how are the passing arguments in amd64?

Question

I want to know how the arguments are passed when we call a libc function in C on amd64.

For example, if I call sqrt(double), is the argument passed by the stack or by a register?

Answer 1

Lets write a sample usecase (test.c):

#include <math.h>

int main()
{
    double val = sqrt(foo(8));
    return 0;
}

double foo(double val)
{
    val+=1;
    return val;
}

Compile it:

gcc test.c -lm

And then check imported symbols:

readelf -Wa a.out |grep sqrt

    4: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND sqrt@GLIBC_2.2.5 (3)
   63: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND sqrt@@GLIBC_2.2.5

Disassemble the object:

objdump -d a.out

0000000000400582 <main>:
  400582:       55                      push   %rbp
  400583:       48 89 e5                mov    %rsp,%rbp
  400586:       48 83 ec 10             sub    $0x10,%rsp
  40058a:       f2 0f 10 05 26 01 00    movsd  0x126(%rip),%xmm0
  400591:       00 
  400592:       e8 bd ff ff ff          callq  400554 <foo>
  400597:       66 0f 28 c8             movapd %xmm0,%xmm1
  40059b:       f2 0f 51 c1             sqrtsd %xmm1,%xmm0
  40059f:       66 0f 2e c0             ucomisd %xmm0,%xmm0
  4005a3:       7a 06                   jp     4005ab <main+0x29>
  4005a5:       66 0f 2e c0             ucomisd %xmm0,%xmm0
  4005a9:       74 09                   je     4005b4 <main+0x32>
  4005ab:       66 0f 28 c1             movapd %xmm1,%xmm0
  4005af:       e8 ac fe ff ff          callq  400460 <sqrt@plt>
  4005b4:       f2 0f 11 45 f8          movsd  %xmm0,-0x8(%rbp)
  4005b9:       b8 00 00 00 00          mov    $0x0,%eax
  4005be:       c9                      leaveq 
  4005bf:       c3                      retq

So you can see that the argument for sqrt is passed by the %xmm0 register.

Answer 2

I've recently created a tool to visualize stack frames of functions, registers usage, memory layout of structured types and other ABI information. You can look at the result for libssh library function ssh_channel_read_timeout on x86_64 here.