What's the difference between strcpy and stpcpy?

Question

While reading the man page for strcpy, I discovered the function stpcpy also exists. However, the only difference I could notice in the man page is

Answer 1

The question you are asking in the title and the question about restrict are actually two different totally unrelated questions. The other answers have already provided you with some good links that will help you to learn more about restrict.

However, the main difference between these two functons in not really the restrict specifier. In fact, in C99 version of C language specification, strcpy also has restrict qualification on its parameters. What you see on your man page for strcpy is simply not updated to conform to C99.

The main difference (which you seem to have missed) is the return value of stpcpy. stpcpy returns the pointer to the terminating \0 character of the target string. This immediately makes clear the purpose of stpcpy and the rationale behind its existence: this function is intended to be used as an intelligent replacement to strcat function in situations when you need to concatenate multiple sub-strings into one string. You see, strcat works pretty poorly in such application (I'd even say that strcat has no meaningful uses in real code). The problem with strcat is that it rescans the destination string every time you add something to it, thus doing lots of unnecessary work and basically generating more heat than light. For example, the following code suffers from that problem

const char *part1, *part2, *part3, *part4;
...
char buffer[size]; /* assume that `size` is calculated properly */

strcpy(buffer, part1);
strcat(buffer, part2);
strcat(buffer, part3);
strcat(buffer, part4);

This code can be reimplemented in a much more reasonable way by using stpcpy

stpcpy(stpcpy(stpcpy(stpcpy(buffer, part1), part2), part3), part4);

And if you don't like chained calls, you can use an intermediate pointer to store the return value of intermediate stpcpy calls

char *end = buffer;

end = stpcpy(end, part1);
end = stpcpy(end, part2);
end = stpcpy(end, part3);
end = stpcpy(end, part4);

Of course it is worth mentioning that strcpy and strcat are standard functions, while stpcpy is not.