A thread safe strtok function compatible with the POSIX strtok_r

mystrtok.h

#ifdef __cplusplus
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cstddef>
#else
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stddef.h>
#endif

char* mystrtokr(char* s, char* token, char** sp){
    char* e;
    if(s == NULL){
        s = *sp;
    }

    if(*s == '\0'){
        *sp = s;
        return NULL;
    }

    s += strspn(s, token);
    if(*s == '\0'){
        *sp = s;
        return NULL;
    }

    e = s + strcspn(s, token);
    if(*e == '\0'){
        *sp = e;
        return s;
    }

    *e = '\0';
    *sp = e + 1;
    return s;
}