C: Why does strcpy return its argument?

Question

Why does strcpy(3) (and strncpy(3)) return their first argument? I don't see how this does add any value. Instead, frequently I'd rather have the number of copied bytes returned.

Addendum: What am I supposed to do when I need also the length of the resulting string? Do I really have to implement my own version?

It would be far more useful if a variant of `strcpy()` returned a pointer to the `'\0'` byte at the end of the string. However, if you length check everything before you do your copying (as you should to be safe), you can use `memmove()` (or maybe `memcpy()`) instead of `strcpy()`. It's only when you don't have a length available that can't use those, but it is arguably not safe to do the copying if you don't know the lengths of the source string and the target buffer. — Jonathan Leffler, May 05 '13 at 16:04
@JonathanLeffler re: _"It would be far more useful if a variant of strcpy() returned a pointer to the '\0' byte at the end of the string"_: There is (in POSIX). It's called [`stpcpy(3)`](https://man7.org/linux/man-pages/man3/stpcpy.3.html). — alx - recommends codidact, Oct 18 '21 at 15:51
And, for chaining memcpy(3), there's GNU's `mempcpy(3)`.; the same with better return value — alx - recommends codidact, Dec 09 '22 at 16:40

score 4 · Answer 1 · answered May 05 '13 at 15:49

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For historical reasons. strcpy and friends date back to the early seventies, and I guess the intended use case for the return value would be a kind of chaining:

// copy src into buf1 and buf2 in a single expression
strcpy(buf1, strcpy(buf2, src));

Or

char *temp = xmalloc(strlen(const_str) + 1);
function_that_takes_mutable_str(strcpy(temp, const_str));

answered May 05 '13 at 15:49

Fred Foo

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Didn't they have comma expressions at the time? `function1((strcpy(buf1, const_str), buf1))`. – Jo So May 05 '13 at 15:56
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@JoSo: I'm not sure when comma expressions were introduced, but I've read quite a lot of old C code and I don't think I ever saw it being used. I personally also never use it except in macros. – Fred Foo May 05 '13 at 16:33

score 1 · Answer 2 · answered May 05 '13 at 15:48

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So that you can do something like

char * str = strcpy(malloc(12), "MyNewString");

answered May 05 '13 at 15:48

user93353

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… the danger of which will not have escaped anyone who saw the version of this answer with `11` where `12` now is. – Pascal Cuoq May 05 '13 at 15:50
Why not just use strdup? Or was it missing at those times? – sashoalm May 05 '13 at 15:51
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`strdup` is, I think, a 1980s addition, when C had already diverged into dialects. It wasn't in V7 Unix, it's still not in ISO C, even though POSIX has always had it. – Fred Foo May 05 '13 at 15:53
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The other danger with chaining with `malloc()` is the crash that occurs when the memory allocation fails! – Jonathan Leffler May 05 '13 at 16:01
@JonathanLeffler: you can do it with a custom `xmalloc`, though. – Fred Foo May 05 '13 at 16:31

score 1 · Answer 3 · answered Apr 23 '17 at 14:32

Most of the string functions in the C library have been designed by amateurs. For instance, in my 25 years of my career, I never used the strcat() function, yet I concatenate strings all the time. Also, if you think the printf(), there is little documentation if you pass NULL for a %s argument. The same goes for for the %c passing a '\0', or a malloc(0).

Sadly, the most useful strcpy() should return a pointer to the end of the destination buffer to chain copying.

C: Why does strcpy return its argument?

3 Answers3