Anton Blanchard wrote:
>Hi,
>
>I was unable to boot 2.5-BK on ppc64 and narrowed it down to the fast
>poll patch. I found:
>
>offsetof(struct poll_list, entries) == 12 but
>sizeof(struct poll_list) == 16
>
>This means pp+1 did not match up with pp->entries. Im not sure what the
>alignment requirements are for a zero length struct (ie is this a
>compiler bug) but the following patch fixes the problem and also changes
>->len to a long to ensure 8 byte alignment of ->entries on 64bit archs.
>
>
That seems to be the root of the problem: ->entries requires 4 byte
alignment, while struct poll_list requires 8 byte alignment. Thus the
compiler aligns entries to a 4-byte boundary, and rounds up
sizeof(struct poll_list) to 16.
Attached is a small test case that shows the problem on both 64-bit and
32-bit platforms. I'm not sure if this is a gcc bug: the Compaq C
compiler on Tru64 and MSVC on Windows generate the same structure layout.
--
Manfred
/* * Testcase for a possible compiler bug: * structure with flexible array member. * offsetof(struct,flexible_member) != sizeof(struct). * * That seems to be a violation of 6.7.2.1, constraint 16 of the C99 * standard: * "First, the size of the structure shall be equal to the offset of the * last element of an otherwise identical structure that replaces the * flexible array member with an array of unspecified length. */ #include <stdio.h>
/* * structure: sizeof() = 4. * alignment: 2, due to the short. */ struct pollfd { short fd; char events; char revents; };
/* * sizeof: 12 * alignment: 4, due to the int. */ struct n2 { /* offset 0 */ int m; /* offset 4 */ char n; /* one byte padding, 'struct pollfd' requires 2 byte alignment */ /* offset 6 */ struct pollfd a[1]; /* offset 10: two bytes padding for 4 byte alignment, * required for the int in this structure */ };
/* * sizeof: 8. * XXX * Is this correct? * offsetof(struct n1, a) is 6 * --> offsetof(struct n1, a) != sizeof(struct n1) * XXX */ struct n1 { /* offset 0 */ int m; /* offset 4 */ char n; /* one byte padding, 'struct pollfd' required 2 byte alignment */ /* offset 6 */ struct pollfd a[]; }; #define offsetof(a,b) \ (int)&((a*)NULL)->b
int main(void) { printf("pollfd: sizeof: %d.\n", sizeof(struct pollfd)); printf("fix: sizeof: %d, offsetof: %d.\n", sizeof(struct n2), offsetof(struct n2, a)); printf("var: sizeof: %d, offsetof: %d.\n", sizeof(struct n1), offsetof(struct n1, a)); return 0; }
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This archive was generated by hypermail 2b29 : Tue Dec 31 2002 - 22:00:09 EST