A misaligned cast
A byte buffer is cast to complex double* at several offsets. Access works at offsets 16 and 8, but offset 4 produces a bus error: the address does not satisfy complex double's alignment requirement. section 12.7
1 #include <stdio.h>
2 #include <inttypes.h>
3 #include <complex.h>
4 #include "crash.h"
5
6 void enable_alignment_check(void);
7 typedef complex double cdbl;
8
9 int main(void) {
10 enable_alignment_check();
11 /* An overlay of complex values and bytes. */
12 union {
13 cdbl val[2];
14 unsigned char buf[sizeof(cdbl[2])];
15 } toocomplex = {
16 .val = { 0.5 + 0.5*I, 0.75 + 0.75*I, },
17 };
18 printf("size/alignment: %zu/%zu\n",
19 sizeof(cdbl), _Alignof(cdbl));
20 /* Run over all offsets, and crash on misalignment. */
21 for (size_t offset = sizeof(cdbl); offset; offset /=2) {
22 printf("offset\t%zu:\t", offset);
23 fflush(stdout);
24 cdbl* bp = (cdbl*)(&toocomplex.buf[offset]); // align!
25 printf("%g\t+%gI\t", creal(*bp), cimag(*bp));
26 fflush(stdout);
27 *bp *= *bp;
28 printf("%g\t+%gI", creal(*bp), cimag(*bp));
29 fputc('\n', stdout);
30 }
31 }
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Inspect the complete program that overlays complex values with bytes, prints size and alignment, and tests progressively smaller offsets. section 12.7
Alignment is the set of byte positions at which an object of a type may start. Most non-character types cannot begin at an arbitrary byte position; they commonly require a word boundary. Casting a character pointer to an object pointer is therefore unsafe when the resulting address is misaligned. section 12.7
TAKEAWAY
- Different architectures tolerate misalignment differently; code that appears to work on i386 can fail elsewhere. section 12.7
- Not every unsigned-char array can safely represent an object of every other type. section 12.8
- The alignas declaration specifier can request stronger alignment for objects such as complex double, float arrays, or unsigned-char arrays. section 12.7
