场景如下

    1. package main
    2. import "fmt"
    3. /*
    4. #define JUMP_PARENT 0x00
    5. #define JUMP_CHILD  0xA0
    6. #define _GNU_SOURCE
    7. #include <stdio.h>
    8. #include <stdlib.h>
    9. #include <unistd.h>
    10. #include <sched.h>
    11. #include <setjmp.h>
    13. char child_stack[4096] __attribute__ ((aligned(16)));
    15. int child_func(void *arg) {
    16.     jmp_buf* env  = (jmp_buf*)arg;
    17.        longjmp(*env, JUMP_CHILD);
    18. }
    20. __attribute__((constructor)) void init(void) {
    21.     printf("init...\n");
    22.     jmp_buf env;
    23.     switch(setjmp(env)) {
    24.         case JUMP_PARENT: 
    25.         printf("JUMP_PARENT\n");
    26.             int child_pid = clone(child_func, child_stack, CLONE_PARENT, env);
    27.             printf("CHILD_PID: %d\n", child_pid);
    28.             exit(0);
    29.         case JUMP_CHILD:
    30.             printf("JUMP_CHILD\n");
    31.             return;
    32.     }
    33. }
    34. */
    35. import "C"
    37. func main() {
    38.     fmt.Println("main...")
    39. }

    使用Cgo在进入Go runtime之前先执行一段C代码。正常情况下child进程应该打印完JUMP_CHILD后开始执行main函数。
    但是在执行完init函数后，无法回到main函数，child进程会阻塞在一个地方。

    1. [root@localhost cgo-practive]# go build .
    2. [root@localhost cgo-practive]# ls
    3. cgo-practive  main.go
    4. [root@localhost cgo-practive]# ./cgo-practive
    5. init...
    6. JUMP_PARENT
    7. CHILD_PID: 14348
    8. [root@localhost cgo-practive]# JUMP_CHILD

    使用GDB查看stack：

    1. (gdb) list
    2. 28                exit(0);
    3. 29            case JUMP_CHILD:
    4. 30                printf("JUMP_CHILD\n");
    5. 31                return;
    6. 32        }
    7. 33    }
    8. 34    */
    9. 35    import "C"
    10. 36
    11. 37    func main() {
    12. (gdb) info stack
    13. #0  0x00007efd6f9684ed in __lll_lock_wait () from /lib64/libpthread.so.0
    14. #1  0x00007efd6f966170 in pthread_cond_broadcast@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
    15. #2  0x00000000004862e6 in x_cgo_notify_runtime_init_done (dummy=<optimized out>) at gcc_libinit.c:69
    16. #3  0x0000000000451070 in runtime.asmcgocall () at /usr/local/go/src/runtime/asm_amd64.s:637
    17. #4  0x00007ffdec4b5c30 in ?? ()
    18. #5  0x000000000044efd1 in runtime.malg.func1 () at /usr/local/go/src/runtime/proc.go:3289
    19. #6  0x000000000044f886 in runtime.systemstack () at /usr/local/go/src/runtime/asm_amd64.s:351
    20. #7  0x000000000042c5b0 in ?? () at /usr/local/go/src/runtime/proc.go:1146
    21. #8  0x000000000044f719 in runtime.rt0_go () at /usr/local/go/src/runtime/asm_amd64.s:201
    22. #9  0x0000000000000000 in ?? ()

    查看Cgo的源码：

    1. // /usr/local/Cellar/go/1.11.5/libexec/src/runtime/cgo/gcc_libinit.c
    2. void
    3. x_cgo_notify_runtime_init_done(void* dummy) {
    4.     pthread_mutex_lock(&runtime_init_mu);
    5.     runtime_init_done = 1;
    6.     pthread_cond_broadcast(&runtime_init_cond); // 阻塞在此处
    7.     pthread_mutex_unlock(&runtime_init_mu);
    8. }