问题
I am trying to understand this inline assembly code which comes from _hypercall0 here.
asm volatile ("call hypercall_page+%c[offset]" \
: "=r" (__res) \
: [offset] "i" (__HYPERVISOR_##name * sizeof(hypercall_page[0])) \
: "memory", "edi", "esi", "edx", "ecx", "ebx", "eax")
I am having trouble finding information on what %c in the first line means. I did not find any information in the most obvious section of the GCC manual, which explains %[name], but not %c[name]. Is there any other place I should look at?
回答1:
From the GCC internals documentation:
`%cdigit' can be used to substitute an operand that is a constant value without the syntax that normally indicates an immediate operand.
回答2:
Check the assembly output (with gcc -S, or maybe disassemble the object file) and it may be clearer.
My guess is that it stands for constant. hypercall_page looks like a table of instructions that each do a syscall. Maybe this will generate a call hypercall_page + {constant based on the expression given}, essentially having computed the address of this offset at compile time.
As an aside, this __HYPERVISOR##name stuff really reminds me of the __NR_name_of_syscall type convention you see for syscalls in Linux's <asm/unistd.h> and similar places.
来源：https://stackoverflow.com/questions/1672900/what-does-c-mean-in-gcc-inline-assembly-code