CODEWITHSUNDEEP
cgccsegmentation-faultinline-assembly
pugnator
pugnator

Reputation: 715

Using union/struct member as gcc inline asm argument

I have several struct/unions, for example: 

union
{
    uint8_t X;
    struct __attribute__((packed, aligned(1)))
    {
    uint8_t A  : 1;
    uint8_t B  : 1;
    uint8_t C  : 1;
    uint8_t D  : 1;
    uint8_t E  : 1;
    uint8_t F  : 1;
    uint8_t G  : 1;
    uint8_t H  : 1;
    };
}DATA;

I need to access their elements in in-line assembly, for example:

asm volatile
(
"\
    mov %1, %%eax       \t\n\
    inc %%eax           \t\n\
    mov $0, %0      \t\n\
"
:"=d"(u->X)
:"d"(temp)      
);

Application segfaults at the last line.

mov    (%rdx),%eax                     
inc    %eax                        
mov    $0x0,%dl                        
mov    %dl,0x1(%rax)

I tried to create a mirror pointer and access struct via it but it doesn't had an effect, just no segfault. So I use temp variable. Is it possible at all or I try to do a weird things?

Upvotes: 1

Views: 555

Answers (1)

Timothy Baldwin
Timothy Baldwin

Reputation: 3675

Inline assembly must not alter registers which are not listed as outputs or in the clobber list.

In your example the compiler has chosen to store u in RAX, which is altered by your code. EAX is the lower half of RAX.

Upvotes: 2

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