CODEWITHSUNDEEP
cassemblygccinline-assemblyatt
perror undefined variable un
perror undefined variable un

Reputation: 47

Bad Instruction - Inline Assembly Language in C Code

I'm trying to exit a program with assembly instructions, but when I compile with gcc it says that mov is a bad instruction, even when I use movl which I don't even know what it is. Is it even possible to exit a program with assembly instructions?

int main(void)
{
    __asm__("movl %rax, $60\n\t"
        "movl %rdi, $0\n\t"
        "syscall\n");
}
// cc main.c -o main && ./main

Upvotes: 3

Views: 771

Answers (1)

Joshua
Joshua

Reputation: 43337

You need movq for 64 bit. Also, your operations are not in the correct order.

The following compiles:

int main(void)
{
    __asm__("movq $60, %rax\n\t"
        "movq $0, %rdi\n\t"
        "syscall\n");
}

Note that for any other system call (which doesn't terminate the whole program), it's necessary to tell the compiler which registers are clobbered, and usually to use a "memory" clobber to make sure memory is in sync with C values before a system call reads or writes memory.

Also, to pass operands, you'll need Extended asm syntax. See How to invoke a system call via sysenter in inline assembly? for an example my_write wrapper. (Which has only "syscall" inside the asm template; we ask the compiler to put the call number and args in the right registers instead of writing mov)

Upvotes: 3

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