Matthieu Moy <matthieu....@grenoble-inp.fr> writes:
> William Duclot <william.duc...@ensimag.grenoble-inp.fr> writes:
>
>> @@ -20,16 +28,37 @@ char strbuf_slopbuf[1];
>>
>> void strbuf_init(struct strbuf *sb, size_t hint)
>> {
>> + sb->flags = 0;
>> sb->alloc = sb->len = 0;
>> sb->buf = strbuf_slopbuf;
>> if (hint)
>> strbuf_grow(sb, hint);
>> }
>
> If you set flags = 0 here, existing callers will have all flags off,
> including OWNS_MEMORY.
>
> I *think* this is OK, as sb->buf is currently pointing to
> strbuf_slopbuf, which the the strbuf doesn't own. But that is too subtle
> to go without an explanatory comment IMHO.
Right
>
> Also, doesn't this make the "new_buf" case useless in strbuf_grow?
>
> With your patch, the code looks like:
>
> void strbuf_grow(struct strbuf *sb, size_t extra)
> {
> int new_buf = !sb->alloc;
> ...
> if (sb->flags & STRBUF_OWNS_MEMORY) {
> if (new_buf) // <---------------------------------------- (1)
> sb->buf = NULL;
> ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
> } else {
> /*
> * The strbuf doesn't own the buffer: to avoid to realloc it,
> * the strbuf needs to use a new buffer without freeing the old
> */
> if (sb->len + extra + 1 > sb->alloc) {
> size_t new_alloc = MAX(sb->len + extra + 1,
> alloc_nr(sb->alloc));
> char *buf = xmalloc(new_alloc);
> memcpy(buf, sb->buf, sb->alloc);
> sb->buf = buf;
> sb->alloc = new_alloc;
> sb->flags |= STRBUF_OWNS_MEMORY;
> }
> }
>
> if (new_buf) // <---------------------------------------- (2)
> sb->buf[0] = '\0';
> }
>
> I think (1) is now dead code, since sb->alloc == 0 implies that
> STRBUF_OWNS_MEMORY is set. (2) seems redundant since you've just
> memcpy-ed the existing '\0' into the buffer.
You're right for (1), I hadn't noticed that.
For (2), we'll still have to set sb->buf[new_alloc-1]='\0' after the memcpy, if
we
have sb->alloc==0 then the memcpy won't copy it.
>> +void strbuf_wrap_preallocated(struct strbuf *sb, char *path_buf,
>> + size_t path_buf_len, size_t alloc_len)
>> +{
>> + if (!path_buf)
>> + die("you try to use a NULL buffer to initialize a strbuf");
>> +
>> + strbuf_init(sb, 0);
>> + strbuf_attach(sb, path_buf, path_buf_len, alloc_len);
>> + sb->flags &= ~STRBUF_OWNS_MEMORY;
>> + sb->flags &= ~STRBUF_FIXED_MEMORY;
>> +}
>
> strbuf_wrap_preallocated seem very close to strbuf_attach. I'd rather
> see a symmetric code sharing like
>
> void strbuf_attach_internal(struct strbuf *sb, ..., unsigned int flags)
>
> and then strbuf_attach() and strbuf_wrap_preallocated() become
> straightforward wrappers around it.
>
> This would avoid setting and then unsetting STRBUF_OWNS_MEMORY (the
> performance impact is probably small, but it looks weird to set the flag
> and then unset it right away).
We'll refactor the code with Johannes' remarks and yours in mind
> After your patch, there are differences between
> strbuf_wrap_preallocated() which I think are inconsistencies:
>
> * strbuf_attach() does not check for NULL buffer, but it doesn't accept
> them either if I read correctly. It would make sense to add the check
> to strbuf_attach(), but it's weird to have the performance-critical
> oriented function do the expensive stuff that the
> non-performance-critical one doesn't.
I agree that strbuf_attach should do the check (it seems strange that it
doesn't already do it, as the "buffer never NULL" invariant is not new).
I don't understand your "but" part, what "expensive stuff" are you talking
about?
