C query string parsing - c

Question

I have the following query string
address=1234&port=1234&username=1234&password=1234&gamename=1234&square=1234&LOGIN=LOGIN
I am trying to parse it into different variables: address,port,username,password,gamename,square and command (which would hold LOGIN)
I was thinking of using strtok but I don't think it would work. How can I parse the string to capture the variables ?
P.S - some of the fields might be empty - no gamename provided or square

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.