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Manual Pages  — GETOPT_LONG


getopt_long, getopt_long_only – get long options from command line argument list



Standard C Library (libc, -lc)


#include <getopt.h>
extern char *optarg;
extern int optind;
extern int optopt;
extern int opterr;
extern int optreset;

getopt_long(int argc, char * const *argv, const char *optstring, const struct option *longopts, int *longindex);

getopt_long_only(int argc, char * const *argv, const char *optstring, const struct option *longopts, int *longindex);


The getopt_long() function is similar to getopt(3) but it accepts options in two forms: words and characters. The getopt_long() function provides a superset of the functionality of getopt(3). The getopt_long() function can be used in two ways. In the first way, every long option understood by the program has a corresponding short option, and the option structure is only used to translate from long options to short options. When used in this fashion, getopt_long() behaves identically to getopt(3). This is a good way to add long option processing to an existing program with the minimum of rewriting.

In the second mechanism, a long option sets a flag in the option structure passed, or will store a pointer to the command line argument in the option structure passed to it for options that take arguments. Additionally, the long option's argument may be specified as a single argument with an equal sign, e.g.,

    myprogram --myoption=somevalue

When a long option is processed, the call to getopt_long() will return 0. For this reason, long option processing without shortcuts is not backwards compatible with getopt(3).

It is possible to combine these methods, providing for long options processing with short option equivalents for some options. Less frequently used options would be processed as long options only.

The getopt_long() call requires a structure to be initialized describing the long options. The structure is:

struct option {
        char *name;
        int has_arg;
        int *flag;
        int val;

The name field should contain the option name without the leading double dash.

The has_arg field should be one of:

no_argument no argument to the option is expected
  an argument to the option is required
  an argument to the option may be presented

If flag is not NULL, then the integer pointed to by it will be set to the value in the val field. If the flag field is NULL, then the val field will be returned. Setting flag to NULL and setting val to the corresponding short option will make this function act just like getopt(3).

If the longindex field is not NULL, then the integer pointed to by it will be set to the index of the long option relative to longopts.

The last element of the longopts array has to be filled with zeroes.

The getopt_long_only() function behaves identically to getopt_long() with the exception that long options may start with ‘-’ in addition to ‘--’. If an option starting with ‘-’ does not match a long option but does match a single-character option, the single-character option is returned.


If the flag field in struct option is NULL, getopt_long() and getopt_long_only() return the value specified in the val field, which is usually just the corresponding short option. If flag is not NULL, these functions return 0 and store val in the location pointed to by flag.

These functions return ‘amp;:’ if there was a missing option argument and error messages are suppressed, ‘amp;?’ if the user specified an unknown or ambiguous option, and -1 when the argument list has been exhausted. The default behavior when a missing option argument is encountered is to write an error and return ‘amp;?’. Specifying ‘amp;:’ in optstr will cause the error message to be suppressed and ‘amp;:’ to be returned instead.

In addition to ‘amp;:’, a leading ‘amp;+’ or ‘amp;-’ in optstr also has special meaning. If either of these are specified, they must appear before ‘amp;:’.

A leading ‘amp;+’ indicates that processing should be halted at the first non-option argument, matching the default behavior of getopt(3). The default behavior without ‘amp;+’ is to permute non-option arguments to the end of argv.

A leading ‘amp;-’ indicates that all non-option arguments should be treated as if they are arguments to a literal ‘amp;1’ flag (i.e., the function call will return the value 1, rather than the char literal '1').


  If set, option processing stops when the first non-option is found and a leading ‘-’ or ‘+’ in the optstring is ignored.


int bflag, ch, fd;
int daggerset;

/* options descriptor */ static struct option longopts[] = {         { "buffy",      no_argument,            NULL,           'b' },         { "fluoride",   required_argument,      NULL,     'f' },         { "daggerset",  no_argument,            daggerset,      1 },         { NULL,         0,                      NULL,           0 } };

bflag = 0; while ((ch = getopt_long(argc, argv, "bf:", longopts, NULL)) != -1) {         switch (ch) {         case 'b':                 bflag = 1;                 break;         case 'f':                 if ((fd = open(optarg, O_RDONLY, 0)) == -1)                         err(1, "unable to open %s", optarg);                 break;         case 0:                 if (daggerset) {                         fprintf(stderr,"Buffy will use her dagger to "                          "apply fluoride to dracula's teeth\n");                 }                 break;         default:                 usage();         } } argc -= optind; argv += optind;


This section describes differences to the GNU implementation found in glibc-2.1.3:




The getopt_long() and getopt_long_only() functions first appeared in the GNU libiberty library. The first BSD implementation of getopt_long() appeared in NetBSD the first BSD implementation of getopt_long_only() in OpenBSD 3.3 . FreeBSD first included getopt_long() in FreeBSD 5.0, getopt_long_only() in FreeBSD 5.2 .


The argv argument is not really const as its elements may be permuted (unless POSIXLY_CORRECT is set).

The implementation can completely replace getopt(3), but right now we are using separate code.

GETOPT_LONG (3) May 2, 2018

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