为什么在使用StrDup时strtok_s可以工作？

Question

I use strtok_s with an allocated string.

The code above handles an exception:

char *string1 = NULL;
string1 = (char*)LocalAlloc(LPTR, 100 * sizeof(char));
// TODO: Verify if string1 != NULL
string1 = "A string\tof ,,tokens\nand some  more tokens";
token1 = strtok_s(cp1, seps, &next_token1);

But when I call StrDup, my code works:

cp1 = StrDup(string1);
token1 = strtok_s(cp1, seps, &next_token1);

However my understanding of StrDup is it uses LocalAlloc to allocate storage space for the copy of the string (according to MSDN ).

So, what is wrong in my first example ? How to correct this code without using StrDup ?

Answer 1

string1 = (char*)LocalAlloc(LPTR, 100 * sizeof(char));
// TODO: Verify if string1 != NULL
string1 = "A string\tof ,,tokens\nand some  more tokens";
token1 = strtok_s(cp1, seps, &next_token1);

You've overwritten your pointer... it starts out NULL and you make it point to allocated space (which is your intention), but you then change it to point to something that is read-only, ac string constant. Since strtok_s needs to be able to write into the buffer, this makes it fail. When you strdup() instead, you create another (writable) copy of the string.

Instead of setting string1 = "some constant string"; you can use strcpy() (one of its length-limiting versions, for safety) to copy the read only string constant into your buffer. This gets you where you need to be without strdup() , but since strdup() performs the allocation for you, your work would be easier by going to it directly and cutting out the allocation / copy.

Answer 2

This line

string1 = "A string\tof ,,tokens\nand some  more tokens";

doesn't do what you expected it to.

You wanted to write the contents of the constant string into the memory buffer pointed to by string1 . What you actually did was change the value of string1 to point at memory that was allocated by the compiler to hold the constant string.

One way of doing what you wanted is

strcpy(string1, "A string\tof ,,tokens\nand some  more tokens");