> * strbuf_attach() calls strbuf_release(), which allows reusing an
> existing strbuf. strbuf_wrap_preallocated() calls strbuf_init which
> would override silently any previous content. I think strbuf_attach()
> does the right thing here.
>
> In any case, you probably want to include calls to strbuf_attach() and
> strbuf_wrap_*() functions on existing non-empty strbufs.
>
>> +void strbuf_wrap_fixed(struct strbuf *sb, char *path_buf,
>> + size_t path_buf_len, size_t alloc_len)
>> +{
>> + strbuf_wrap_preallocated(sb, path_buf, path_buf_len, alloc_len);
>> + sb->flags |= STRBUF_FIXED_MEMORY;
>> +}
>
> And this could become a 3rd caller of strbuf_attach_internal().
True. We'll take care of that when refactoring
>> @@ -61,9 +96,32 @@ void strbuf_grow(struct strbuf *sb, size_t extra)
>> if (unsigned_add_overflows(extra, 1) ||
>> unsigned_add_overflows(sb->len, extra + 1))
>> die("you want to use way too much memory");
>> - if (new_buf)
>> - sb->buf = NULL;
>> - ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
>> + if ((sb->flags & STRBUF_FIXED_MEMORY) && sb->len + extra + 1 >
>> sb->alloc)
>> + die("you try to make a string overflow the buffer of a fixed
>> strbuf");
>> +
>> + /*
>> + * ALLOC_GROW may do a realloc() if needed, so we must not use it on
>> + * a buffer the strbuf doesn't own
>> + */
>> + if (sb->flags & STRBUF_OWNS_MEMORY) {
>> + if (new_buf)
>> + sb->buf = NULL;
>> + ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
>> + } else {
>> + /*
>> + * The strbuf doesn't own the buffer: to avoid to realloc it,
>> + * the strbuf needs to use a new buffer without freeing the old
>> + */
>> + if (sb->len + extra + 1 > sb->alloc) {
>> + size_t new_alloc = MAX(sb->len + extra + 1,
>> alloc_nr(sb->alloc));
>> + char *buf = xmalloc(new_alloc);
>> + memcpy(buf, sb->buf, sb->alloc);
>
> I think you want to memcpy only sb->len + 1 bytes. Here, you're
> memcpy-ing the allocated-but-not-initialized part of the array.
>
> xmemdupz can probably simplify the code too (either you include the '\0'
> in what memcpy copies, or you let xmemdupz add it).
memcpying sb->len+1 bytes is indeed enough.
xmemdupz can only allocate the same size it will copy. Here we want to
allocate `new_alloc` bytes but only copy `sb->alloc` bytes (or sb->len+1
bytes, as you noticed)
>> +/**
>> + * Allow the caller to give a pre-allocated piece of memory for the strbuf
>> + * to use. It is possible then to strbuf_grow() the string past the size
>> of the
>> + * pre-allocated buffer: a new buffer will be allocated. The pre-allocated
>
> To make it clearer: "a new buffer will then be allocated"?
OK
>> +/**
>> + * Allow the caller to give a pre-allocated piece of memory for the strbuf
>> + * to use and indicate that the strbuf must use exclusively this buffer,
>> + * never realloc() it or allocate a new one. It means that the string can
>> + * be manipulated but cannot overflow the pre-allocated buffer. The
>> + * pre-allocated buffer will never be freed.
>> + */
>
> Perhaps say explicitly that although the allocated buffer has a fixed
> size, the string itself can grow as long as it does not overflow the
> buffer?
That's what I meant by "the string can be manipulated but cannot overflow
the pre-allocated buffer". I'll try to reformulate
>> @@ -91,6 +116,8 @@ extern void strbuf_release(struct strbuf *);
>> * Detach the string from the strbuf and returns it; you now own the
>> * storage the string occupies and it is your responsibility from then on
>> * to release it with `free(3)` when you are done with it.
>> + * Must allocate a copy of the buffer in case of a preallocated/fixed
>> buffer.
>> + * Performance-critical operations have to be aware of this.
>
> Better than just warn about performance, you can give the alternative.
I'm not sure what you mean, I don't think there really is an alternative for
detaching a string?
--
To unsubscribe from this list: send the line "unsubscribe git" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
