Alignment describes the byte positions at which an object of a type can start. Objects of many noncharacter types cannot start at an arbitrary byte; they typically require an address suitable for their type. section 12.7
Structure members may need to start at particular positions, such as word boundaries. The compiler can insert byte padding between members to achieve this; there is no padding at the beginning of a structure. section 6.3.4
A pointer cast does not make an address suitably aligned for the target type. Converting a pointer to a differently typed pointer when it is not properly aligned is invalid; misaligned access can also cause a bus error on some systems. section 12.7 section 15.1.2
int main(void) {
enable_alignment_check();
/* An overlay of complex values and bytes. */
union {
cdbl val[2];
unsigned char buf[sizeof(cdbl[2])];
} toocomplex = {
.val = { 0.5 + 0.5*I, 0.75 + 0.75*I, },
};
printf("size/alignment: %zu/%zu\n",
sizeof(cdbl), alignof(cdbl));
/* Run over all offsets, and crash on misalignment. */
for (size_t offset = sizeof(cdbl); offset; offset /=2) {
printf("offset\t%zu:\t", offset);
fflush(stdout);
cdbl* bp = (cdbl*)(&toocomplex.buf[offset]); // align!
printf("%g\t+%gI\t", creal(*bp), cimag(*bp));
fflush(stdout);
*bp *= *bp;
printf("%g\t+%gI", creal(*bp), cimag(*bp));
fputc('\n', stdout);
}
}
This program overlays complex values with bytes, then casts pointers at progressively smaller offsets to demonstrate the consequences of misalignment. section 12.7
C's alignof and alignas notation appears in declarations that inspect a type's alignment or request alignment for an object. section 12.7
