What defines strscpy in C?

Question

From this video on youtube by Kees Cook from linux.conf.au 2019 he mentions strscpy as being preferred but generally doing what users want (less NUL-padding). However, he doesn't say what defines this (spec or header),

Slide from the video,

I can't find strscpy() with man

$ for i in strcpy strlcpy strscpy; do man -w $i; done;
/usr/share/man/man3/strcpy.3.gz
/usr/share/man/man3/strlcpy.3bsd.gz
No manual entry for strscpy

https://mafford.com/text/the-many-ways-to-copy-a-string-in-c/ — Iłya Bursov, Feb 08 '19 at 22:45
`strscpy()` it not part of C, nor of POSIX. It is an extension. Using it make the code unportable. — alk, Feb 09 '19 at 09:08
strscpy does its thing safely, but I don't think silent truncation is what users generally want. It is nit standard anyway. The standard (but optional and not supported by glibc) function that does the same thing is strcpy_s. — n. m. could be an AI, Aug 25 '19 at 13:40

score 5 · Accepted Answer · answered Feb 08 '19 at 22:49

5

It's in the linux source, not in the standard library. Online man page

answered Feb 08 '19 at 22:49

AShelly

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1

Shoulf we expect it to be added into GLibc? – Some Name Feb 09 '19 at 05:37
3

@SomeName: Not all functions, provided in the Linux kernel, are really needed for user-space programs. And, in my opinion, `strscpy` is one of them: it very cares about non-trusted strings, which may come from the user space (and shouldn't make the kernel to crash in any case), but this feature is not so crucial for user-space programs. See also [this reasoning](https://mafford.com/text/the-many-ways-to-copy-a-string-in-c/) about some functions added to the Linux kernel. – Tsyvarev Feb 09 '19 at 18:56

score 1 · Answer 2 · answered Aug 25 '19 at 13:23

Given that there isn't any library that provides this function (AFAIK), you can write it yourself. I'm sure it can be optimized, but a very simple definition using GNU C11 for it is this one, which I use:

#pragma once    /* libalx/base/string/strcpy/strscpy.h */


#include <stddef.h>


__attribute__((nonnull))
ptrdiff_t strscpy       (char dest[restrict /*size*/],
                         const char src[restrict /*size*/],
                         ptrdiff_t size);

#include "libalx/base/string/strcpy/strscpy.h"

#include <errno.h>
#include <stddef.h>
#include <string.h>


ptrdiff_t strscpy       (char dest[restrict /*size*/],
                         const char src[restrict /*size*/],
                         ptrdiff_t size)
{
        ptrdiff_t   len;

        if (size <= 0)
                return  -E2BIG;

        len     = strnlen(src, size - 1);
        memcpy(dest, src, len);
        dest[len] = '\0';

        return  len;
}

Note: I prefer ptrdiff_t for array sizes, but you can use size_t/ssize_t as the linux version does.

What defines strscpy in C?

2 Answers2