CODEWITHSUNDEEP
c++c
phillythompson
phillythompson

Reputation: 111

How can I use the secure strcpy_s function?

I am trying to update a call to strcpy with the more secure strcpy_s. The old function call looks like this:

char buf[6];
strcpy(buf+1, "%#c");

In order to turn the above code into the more secure version, what would I put in for the size parameter? 

char buf[6];
strcpy_s(buf+1, /*SIZE GOES HERE*/, "%#c");

I suppose I am getting confused with the buf+1 portion of the code. Is that essentially just a pointer that points to the register ONE block over from buf? If so, what would I put in for the SIZE parameter of strcpy_s? I have tried:

  strcpy_s(buf+1, sizeof(buf+1), "%#c");

which seems to work, but I wonder if there is a better way to do it.

Upvotes: 1

Views: 466

Answers (3)

eerorika
eerorika

Reputation: 238351

The size should be 5 (assuming you want to skip the first index). A proper way to calculate is:

int offset = 1; // not necessary, but shows the connection between the size and offset
strcpy_s(buf+offset, (sizeof(buf)-offset) / sizeof(buf[0]), "%#c");

Division by char size is redundant, but it's good to remember because it's necessary if you use a wide characters.

Upvotes: 1

Rakib
Rakib

Reputation: 7625

buf+1 points to the memory location one past the starting address of buf, so effectively the copy starts from that location. But the size parameter specifies the size of the destination. It does not make sense to use sizeof(buf+1) for that parameter. You should pass the available space of the destination ( should be <= (total size - starting offset)).

In your case, it should be

strcpy_s(buf+1, sizeof(buf)/sizeof(*buf) - 1, "%#c");

look here for details

Upvotes: 0

Paul Roub
Paul Roub

Reputation: 36438

If you were copying into the start of buf, you'd say:

strcpy_s(buf, sizeof(buf), "%#c");

since you're skipping the first character, you have one less character to work with, so:

strcpy_s(buf + 1, sizeof(buf) - 1, "%#c");

Upvotes: 5

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