golang syscall 系统调用认知

1 本文整体结构

C语言中syscall如何使用？
golang中如何使用？
syscall 手册

2 C语言中syscall如何使用？

        #define _GNU_SOURCE#include <unistd.h>#include <sys/syscall.h>#include <sys/types.h>#include <signal.h>int main(int argc, char *argv[]){pid_t tid;tid = syscall(SYS_gettid);syscall(SYS_tgkill, getpid(), tid, SIGHUP);}

执行结果：fish: Job 1, './a.out' terminated by signal SIGHUP (Terminal hung up)

3 golang中如何使用

// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDITfunc EpollCtl(epfd int, op int, fd int, event *EpollEvent) (err error) {_, _, e1 := RawSyscall6(SYS_EPOLL_CTL, uintptr(epfd), uintptr(op), uintptr(fd), uintptr(unsafe.Pointer(event)), 0, 0)if e1 != 0 {err = errnoErr(e1)}return
}

func syscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
func syscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
func syscall6X(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
func rawSyscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
func rawSyscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
func syscallPtr(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)

Syscall在开始和结束的时候，会分别调用runtime中的进入系统和退出系统的函数，所以Syscall是受调度器控制的，因为调度器有开始和结束的的事件。而RawSyscall则不受调度器控制，
RawSyscall 可能会导致其他正在运行的线程（协程）阻塞，调度器可能会在一段时间后运行它们，但是也有可能不会。所以，我们在进行系统调用的时候，应该极力避免使用RawSyscall，除非你确定这个操作是非阻塞的。

