument to be processed by getopt, initializes to 1, and becomes the doorway to non-option arguments once option parsing completes.