discount/gethopt.3

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.Dd Jan 23, 2017
.Dt GETHOPT 3
.Os Mastodon
.Sh NAME
.Nm gethopt
.Nd get option letter or word from argv
.Sh SYNOPSIS
.Fd #include "gethopt.h"

.Bd -literal -compact
struct h_opt {
    int  option;
    char *optword;
    char optchar;
    int  opthasarg;
    char *optdesc;
} ;
.Ed
.Ft char*
.Fn hoptarg "struct h_context* argctx"
.Ft int
.Fn hoptind "struct h_context* argctx"
.Ft int
.Fn hopterr "struct h_context* argctx" "int flag"
.Ft char
.Fn hoptopt "struct h_context* argctx"
.Ft void
.Fn hoptset "struct h_context* argctx" "int argc" "char** argv"
.Ft struct h_opt*
.Fn gethopt "struct h_context* argctx" "struct h_opt* optarray" "int nropts"

.Sh DESCRIPTION
The
.Fn gethopt
function gets 
the next
.Em known
option word or character from
.Fa argctx .
An option is
.Em known
if it has been specified in the array of accepted options
.Fa optarray .
.Pp
The option array
.Fa optarray
contains records with either an option word, character, or both,
a flag saying the option needs an argument, a short description
of the option, and an
.Va option
key (which is there for the convenience of the calling program;
.Fn gethopt
does not use it in any way.).
It does not matter to
.Fn getopt
if a following argument has leading white space.
.Pp
On return from
.Fn gethopt ,
.Fn hoptarg
returns an option argument, if it is anticipated,
and the variable
.Fn hoptind
contains the index to the next
.Fa argv
argument for a subsequent call
to
.Fn gethopt .
.Pp
.Fn
gethopt
uses a (semi) opaque data blob to hold the current state, which
must be initialized by the
.Fn hoptset
function.
.Pp
The
.Fn gethopt
function
returns
.Dv HOPTERR
if a non-recognized
option is encountered,
and NULL when it reaches the end of the options on the command line..
.Pp
The interpretation of options in the argument list may be cancelled
by 
.Ql -
(single dash) or
.Ql --
(double dash) which causes
.Fn getopt
to signal the end of argument processing and return an
.Dv NULL . 
When all options have been processed (i.e., up to the first non-option
argument),
.Fn gethopt
returns
.Dv NULL .
.Sh DIAGNOSTICS
If the
.Fn gethopt
function encounters a character not found in
.Va optarray
or detects
a missing option argument
it returns
.Dv HOPTERR
(and writes an error message to the
.Em stderr 
if
.Fn hopterr
is used to turn error reporting on.)
.Sh EXAMPLE
.Bd -literal -compact
struct h_opt opts[] = {
    { 0, "css",    0,  1, "css file" },
    { 1, "header", 0,  1, "header file" },
    { 2, 0,       'a', 0, "option a (no arg)" },
    { 3, 0,       'b', 1, "option B (with arg)" },
    { 4, "help",  '?', 0, "help message" },
} ;

#define NROPT (sizeof opts/sizeof opts[0])


int
main(int argc, char **argv)
{
    struct h_opt *ret;
    struct h_context ctx;

    hoptset(&ctx, argc, argv);
    hopterr(&ctx, 1);

    while (( ret = gethopt(&ctx, opts, NROPT) )) {

	if ( ret != HOPTERR ) {
	    if ( ret->optword )
		printf("%s", ret->optword);
	    else
		printf("%c", ret->optchar);

	    if ( ret->opthasarg ) {
		if ( hoptarg(&ctx) )
		    printf(" = %s", hoptarg(&ctx));
		else
		    printf(" with no argument?");
	    }
	    puts(ret->optdesc ? ret->optdesc : "");
	}
    }

    argc -= hoptind(&ctx);
    argv += hoptind(&ctx);

.Ed
.Sh HISTORY
The
.Fn gethopt
function was a quick hack to replace manually parsing full-word arguments
in 
.Va discount .