What is the difference between 'asm', '__asm' and '__asm__'?

Question

As far as I can tell, the only difference between __asm { ... }; and __asm__(\"...\"); is that the first uses mov eax, var and the sec

Answer 1

asm vs __asm__ in GCC

asm does not work with -std=c99, you have two alternatives:

• use __asm__
• use -std=gnu99

More details: error: ‘asm’ undeclared (first use in this function)

__asm vs __asm__ in GCC

I could not find where __asm is documented (notably not mentioned at https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Alternate-Keywords.html#Alternate-Keywords ), but from the GCC 8.1 source they are exactly the same:

  { "__asm",        RID_ASM,    0 },
  { "__asm__",      RID_ASM,    0 },

so I would just use __asm__ which is documented.