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cgccassemblyinline-assemblyxen

What does %c mean in GCC inline assembly code?

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?

Solution

  • 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.