4 syscall 手册

SYSCALL(2)                                                                      Linux Programmer's Manual                                                                      SYSCALL(2)NAMEsyscall - indirect system callSYNOPSIS#define _GNU_SOURCE         /* See feature_test_macros(7) */#include <unistd.h>#include <sys/syscall.h>   /* For SYS_xxx definitions */long syscall(long number, ...);DESCRIPTIONsyscall()  is  a small library function that invokes the system call whose assembly language interface has the specified number with the specified arguments.  Employing syscall()is useful, for example, when invoking a system call that has no wrapper function in the C library.syscall() saves CPU registers before making the system call, restores the registers upon return from the system call, and stores any error code returned by  the  system  call  inerrno(3) if an error occurs.Symbolic constants for system call numbers can be found in the header file <sys/syscall.h>.RETURN VALUEThe  return value is defined by the system call being invoked.  In general, a 0 return value indicates success.  A -1 return value indicates an error, and an error code is storedin errno.NOTESsyscall() first appeared in 4BSD.Architecture-specific requirementsEach architecture ABI has its own requirements on how system call arguments are passed to the kernel.  For system calls that have a glibc wrapper (e.g., most system calls), glibchandles  the details of copying arguments to the right registers in a manner suitable for the architecture.  However, when using syscall() to make a system call, the caller mightneed to handle architecture-dependent details; this requirement is most commonly encountered on certain 32-bit architectures.For example, on the ARM architecture Embedded ABI (EABI), a 64-bit value (e.g., long long) must be aligned to an even register pair.  Thus, using syscall() instead of the wrapperprovided by glibc, the readahead() system call would be invoked as follows on the ARM architecture with the EABI in little endian mode:syscall(SYS_readahead, fd, 0,(unsigned int) (offset & 0xFFFFFFFF),(unsigned int) (offset >> 32),count);Since  the  offset  argument  is 64 bits, and the first argument (fd) is passed in r0, the caller must manually split and align the 64-bit value so that it is passed in the r2/r3register pair.  That means inserting a dummy value into r1 (the second argument of 0).  Care also must be taken so that the split follows endian conventions (according to  the  CABI for the platform).Similar issues can occur on MIPS with the O32 ABI, on PowerPC with the 32-bit ABI, and on Xtensa.Note that while the parisc C ABI also uses aligned register pairs, it uses a shim layer to hide the issue from userspace.The affected system calls are fadvise64_64(2), ftruncate64(2), posix_fadvise(2), pread64(2), pwrite64(2), readahead(2), sync_file_range(2), and truncate64(2).This  does  not  affect syscalls that manually split and assemble 64-bit values such as _llseek(2), preadv(2), preadv2(2), pwritev(2).  and pwritev2(2).  Welcome to the wonderfulworld of historical baggage.Architecture calling conventionsEvery architecture has its own way of invoking and passing arguments to the kernel.  The details for various architectures are listed in the two tables below.The first table lists the instruction used to transition to kernel mode (which might not be the fastest or best way to transition to the kernel, so you might  have  to  refer  tovdso(7)), the register used to indicate the system call number, the register used to return the system call result, and the register used to signal an error.arch/ABI    instruction           syscall #  retval  error    Notes────────────────────────────────────────────────────────────────────alpha       callsys               v0         a0      a3       [1]arc         trap0                 r8         r0      -arm/OABI    swi NR                -          a1      -        [2]arm/EABI    swi 0x0               r7         r0      -arm64       svc #0                x8         x0      -blackfin    excpt 0x0             P0         R0      -i386        int $0x80             eax        eax     -ia64        break 0x100000        r15        r8      r10      [1]m68k        trap #0               d0         d0      -microblaze  brki r14,8            r12        r3      -mips        syscall               v0         v0      a3       [1]nios2       trap                  r2         r2      r7parisc      ble 0x100(%sr2, %r0)  r20        r28     -powerpc     sc                    r0         r3      r0       [1]s390        svc 0                 r1         r2      -        [3]s390x       svc 0                 r1         r2      -        [3]superh      trap #0x17            r3         r0      -        [4]sparc/32    t 0x10                g1         o0      psr/csr  [1]sparc/64    t 0x6d                g1         o0      psr/csr  [1]tile        swint1                R10        R00     R01      [1]x86_64      syscall               rax        rax     -        [5]x32         syscall               rax        rax     -        [5]xtensa      syscall               a2         a2      -Notes:[1] On a few architectures, a register is used as a boolean (0 indicating no error, and -1 indicating an error) to signal that the system call failed.  The actual error valueis still contained in the return register.  On sparc, the carry bit (csr) in the processor status register (psr) is used instead of a full register.[2] NR is the system call number.[3] For s390 and s390x, NR (the system call number) may be passed directly with svc NR if it is less than 256.[4] On SuperH, the trap number controls the maximum number of arguments passed.  A trap #0x10 can be used with only 0-argument system calls, a trap #0x11 can be used with  0-or 1-argument system calls, and so on up to trap #0x17 for 7-argument system calls.[5] The x32 ABI uses the same instruction as the x86_64 ABI and is used on the same processors.  To differentiate between them, the bit mask __X32_SYSCALL_BIT is bitwise-ORedinto the system call number for system calls under the x32 ABI.  Both system call tables are available though, so setting the bit is not a hard requirement.The second table shows the registers used to pass the system call arguments.arch/ABI      arg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes──────────────────────────────────────────────────────────────alpha         a0    a1    a2    a3    a4    a5    -arc           r0    r1    r2    r3    r4    r5    -arm/OABI      a1    a2    a3    a4    v1    v2    v3arm/EABI      r0    r1    r2    r3    r4    r5    r6arm64         x0    x1    x2    x3    x4    x5    -blackfin      R0    R1    R2    R3    R4    R5    -i386          ebx   ecx   edx   esi   edi   ebp   -ia64          out0  out1  out2  out3  out4  out5  -m68k          d1    d2    d3    d4    d5    a0    -microblaze    r5    r6    r7    r8    r9    r10   -mips/o32      a0    a1    a2    a3    -     -     -     [1]mips/n32,64   a0    a1    a2    a3    a4    a5    -nios2         r4    r5    r6    r7    r8    r9    -parisc        r26   r25   r24   r23   r22   r21   -powerpc       r3    r4    r5    r6    r7    r8    r9s390          r2    r3    r4    r5    r6    r7    -s390x         r2    r3    r4    r5    r6    r7    -superh        r4    r5    r6    r7    r0    r1    r2sparc/32      o0    o1    o2    o3    o4    o5    -sparc/64      o0    o1    o2    o3    o4    o5    -tile          R00   R01   R02   R03   R04   R05   -x86_64        rdi   rsi   rdx   r10   r8    r9    -x32           rdi   rsi   rdx   r10   r8    r9    -xtensa        a6    a3    a4    a5    a8    a9    -Notes:[1] The mips/o32 system call convention passes arguments 5 through 8 on the user stack.Note that these tables don't cover the entire calling convention—some architectures may indiscriminately clobber other registers not listed here.

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