This function is identical to nxt_ustr2str(), except that it takes
a nxt_str_t structure as input, instead of a 'u_char *' and a size.

The documentation of the function:

/*
 * SYNOPSIS
 *   void nxt_usts2str(char dst[restrict .src->length+1],
 *                     const nxt_str_t *restrict src);
 *
 * ARGUMENTS
 *      dst     Pointer to the first byte of the destination buffer.
 *      src     Pointer to the source Unterminated STring Structure.
 *
 * DESCRIPTION
 *      Copy a string from the source nxt_str_t, which may be
 *      not-NUL-terminated, into a NUL-terminated string in the
 *      destination buffer.
 *
 * CAVEATS
 *      If the destination buffer is not wider than the source buffer
 *      at least by 1 byte, the behavior is undefined.
 *
 * EXAMPLES
 *      nxt_str_t  src = nxt_string("0123456789");
 *      char       dst[src.length + 1];
 *
 *      nxt_usts2str(dst, &src);
 *
 * SEE ALSO
 *      ustr2str(3), strlcpy(3), strscpy(9)
 */

Suggested-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

This function makes it easy to transform a fixed-width buffer
(which is how we represent strings in Unit most of the time) into
a proper C string (NUL-terminated).  We need to do this when
interfacing libraries or the kernel, where most APIs expect
NUL-terminated strings.

The implementation is similar to strncpy_s(3), but avoids the
unnecessary runtime checks.  It's better to wrap the function in a
macro and do as many static_assert(3)s as one considers necessary;
in fact, if in the future C allows backwards VLA syntax, static
analysis could be better than those static_assert(3)s.

We use char for NUL-terminated strings, and u_char for the
*u*nterminated strings.

The documentation for the function:

/*
 * SYNOPSIS
 *   void ustr2str(char dst[restrict .n+1],
 *                 const u_char src[restrict .n],
 *                 size_t n);
 *
 * ARGUMENTS
 *	dst	Pointer to the first byte of the destination buffer.
 *	src	Pointer to the first byte of the source string.
 *	n	Size of 'src'.
 *
 * DESCRIPTION
 *	Copy a string from the fixed-width source string, which may be
 *	not-NUL-terminated, into a NUL-terminated string in the
 *	destination buffer.
 *
 * CAVEATS
 *	If the destination buffer is not wider than the source buffer
 *	at least by 1 byte, the behavior is undefined.
 *
 *	Use of this function normally indicates a problem in the design
 *	of the strings, since normally it's better to guarantee that all
 *	strings are properly terminated.  The main use for this function
 *	is to interface with some standard buffers, such as those
 *	defined in utmp(7), which for historical reasons are not
 *	guaranteed to be terminated.
 *
 * EXAMPLES
 *	u_char  src[10] = "0123456789";  // not NUL-terminated
 *	char    dst[sizeof(src) + 1];
 *
 *	static_assert(lengthof(src) < lengthof(dst))
 *	ustr2str(dst, src, lengthof(src));
 *
 * SEE ALSO
 *	nxt_sts2str(3), strlcpy(3), strscpy(9)
 */

Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

Ruby redefines memcpy(3) in public headers.  Or at least they did until
they fixed it 4 months ago.  We need to undefine their broken definition.

Link: <https://bugs.ruby-lang.org/issues/18893>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

This allows using the type in declarations before it's actually
defined, and also to move the typedef to another file.

Signed-off-by: Alejandro Colomar <alx@nginx.com>
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>

This macro is similar to C++'s static_cast().  It allows a very-limited
set of casts to be performed, but rejects most casts.  Basically, it
only allows converting from char to u_char and vice-versa, while
respecting the const qualifier.

See the _Generic(3) manual page for more details and examples.

Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

Test if _Generic(3) is available.  Although we require GNU C11, some
old compilers, like the one in CentOS 7 still don't have full support
for C11, and _Generic(3) is not available there.

Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

sizeof() should never be used to get the size of an array.  It is
very unsafe, since arrays easily decay to pointers, and sizeof()
applied to a pointer gives false results that compile and produce
silent bugs.

It's better to use nxt_items(), which implements sizeof()
division, which recent compilers warn when used with pointers.

This change would have avoided a bug that we almost introduced
recently by using:

    nxt_str_set(&port, (r->tls ? "https://" : "http://"));

which in the macro expansion runs:

    (&port)->length = nxt_length((r->tls ? : "https://" : "http://"));

which evaluates to:

    port.length = sizeof(r->tls ? "https://" : "http://") - 1;

which evaluates to:

    port.length = 8 - 1;

Of course, we didn't want a compile-time-constant 8 there, but
rather the length of the string.

Link: <https://stackoverflow.com/a/57537491>
Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

Suggested-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

Refs: https://github.com/nginx/unit/issues/778

The casts are unnecessary, since memchr(3)'s argument is 'const void *'.
It might have been necessary in the times of K&R, where 'void *' didn't
exist.  Nowadays, it's unnecessary, and _very_ unsafe, since casts can
hide all classes of bugs by silencing most compiler warnings.

The changes from nxt_memchr() to memchr(3) were scripted:

$ find src/ -type f \
  | grep '\.[ch]$' \
  | xargs sed -i 's/nxt_memchr/memchr/'

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>