syscall 是 x86_64 上的指令吗？

Question

I wanted to check the code for performing system calls in glibc. I found something like this:

ENTRY (syscall)
    movq %rdi, %rax     /* Syscall number -> rax.  */
    movq %rsi, %rdi     /* shift arg1 - arg5.  */
    movq %rdx, %rsi
    movq %rcx, %rdx
    movq %r8, %r10
    movq %r9, %r8
    movq 8(%rsp),%r9    /* arg6 is on the stack.  */
    syscall         /* Do the system call.  */
    cmpq $-4095, %rax   /* Check %rax for error.  */
    jae SYSCALL_ERROR_LABEL /* Jump to error handler if error.  */
L(pseudo_end):
    ret         /* Return to caller.  */

Now my questions are:

1. Is syscall (before the cmpq instruction) an instruction?
2. If it is an instruction, what is the meaning of ENTRY (syscall) ? The same name for an ENTRY (I don't know what an ENTRY is) and instruction?
3. What is L(pseudo_end) ?

Answer 1

syscall is an instruction in x86-64, and is used as part of the ABI for making system calls . (The 32-bit ABI uses int 80h or sysenter , and is also available in 64-bit mode, but using the 32-bit ABI from 64-bit code is a bad idea, especially for calls with pointer arguments.)

But there is also a C library function named syscall(2) , a generic wrapper for the system-call ABI. Your code shows the dump of that function, including its decoding of the return value into errno -setting. ENTRY(syscall) just means that the function starts there.

L() and ENTRY() are CPP macros.

L(pseudo_end) is just a Label that can be a jump target. Maybe the code at SYSCALL_ERROR_LABEL jumps back to there, although it would be more efficient for that block of code to just ret , so maybe it's a relic from a former version, or used for something else.

Answer 2

Yes, syscall is an instruction on x86-64. There is a similar instruction sysenter on i686.

ENTRY(syscall) would be a macro. Probably expands to the symbol definition, you have to grep for that.