我的 getToken 函数在第一次调用时不起作用

Question

I've written this function for a homework assignment I'm doing in school:

char* getToken(char buffer[], int pos)
{
    int i; 
    char copy[350], *token, *del = ",\n"; 

    strcpy(copy, buffer); 
    token = strtok(copy, del); 

    for (i = 1; i < pos; i++) 
        token = strtok(NULL, del); 

    return token; 
}

I want it to return a token at a given position without corrupting the original character array. The issue is that it return garbage on the first call, yet it works as expected on all subsequent calls. This should be a very simple fix, but I've been coding all day and I need a fresh set of eyes to back me up. (The hardcoded 350 is a given in this homework assignment, buffer shouldn't exceed 349 characters)

Answer 1

You are returning pointer pointing non-static local variable, which will vanish on returning from the function and dereferencing the returned pointer from caller will invoke undefined behavior .

I guess you should copy the token before returning. Add #include <stdlib.h> to use malloc() and free() .

char* getToken(const char buffer[], int pos)
{
    int i; 
    char *copy, *token, *ret, *del = ",\n"; 

    copy = malloc(strlen(buffer) + 1); /* for string longer than 349 bytes is passed */
    if (copy == NULL) return NULL;
    strcpy(copy, buffer); 
    token = strtok(copy, del); 

    for (i = 1; i < pos; i++) 
        token = strtok(NULL, del); 

    ret = malloc(strlen(token) + 1);
    if (ret != NULL) strcpy(ret, token); /* copy string before freeing it */
    free(copy); /* if you use malloc(), use free() */
    return ret;
}