getsubopt(3)

getsubopt(3) 💖 — parsing suboption arguments like the queen you are

Okay so bestie, gather round, because getsubopt is literally that girl when it comes to breaking down comma-separated suboption strings. No cap. She's the one you call when getopt(3) hands you a messy -o argument and you need to make sense of it. Let's get into it. ✨

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🎀 LIBRARY 🎀

Standard C library (libc, -lc) 💅

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💅 SYNOPSIS 💅

#include <stdlib.h>

int getsubopt(char **restrict optionp, char *const *restrict tokens,
              char **restrict valuep);

🦋 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

getsubopt():
    _XOPEN_SOURCE >= 500
        || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

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🌸 DESCRIPTION 🌸

So like, getsubopt() parses the list of comma-separated suboptions that you pass in through optionp. (This kind of suboption list is typically produced when getopt(3) is used to parse a command line; see for example the -o option of mount(8).) Each suboption can have a little value attached to it, separated from the suboption name by an equal sign. Here's the kind of string that might be passed in optionp, bestie:

ro,name=xyz

The tokens argument is a pointer to a NULL-terminated array of pointers to the tokens that getsubopt() will look for in optionp. The tokens should be distinct, null-terminated strings containing at least one character, with no embedded equal signs or commas. Like, don't do that. Periodt. 💀

Every single call to getsubopt() returns info about the next unprocessed suboption in optionp. The first equal sign in a suboption (if there even is one) is interpreted as a separator between the name and the value of that suboption. The value extends to the next comma, or (for the last suboption) all the way to the end of the string. If the name of the suboption matches a known name from tokens, and a value string was found, getsubopt() sets *valuep to the address of that string. The first comma in optionp gets overwritten with a null byte, so *valuep is precisely the "value string" for that suboption. Iconic behavior, honestly. 👑

If the suboption is recognized but no value string was found, *valuep is set to NULL.

When getsubopt() returns, optionp points to the next suboption, or to the null byte ('\0') at the end of the string if the last suboption was just processed. She's always keeping track, fr fr. 🫶

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🔥 RETURN VALUE 🔥

If the first suboption in optionp is recognized, getsubopt() returns the index of the matching suboption element in tokens. Otherwise, -1 is returned and *valuep is the entire name[=value] string. No match? She's still giving you the whole thing so you can debug.

Since *optionp is changed, the first suboption before the call to getsubopt() is not (necessarily) the same as the first suboption after getsubopt(). Lowkey important to remember. 💅

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⭐ ATTRIBUTES ⭐

For an explanation of the terms used in this section, see attributes(7).

• Interface: getsubopt() — Attribute: Thread safety — Value: MT-Safe 🌈

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👑 STANDARDS 👑

POSIX.1-2008. She's certified, bestie. ✅

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🧚 HISTORY 🧚

POSIX.1-2001. An OG. 💜

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💀 NOTES 💀

Since getsubopt() overwrites any commas it finds in the string *optionp, that string needs to be writable; it cannot be a string constant. Like, don't come for her with a const char * and expect things to go well. It will not. Bestie, it will not. 🙈

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🍓 EXAMPLES 🍓

The following program expects suboptions following a "-o" option. Look at her go:

#define _XOPEN_SOURCE 500
#include <stdio.h>
#include <stdlib.h>

#include <assert.h>

int
main(int argc, char *argv[])
{
    enum {
        RO_OPT = 0,
        RW_OPT,
        NAME_OPT
    };
    char *const token[] = {
        [RO_OPT]   = "ro",
        [RW_OPT]   = "rw",
        [NAME_OPT] = "name",
        NULL
    };
    char *subopts;
    char *value;
    int opt;

    int readonly = 0;
    int readwrite = 0;
    char *name = NULL;
    int errfnd = 0;

    while ((opt = getopt(argc, argv, "o:")) != -1) {
        switch (opt) {
        case 'o':
            subopts = optarg;
            while (*subopts != '\0' && !errfnd) {

                switch (getsubopt(&subopts, token, &value)) {
                case RO_OPT:
                    readonly = 1;
                    break;

                case RW_OPT:
                    readwrite = 1;
                    break;

                case NAME_OPT:
                    if (value == NULL) {
                        fprintf(stderr,
                                "Missing value for suboption '%s'\n",
                                token[NAME_OPT]);
                        errfnd = 1;
                        continue;
                    }

                    name = value;
                    break;

                default:
                    fprintf(stderr,
                            "No match found for token: /%s/\n", value);
                    errfnd = 1;
                    break;
                }
            }
            if (readwrite && readonly) {
                fprintf(stderr,
                        "Only one of '%s' and '%s' can be specified\n",
                        token[RO_OPT], token[RW_OPT]);
                errfnd = 1;
            }
            break;

        default:
            errfnd = 1;
        }
    }

    if (errfnd || argc == 1) {
        fprintf(stderr, "\nUsage: %s -o <suboptstring>\n", argv[0]);
        fprintf(stderr,
                "suboptions are 'ro', 'rw', and 'name=<value>'\n");
        exit(EXIT_FAILURE);
    }

    /* Remainder of program...  */

    exit(EXIT_SUCCESS);
}

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🌈 SEE ALSO 🌈

getopt(3)

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That's the whole vibe, bestie. This page came straight from Linux man-pages 6.20, dated 2026-02-08. Slay. 💅💖✨