myString.h

#ifndef MYSTRING_H
#define MYSTRING_H

char* my_Strchr(const char* s, int c);
size_t my_Strspn(const char* s1, const char* s2);
size_t my_Strcspn(const char* s1, const char* s2);
char* my_Strtok(char* str, const char* delim);

char* my_Strchr(const char* s, int c) {
    while (*s != (char) c)
        if (!*s++)
            return 0;
    return (char*) s;
}

size_t my_Strspn(const char* s1, const char* s2) {
    size_t ret = 0;
    while (*s1 && my_Strchr(s2, *s1++))
        ret++;
    return ret;
}

size_t my_Strcspn(const char* s1, const char* s2) {
    size_t ret = 0;
    while (*s1)
        if (my_Strchr(s2, *s1))
            return ret;
        else
            s1++, ret++;
    return ret;
}

char* my_Strtok(char* str, const char* delim) {
    static char* p = 0;
    if (str)
        p = str;
    else if (!p)
        return 0;
    str = p + my_Strspn(p, delim);
    p = str + my_Strcspn(str, delim);
    if (p == str)
        return p = 0;
    p = *p ? *p = 0, p + 1 : 0;
    return str;
}

#endif  /* MYSTRING_H */