GETSUBOPT(3POSIX)

📦 version 2017a, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu resolute, ubuntu stonking

💖 GETSUBOPT(3POSIX) 💖

okay bestie, we're in the POSIX Programmer's Manual today. This is getsubopt(3POSIX), and she's the girl who parses your suboption arguments out of a string. No cap.

🌸 PROLOG 🌸

Real talk before we start: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ — so if you're on Linux, go check the corresponding Linux manual page for the actual Linux behavior. Or, plot twist, the interface might not even be implemented on Linux at all. 🙈

✨ NAME ✨

getsubopt — parse suboption arguments from a string. She's literally just doing one job and doing it gorgeously. 💅

🦋 SYNOPSIS 🦋

#include <stdlib.h>

int getsubopt(char **optionp, char * const *keylistp, char **valuep);

💜 DESCRIPTION 💜

So the getsubopt() function shall parse suboption arguments in a flag argument. And like, where do these come from? Usually they pop off when you've been using getopt(). It's giving "getopt's chaotic little sister." 🔥

The getsubopt() argument optionp is a pointer to a pointer to the option argument string. The suboption arguments shall be separated by <comma> characters, and each one may consist of either:

  • a single token, or
  • a token-value pair separated by an <equals-sign>.

The keylistp argument shall be a pointer to a vector of strings. The end of the vector is identified by a null pointer. Each entry in the vector is one of the possible tokens that might be found in *optionp. And listen — since <comma> characters delimit suboption arguments in optionp, they should not appear in any of the strings pointed to by keylistp. Same energy with the <equals-sign>: because an <equals-sign> separates a token from its value, the application should not include an <equals-sign> in any of the strings pointed to by keylistp. Also, iconically, the getsubopt() function shall not modify the keylistp vector. She looks, she does not touch. 👑

The valuep argument is the address of a value string pointer.

Now for the parsing lore, because this is where it gets juicy:

  • If a <comma> appears in optionp, it shall be interpreted as a suboption separator.
  • After <comma> characters have been processed, if there are one or more <equals-sign> characters in a suboption string, the first <equals-sign> in any suboption string shall be interpreted as a separator between a token and a value.
  • Subsequent <equals-sign> characters in a suboption string shall be interpreted as part of the value. (Yes bestie, the rest of them just get absorbed into the value. That's the vibe.)

If the string at *optionp contains only one suboption argument (equivalently, no <comma> characters), getsubopt() shall update *optionp to point to the null character at the end of the string. Otherwise, it shall isolate the suboption argument by replacing the <comma> separator with a null character, and shall update *optionp to point to the start of the next suboption argument.

If the suboption argument has an associated value (equivalently, contains an <equals-sign>), getsubopt() shall update *valuep to point to the value's first character. Otherwise, it shall set *valuep to a null pointer. The calling application may use this information to determine whether the presence or absence of a value for the suboption is an error. That's on you, bestie — she's just handing you the receipts. 🧾

Additionally, when getsubopt() fails to match the suboption argument with a token in the keylistp array, the calling application should decide if this is an error, or if the unrecognized option should be processed in another way. Her hands are clean, it's your call. 🫶

⭐ RETURN VALUE ⭐

The getsubopt() function shall return the index of the matched token string, or -1 if no token strings were matched. Simple as that. Slay.

🌈 ERRORS 🌈

No errors are defined. Not one. Zero. She's unbothered.

The following sections are informative. 💅

🎀 EXAMPLES 🎀

🌸 Parsing Suboptions 🌸

The following example uses the getsubopt() function to parse a value argument in the optarg external variable returned by a call to getopt(). Look at her go:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int do_all;
const char *type;
int read_size;
int write_size;
int read_only;

enum
{
    RO_OPTION = 0,
    RW_OPTION,
    READ_SIZE_OPTION,
    WRITE_SIZE_OPTION
};

const char *mount_opts[] =
{
    [RO_OPTION] = "ro",
    [RW_OPTION] = "rw",
    [READ_SIZE_OPTION] = "rsize",
    [WRITE_SIZE_OPTION] = "wsize",
    NULL
};

int
main(int argc, char *argv[])
{
    char *subopts, *value;
    int opt;

    while ((opt = getopt(argc, argv, "at:o:")) != -1)
        switch(opt)
            {
            case 'a':
                do_all = 1;
                break;
            case 't':
                type = optarg;
                break;
            case 'o':
                subopts = optarg;
                while (*subopts != ' ')
                {
                    char *saved = subopts;
                    switch(getsubopt(&subopts, (char **)mount_opts,
                        &value))
                    {
                    case RO_OPTION:
                        read_only = 1;
                        break;
                    case RW_OPTION:
                        read_only = 0;
                        break;
                    case READ_SIZE_OPTION:
                        if (value == NULL)
                            abort();
                        read_size = atoi(value);
                        break;
                    case WRITE_SIZE_OPTION:
                        if (value == NULL)
                            abort();
                        write_size = atoi(value);
                        break;
                    default:
                        /* Unknown suboption. */
                        printf("Unknown suboption `%s'\n", saved);
                        abort();
                    }
                }
                break;
            default:
                abort();
            }

    /* Do the real work. */

    return 0;
}

If the above example is invoked with:

program -o ro,rsize=512

then after option parsing, the variable do_all will be 0, type will be a null pointer, read_size will be 512, write_size will be 0, and read_only will be 1. Chef's kiss. 👨‍🍳💋

If it is invoked with:

program -o oops

it will print:

"Unknown suboption `oops'"

before aborting. Not me getting caught lacking by an unrecognized suboption. 💀

🧚 APPLICATION USAGE 🧚

The value of *valuep when getsubopt() returns -1 is unspecified. And, historical implementations provide various incompatible extensions to allow an application to access the suboption text that was not found in the keylistp array. So like... don't count on any specific behavior there, bestie. It's giving "undefined, depends on the girl, depends on the era." 😭

🍓 RATIONALE 🍓

The keylistp argument of getsubopt() is typed as char * const * to match historical practice. However, the standard is clear that implementations will not modify either the array or the strings contained in the array, as if the argument had been typed const char * const *. So it's a typing thing, not a "she's gonna rearrange your array" thing. She would never. 💖

💀 FUTURE DIRECTIONS 💀

None. She said what she said and she's not changing. Periodt.

👑 SEE ALSO 👑

  • getopt()
  • The Base Definitions volume of POSIX.1‐2017, <stdlib.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

and that's the whole page, bestie!! ✨ this one came to you courtesy of IEEE/The Open Group, dated 2017, and she's the GETSUBOPT(3POSIX) girlie forever. go forth and parse those suboptions. mwah 💅💖🎀

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