const char *optionsfrom(char *filename, int *argcp,
char ***argvp, int optind, FILE *errsto);
Optionsfrom reads the additional arguments from the specified filename, allocates a new argument vector to hold pointers to the existing arguments plus the new ones, and amends argc and argv (via the pointers argcp and argvp, which must point to the argc and argv being supplied to getopt_long(3)) accordingly. Optind must be the index, in the original argument vector, of the next argument.
If errsto is NULL, optionsfrom returns NULL for success and a pointer to a string-literal error message for failure; see DIAGNOSTICS. If errsto is non-NULL and an error occurs, optionsfrom prints a suitable complaint onto the errsto descriptor and invokes exit with an exit status of 2; this is a convenience for cases where more sophisticated responses are not required.
The text of existing arguments is not disturbed by optionsfrom, so pointers to them and into them remain valid.
The file of additional arguments is an ASCII text file. Lines consisting solely of white space, and lines beginning with #, are comments and are ignored. Otherwise, a line which does not begin with - is taken to be a single argument; if it both begins and ends with double-quote ("), those quotes are stripped off (note, no other processing is done within the line!). A line beginning with - is considered to contain multiple arguments separated by white space.
Because optionsfrom reads its entire file before the getopt_long(3) scan is resumed, an optionsfrom file can contain another --optionsfrom option. Obviously, infinite loops are possible here. If errsto is non-NULL, optionsfrom considers it an error to be called more than 100 times. If errsto is NULL, loop detection is up to the caller (and the internal loop counter is zeroed out).
#include <getopt.h> struct option opts[] = { /* ... */ "optionsfrom", 1, NULL, '+', /* ... */ }; int main(argc, argv) int argc; char *argv[]; { int opt; extern char *optarg; extern int optind; while ((opt = getopt_long(argc, argv, "", opts, NULL)) != EOF) switch (opt) { /* ... */ case '+': /* optionsfrom */ optionsfrom(optarg, &argc, &argv, optind, stderr); /* does not return on error */ break; /* ... */ } /* ... */
Line length is currently limited to 1023 bytes, and there is no continuation convention.
The restriction of error reports to literal strings (so that callers don't need to worry about freeing them or copying them) does limit the precision of error reporting.
The error-reporting convention lends itself to slightly obscure code, because many readers will not think of NULL as signifying success.
There is a certain element of unwarranted chumminess with the insides of getopt_long(3) here. No non-public interfaces are actually used, but optionsfrom does rely on getopt_long(3) being well-behaved in certain ways that are not actually promised by the specs.