Split string with strtok (nested) - c++

Question

I use an arduino MEGA to parse a the param part of an url.
It should not matter how the order of the parameters are. I have following code and I tried that with strtok.
char text[] = "ssid=SSID&pwd=PASSWORD&userId=1234"
and will split in
ssid=SSID
pwd=PASSWORD
userId=1234
and split it again in key and values
ssid
SSID
pwd
PASSWORD
userId
1234
I tried to use strtok for the first split.
char *ptr;
ptr = strtok(params, "&");
while (ptr != NULL) {
Serial.println(urlParam);
ptr = strtok(NULL, "&");
}
and the output is as expected:
Output:
ssid=SSID
pwd=PASSWORD
userId=1234
then the next split:
char *ptr;
ptr = strtok(params, "&");
while (ptr != NULL) {
char *paramKey;
char *paramValue;
paramKey = strtok(ptr, "=");
Serial.println(paramKey);
if (paramKey == 'ssid'){
paramValue = strtok(NULL, "=");
Serial.println(paramValue);
ssidName = paramValue;
}
if (paramKey == 'pwd'){
...
}
if (paramKey == 'userId'){
...
}
ptr = strtok(NULL, "&");
}
But the output is just
ssid
SSID
Looks like the loop is not working properly.
Where do I make the mistake?
Is there any other way to resolve this string?

Answer 1

The strtok function uses an internal static variable to keep track of its current state. When you use the function for multiple different substrings interleaved like you're doing, you step on the internal state.
You need to instead use strtok_r, which uses an external variable to keep track of state.
char *ptr, *sav1 = NULL;
ptr = strtok_r(params, "&", &sav1); // outer strtok_r, use sav1
while (ptr != NULL) {
char *paramKey;
char *paramValue;
char *sav2 = NULL;
paramKey = strtok_r(ptr, "=", &sav2); // inner strtok_r, use sav2
Serial.println(paramKey);
if (!strcmp(paramKey, "ssid")) {
paramValue = strtok_r(NULL, "=", &sav2); // inner strtok_r, use sav2
Serial.println(paramValue);
ssidName = paramValue;
}
if (!strcmp(paramKey, "pwd")) {
...
}
if (!strcmp(paramKey, "userId")) {
...
}
ptr = strtok_r(NULL, "&", &sav1); // outer strtok_r, use sav1
}
Not related to the parsing issue, you also can't compare strings with ==. You need to use strcmp instead, and string constants are surrounded in double quotes, not single quotes.

Answer 2

You have three problems:
The first is that you use implementation-specific multi-character literals like e.g. 'ssid', when you should be using strings like "ssid".
The second problem is that you use == to compare strings. That's almost impossible to get to work, as then you compare the pointers instead of the string contents. To compare strings you need to use strcmp.
The strtok function is not reentrant. You can't have multiple strtok parsings going simultaneously. Either separate the steps, or use strtok_s (or strtok_r if such a function is available).

Answer 3

#include <stdio.h>
#include <string.h>
struct split_result {
char first_part[64];
char second_part[64];
};
split_result return_left(char splitter, const char *str);
int main() {
/*
ssid=SSID
pwd=PASSWORD
userId=1234
*/
char *str = "ssid=109304905995";
split_result result = return_left('=', str);
printf("first part: %s \n second part: %s\n", result.first_part, result.second_part);
return 0;
}
split_result return_left(char splitter, const char *str) {
int i = 0;
split_result split_ptr;
while (str[i] != 0) {
//we found splitter
if ((char)str[i] == splitter) {
//remove first half
strncpy_s(split_ptr.first_part, str, (int)(&str[i]-str));
break;
}
//we need to remove second half now
i++;
}
strcpy_s(split_ptr.second_part, &str[i+1]);
return split_ptr;
}
I hope this will be helpful to anyone.