Re: [PATCH] git-compat-util.h: introduce CALLOC(x)
From: René Scharfe <hidden>
Date: 2022-12-05 21:03:42
Am 05.12.22 um 19:54 schrieb Taylor Blau:
When zero-initializing a struct without the use of static initializers
like "{ 0 }", it is common to write one of:
T *ptr = xcalloc(1, sizeof(T));
T *ptr = xcalloc(1, sizeof(*ptr));
These correctly initialize "*ptr" to the all-zeros value, but have a
couple of drawbacks. Notably, both initializations are verbose, but the
former is a foot-gun. If "*ptr"'s type changes to something other than
"T", the programmer has to remember to update not only the type of the
variable, but the right-hand side of its initialization, too.
In git.git, it is sometimes common to write something like:
T *ptr;
CALLOC_ARRAY(ptr, 1);
...but that is confusing, since we're not initializing an array. Rather,
we're using the CALLOC_ARRAY() macro to pretend like we are. But since
the array length is 1, the effect is zero initializing a single element.An object and a single-element array of objects allocated on the heap are indistinguishable. In that sense there is no confusion -- we are indeed allocating a single-element array. But if the intent is to only get one thing then having to fill in 1 in the bulk order form is a bit tedious, especially since this is the most common kind of request. A shortcut for the frequent case makes sense.
Introduce a new variant, CALLOC(x), which initializes "x" to the all-zeros value, without exposing the confusing use of CALLOC_ARRAY(), or the foot-guns available when using xcalloc() directly.
AFAIK the first "c" in "calloc" is for "continuous", not "zeroed". A single object is always contiguous, so the "C" in "CALLOC" is tautologic if read in that way. It fits our naming scheme for ALLOC_ARRAY and CALLOC_ARRAY, though, so that might not be much of a problem. And there are lots of occurrences of xmalloc(sizeof(T)) and xmalloc(sizeof(*ptr)) that would benefit from the automatic size inference of ALLOC_ARRAY -- an ALLOC macro would complement CALLOC nicely.
quoted hunk ↗ jump to hunk
Write a Coccinelle patch which codifies these rules, but mark it as "pending" since the resulting diff is too large to include in a single patch: $ git apply .build/contrib/coccinelle/xcalloc.pending.cocci.patch $ git diff --shortstat 89 files changed, 221 insertions(+), 178 deletions(-) Signed-off-by: Taylor Blau <redacted> --- This is a follow-up on [1], where introducing CALLOC(x) as the preferred alternative to CALLOC_ARRAY(x, 1) was first suggested. The patch is straightforward, and I am pretty sure that the Coccinelle rules are right, too ;-). But as a first-time Coccinelle user, any extra scrutiny on those bits would be appreciated. The main point of discussion I think is whether we should consider adopting this rule. I am biased, of course, but I think that we should. In any case, we should focus our efforts on polishing v2.39.0, but I wanted to send this out to the list before I forgot about it. [1]: https://lore.kernel.org/git/Y1MrXoobkVKngYL1@coredump.intra.peff.net/ (local) contrib/coccinelle/xcalloc.pending.cocci | 21 +++++++++++++++++++++ git-compat-util.h | 8 ++++++++ 2 files changed, 29 insertions(+) create mode 100644 contrib/coccinelle/xcalloc.pending.coccidiff --git a/contrib/coccinelle/xcalloc.pending.cocci b/contrib/coccinelle/xcalloc.pending.cocci new file mode 100644 index 0000000000..83e4ca1a68 --- /dev/null +++ b/contrib/coccinelle/xcalloc.pending.cocci@@ -0,0 +1,21 @@ +@@ +type T; +T *ptr; +@@ +- ptr = xcalloc(1, \( sizeof(T) \| sizeof(*ptr) \) ) ++ CALLOC(ptr) + +@@ +type T; +identifier ptr; +@@ +- T ptr = xcalloc(1, \( sizeof(T) \| sizeof(*ptr) \) ); ++ T ptr; ++ CALLOC(ptr);
This rule would turn this code: struct foo *bar = xcalloc(1, sizeof(*bar)); int i; ... into: struct foo *bar; CALLOC(bar); int i; ... which violates the coding guideline to not mix declarations and statements (-Wdeclaration-after-statement).
quoted hunk ↗ jump to hunk
+ +@@ +type T; +T *ptr; +@@ +- CALLOC_ARRAY(ptr, 1) ++ CALLOC(ptr)diff --git a/git-compat-util.h b/git-compat-util.h index a76d0526f7..827e5be89c 100644 --- a/git-compat-util.h +++ b/git-compat-util.h@@ -1107,6 +1107,14 @@ static inline void move_array(void *dst, const void *src, size_t n, size_t size) memmove(dst, src, st_mult(size, n)); } +/* + * Like CALLOC_ARRAY, but the argument is treated as a pointer to a + * single struct. + * + * Preferable over xcalloc(1, sizeof(...)), or CALLOC_ARRAY(..., 1). + */ +#define CALLOC(x) (x) = CALLOC_ARRAY((x), 1) + /* * These functions help you allocate structs with flex arrays, and copy * the data directly into the array. For example, if you had: --2.38.0.16.g393fd4c6db