RobertJoseph RobertJoseph - 3 months ago 10
C Question

Compile c source with 'asm' directive

I'm trying to compile this c source file using cl.exe (from Visual Studio). The specific code in question that fails to compile is:

#include <stdio.h>

static inline void native_cpuid(unsigned int *eax, unsigned int *ebx,
unsigned int *ecx, unsigned int *edx)
{
/* ecx is often an input as well as an output. */
asm volatile("cpuid"
: "=a" (*eax),
"=b" (*ebx),
"=c" (*ecx),
"=d" (*edx)
: "0" (*eax), "2" (*ecx));
}


I'm seeing this failure:

C:\>cl sgx.c
Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24213.1 for x86
Copyright (C) Microsoft Corporation. All rights reserved.

sgx.c
sgx.c(7): error C2065: 'asm': undeclared identifier
sgx.c(7): error C2143: syntax error: missing ';' before 'volatile'

Answer

Instead of direct assembly, you can use compiler intrinsics. Microsoft compilers support __cpuid() documented here: https://msdn.microsoft.com/en-us/library/hskdteyh.aspx

GCC instead supports __get_cpuid() via cpuid.h as described by this answer (http://stackoverflow.com/a/14266932/1401351).

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