http://llvm.org/bugs/show_bug.cgi?id=18132
Bug ID: 18132
Summary: -Wpacked warns even though packed structure has
different size/memory layout
Product: clang
Version: 3.3
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Frontend
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Consider the following rather simple code snippet:
----- packed.c ----
#include <stdint.h>
#include <stdio.h>
struct S1 {
uint16_t addr;
uint32_t value;
};
struct S2 {
uint16_t addr;
uint32_t value;
} __attribute__((packed));
int main (int argc, char ** argv) {
struct S1 s1;
struct S2 s2;
printf("%zu - %zu\n", sizeof(s1), sizeof(s2));
return 0;
};
-------------------
Compiling and running this code should produce the following output on a 32/64
bit system:
8 - 6
(I think it may be 6 - 6 on a 16 bit system)
So whether the struct is packed or not *DOES* make a difference here. Yet,
compiling the code above using "-Wpacked", I get the following:
packed.c:10:11: warning: packed attribute is unnecessary for 'addr' [-Wpacked]
uint16_t addr;
^
This doesn't seem correct to me. As I understand "-Wpacked", it should only
produce a warning if the presence of the packed attribute has no influence on
the structure size and thus also no influence on the relative memory layout of
the fields within the structure.
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