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Use offsetof for flexible struct allocation #390
Description
We are doing allocation of structs with flexible array members wrong. It should be:
palloc( offsetof(MyStruct, last_member) + num_items * sizeof(ArrayMember) );See https://github.com/postgres/postgres/blob/master/src/include/c.h#L342-L350.
The reason is that padding may give a different array start location compared to sizeof.
typedef struct MyStruct {
double x;
char y;
int z[];
} MyStruct;
int main(void) {
printf("sizeof: %lu\n", sizeof(MyStruct));
printf("offsetof: %lu\n", offsetof(MyStruct, z));
}The above prints:
sizeof: 16
offsetof: 12
Activity
cristianberneanu commentedon Jun 27, 2022
It doesn't look to me that we have a problem:
sizeof(MyStruct)is always greater than or equal tooffsetof(MyStruct, z), so, in the worst case, we just waste the padding bytes.edongashi commentedon Jun 27, 2022
You're right. For a moment I saw it the other way around. Nonetheless, it's a good small fix to have.