C query string parsing

Question

I have the following query string

address=1234&port=1234&username=1234&password=1234&gamename=1234&square=1234&LOGIN=LOGIN


        

Answer 1

When parsing a sting that may contain an empty-field between delimiters, strtok cannot be used, because strtok will treat any number of sequential delimiters as a single delimiter.

So in your case, if the variable=values fields may also contain an empty-field between the '&' delimiters, you must use strsep, or other functions such as strcspn, strpbrk or simply strchr and a couple of pointers to work your way down the string.

The strsep function is a BSD function and may not be included with your C library. GNU includes strsep and it was envisioned as a replacement for strtok simply because strtok cannot handle empty-fields.

(If you do not have strsep available, you will simply need to keep a start and end pointer and use a function like strchr to locate each occurrence of '&' setting the end pointer to one before the delimiter and then obtaining the var=value information from the characters between start and end pointer, then updating both to point one past the delimiter and repeating.)

Here, you can use strsep with a delimiter of "&\n" to locate each '&' (the '\n' char included presuming the line was read from a file with a line-oriented input function such as fgets or POSIX getline). You can then simply call strtok to parse the var=value text from each token returned by strsep using "=" as the delimiter (the '\n' having already been removed from the last token when parsing with strsep)

An example inserting a specific empty-field for handling between "...gamename=1234&&square=1234...", could be as follows:

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

int main (void) {

    char array[] =  "address=1234&port=1234&username=1234&password=1234"
                    "&gamename=1234&&square=1234&LOGIN=LOGIN",
        *query = strdup (array),  /* duplicate array, &array is not char** */
        *tokens = query,
        *p = query;

    while ((p = strsep (&tokens, "&\n"))) {
        char *var = strtok (p, "="),
             *val = NULL;
        if (var && (val = strtok (NULL, "=")))
            printf ("%-8s    %s\n", var, val);
        else
            fputs ("<empty field>\n", stderr);
    }

    free (query);
}

(note: strsep takes a char** parameter as its first argument and will modify the argument to point one past the delimiter, so you must preserve a reference to the start of the original allocated string (query above)).

Example Use/Output

$ ./bin/strsep_query
address     1234
port        1234
username    1234
password    1234
gamename    1234
<empty field>
square      1234
LOGIN       LOGIN

(note: the conversion of "1234" to a numeric value has been left to you)

Look things over and let me know if you have further questions.