[英]What does %c mean in GCC inline assembly code?
I am trying to understand this inline assembly code which comes from _hypercall0
here . 我想了解哪些来源于此联汇编代码_hypercall0
这里 。
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. 我无法找到第一行中%c
的含义的信息。 I did not find any information in the most obvious section of the GCC manual , which explains %[name]
, but not %c[name]
. 我没有在GCC手册中最明显的部分找到任何信息,这解释了%[name]
,但没有找到%c[name]
。 Is there any other place I should look at? 还有其他我应该看的地方吗?
From the GCC internals documentation : 来自GCC内部文件 :
`%c digit ' can be used to substitute an operand that is a constant value without the syntax that normally indicates an immediate operand. `%c digit '可用于替换常量值的操作数,而不使用通常表示立即操作数的语法。
Check the assembly output (with gcc -S
, or maybe disassemble the object file) and it may be clearer. 检查程序集输出(使用gcc -S
,或者可能反汇编目标文件),它可能更清晰。
My guess is that it stands for constant . 我的猜测是它代表不变 。 hypercall_page
looks like a table of instructions that each do a syscall. hypercall_page
看起来像是一个指令表,每个指令执行系统调用。 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. 也许这会产生一个call hypercall_page + {constant based on the expression given}
,基本上在编译时计算了这个偏移的地址。
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. __HYPERVISOR##name
一下,这个__HYPERVISOR##name
真的让我想起了你在Linux的<asm / unistd.h>和类似的地方看到的系统调用的__NR_name_of_syscall
类型约定。
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