The problem isn’t solvable in general; that’s because the comment, // call the function & handle the return value. use __stdcall calling convention indicates reliance on 32bit calling conventions. In 32bit x86, stdcall means that all arguments are passed on the stack, in reverse order (i.e. the last arg is pushed first. That is, if … Read more
__asm__ itself does not return a value. C standard does not define how __asm__ should handle the return value, so the behavior might be different between compilers. You stated that Visual Studio example is valid, but Visual Studio uses __asm. __asm__ is used at least by GCC. Visual Studio To get the result in a … Read more
Well, you don’t say specifically, but by your post, it appears like you’re using gcc and its inline asm with constraints syntax (other C compilers have very different inline syntax). That said, you probably need to use AT&T assembler syntax rather than Intel, as that’s what gets used with gcc. So with the above said, … Read more
I always find some examples on Internet that the guy manually insert a tab and new-line instead of \t and \n, however it doesn’t work for me. Not very sure if your example even compile.. but this is how I do: asm volatile( // note the backslash line-continuation “xor %eax,%eax \n\t\ mov $0x7c802446, %ebx \n\t\ … Read more
Quoting from Intel® 64 and IA-32 Architectures Software Developer Manuals 3-650 Vol. 2A on moving to and from control registers: This instruction can be executed only when the current privilege level is 0. Which means the instruction can only be executed in kernel mode. A minimal kernel module, that logs the contents of cr0, cr2 … Read more
See the “Extended Asm” page in the GCC documentation. You can prevent an asm instruction from being deleted by writing the keyword volatile after the asm. […] The volatile keyword indicates that the instruction has important side-effects. GCC will not delete a volatile asm if it is reachable. and An asm instruction without any output … Read more
The problem with using GCC’s inline asm to learn assembly is that you spend half your time learning about how gcc’s inline assembly works instead of actually learning assembly. For example here’s how I might write this same code: #include <stdio.h> int getStringLength(const char *pStr){ int len; __asm__ ( “repne scasb\n\t” “not %%ecx\n\t” “dec %%ecx” … Read more
Compile with gcc -masm=intel and don’t try to switch modes inside the asm template string. AFAIK there’s no equivalent before clang14 (Note: MacOS installs clang as gcc / g++ by default.) Also, of course you need to use valid GNU C inline asm, using operands to tell the compiler which C objects you want to … Read more
The problems you’d see from failing to tell the compiler about registers you modify would be exactly the same as if you wrote a function in asm that modified some call-preserved registers1. See more explanation and a partial example in Why should certain registers be saved? What could go wrong if not? In GNU inline-asm, … Read more
From your original question I did not realize gcc limited red-zone use to leaf functions. I don’t think that’s required by the x86_64 ABI, but it is a reasonable simplifying assumption for a compiler. In that case you only need to make the function calling your assembly routine a non-leaf for purposes of compilation: int … Read more