getsubopt(3) Library Functions Manual getsubopt(3)

getsubopt - parse suboption arguments from a string

Standard C library (libc, -lc)

#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)):


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

getsubopt() parses the list of comma-separated suboptions provided in optionp. (Such a 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 may include an associated value, which is separated from the suboption name by an equal sign. The following is an example of the kind of string that might be passed in optionp:


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.

Each call to getsubopt() returns information about the next unprocessed suboption in optionp. The first equal sign in a suboption (if any) 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) 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 is overwritten with a null byte, so *valuep is precisely the "value string" for that suboption.

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.

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.

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

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

Interface Attribute Value
getsubopt () Thread safety MT-Safe



Since getsubopt() overwrites any commas it finds in the string *optionp, that string must be writable; it cannot be a string constant.

The following program expects suboptions following a "-o" option.

#define _XOPEN_SOURCE 500
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
main(int argc, char *argv[])
    enum {
        RO_OPT = 0,
    char *const token[] = {
        [RO_OPT]   = "ro",
        [RW_OPT]   = "rw",
        [NAME_OPT] = "name",
    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;
                case RW_OPT:
                    readwrite = 1;
                case NAME_OPT:
                    if (value == NULL) {
                                "Missing value for suboption '%s'\n",
                        errfnd = 1;
                    name = value;
                            "No match found for token: /%s/\n", value);
                    errfnd = 1;
            if (readwrite && readonly) {
                        "Only one of '%s' and '%s' can be specified\n",
                        token[RO_OPT], token[RW_OPT]);
                errfnd = 1;
            errfnd = 1;
    if (errfnd || argc == 1) {
        fprintf(stderr, "\nUsage: %s -o <suboptstring>\n", argv[0]);
                "suboptions are 'ro', 'rw', and 'name=<value>'\n");
    /* Remainder of program... */


2023-10-31 Linux man-pages 6.7