如何解决函数调用中指针值的变化？这是一个cgo的错误吗？

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英文:

How to resolve changing pointer value in function call? Is this a cgo bug?

问题

我正在通过cgo调用一个C函数，代码如下：

  1. var _outptr_7 C.double 
  2. var _outptr_8 C.double  
  3. var kk uintptr = uintptr(unsafe.Pointer(&_outptr_7))
  4. gogsl.InitializeGslFunction(f)
  5. _result := int32(C.gsl_integration_qags((*C.gsl_function)(unsafe.Pointer(f.CPtr())), C.double(a), C.double(b), C.double(epsabs), C.double(epsrel), C.size_t(limit), (*C.gsl_integration_workspace)(unsafe.Pointer(workspace.Ptr())), (*C.double)(&_outptr_7), (*C.double)(&_outptr_8)))
  6. fmt.Printf("%10.10X\n",kk)
  7. return _result, *(*float64)(unsafe.Pointer(&_outptr_7)), *(*float64)(unsafe.Pointer(&_outptr_8))

现在，kk是用于调试目的的。我修改了C函数gsl_integration_qags，使其输出它接收到的第8个参数（即&_outptr_7，从0开始计数的_n！）

（这里可以忽略InitializeGslFunction函数...）

kk和C函数的输出结果完全一致。

然而，我得到的结果与C函数返回的结果不同 - 它通过间接方式设置第8个参数。我确信这是正确的，我在gdb中观察过它的工作。

具体情况如下：（省略了只读取(gdb)的行，我按回车键重复命令）

  1. Breakpoint 3, qags (f=0xc20800a260, a=0, b=1, epsabs=0, epsrel=9.9999999999999995e-08, limit=1000, workspace=0x782ec0, result=0xc208031e28, abserr=0xc208031e20, q=0x7ffff79ad12c <gsl_integration_qk21>)
  2.  at qags.c:479
  3. 479	  *result = res_ext;
  4. (gdb) n
  5. 480	  *abserr = err_ext;
  6. (gdb) p *result
  7. $1 = -4.0000000000000853
  8. (gdb) p result
  9. $2 = (double *) 0xc208031e28
  10. (gdb) n
  11. 482	  if (err_ext == GSL_DBL_MAX)
  12. 485	  if (error_type || error_type2)
  13. 513	    double max_area = GSL_MAX_DBL (fabs (res_ext), fabs (area)); 
  14. 515	    if (!positive_integrand && max_area < 0.01 * resabs0)
  15. 520	    double ratio = res_ext / area;
  16. 522	    if (ratio < 0.01 || ratio > 100.0 || errsum > fabs (area))
  17. 526	  goto return_error;
  18. 535	  if (error_type > 2) 
  19. 540	  if (error_type == 0) 
  20. 542	      return GSL_SUCCESS;
  21. 573	}
  22. gsl_integration_qags (f=0xc20800a260, a=0, b=1, epsabs=0, epsrel=9.9999999999999995e-08, limit=1000, workspace=0x782ec0, result=0xc208031e28, abserr=0xc208031e20) at qags.c:53
  23. 53	  return status ;
  24. 54	}
  26. // 正确的值（-4.0）从C函数返回。
  28. _cgo_3ce45c051e63_Cfunc_gsl_integration_qags (v=0xc208031db8) at /home/dtrombley/go/src/github.com/dtromb/gogsl/numint/numint.go:241
  29. asmcgocall () at /usr/lib/golang/src/runtime/asm_amd64.s:669
  30. 669		MOVQ	48(SP), DI
  31. 670		MOVQ	(g_stack+stack_hi)(DI), SI
  32. 671		SUBQ	40(SP), SI
  33. 672		MOVQ	DI, g(CX)
  34. 673		MOVQ	SI, SP
  35. asmcgocall () at /usr/lib/golang/src/runtime/asm_amd64.s:674
  36. 674		RET
  37. runtime.asmcgocall_errno () at /usr/lib/golang/src/runtime/asm_amd64.s:627
  38. 627		MOVL	AX, ret+16(FP)
  39. runtime.asmcgocall_errno () at /usr/lib/golang/src/runtime/asm_amd64.s:628
  40. 628		RET
  41. runtime.cgocall_errno (fn=0x405210 <_cgo_3ce45c051e63_Cfunc_gsl_integration_qags>, arg=0xc20805fdb8, ~r2=0) at /usr/lib/golang/src/runtime/cgocall.go:132
  42. 132		exitsyscall()
  43. 134		return errno
  45. // cgo C绑定即将从Go调用者返回
  47. github.com/dtromb/gogsl/numint._Cfunc_gsl_integration_qags (p0=0xc20800a260, p1=0, p2=1, p3=0, p4=9.9999999999999995e-08, p5=1000, p6=0x782ec0, p7=0xc20805fe28, p8=0xc20805fe20, r1=0)
  48. at /home/dtrombley/go/src/github.com/dtromb/gogsl/numint/:92
  49. 92	/home/dtrombley/go/src/github.com/dtromb/gogsl/numint/: No such file or directory.
  51. // 糟糕，出错了。返回值结构体中的p7值与输入值不同... **_outptr_7的位置已经改变**。
  53. (gdb) p p7
  54. $3 = (github.com/dtromb/gogsl/numint._Ctype_double *) 0xc20805fe28
  55. (gdb) p *p7
  56. $4 = 0
  57. (gdb) n
  58. github.com/dtromb/gogsl/numint.Qags (f=0xc20805ff58, a=0, b=1, epsabs=0, epsrel=9.9999999999999995e-08, limit=1000, workspace=0xc20800a1d0, ~r7=7753968, ~r8=0, ~r9=0)
  59. at /home/dtrombley/go/src/github.com/dtromb/gogsl/numint/numint.go:75
  60. 75	   fmt.Printf("%10.10X\n",kk)
  61. C208031E28
  62. 76	   return _result, *(*float64)(unsafe.Pointer(&_outptr_7)), *(*float64)(unsafe.Pointer(&_outptr_8))
  63. (gdb) p _outptr_7
  64. $5 = 0
  65. (gdb) p &_outptr_7
  66. $6 = (float64 *) 0xc20805fe28
  68. // 但是正确的值仍然存在。
  70. (gdb) p $1
  71. $7 = -4.0000000000000853
  74. 那么，出了什么问题？这里发生了什么？如何修复/解决？
  76. 为了完整起见，这是cgo绑定显示的结构体映射：
  78. ```c
  79. void
  80. _cgo_a9ebceabba03_Cfunc_gsl_integration_qags(void *v)
  81. {
  82.     struct {
  83.             gsl_function* p0;
  84.             double p1;
  85.             double p2;
  86.             double p3;
  87.             double p4;
  88.             size_t p5;
  89.             gsl_integration_workspace* p6;
  90.             double* p7;
  91.             double* p8;
  92.             int r;
  93.             char __pad76[4];
  94.     } __attribute__((__packed__, __gcc_struct__)) *a = v;
  95.     char *stktop = _cgo_topofstack();
  96.     __typeof__(a->r) r = gsl_integration_qags((void*)a->p0, a->p1, a->p2, a->p3, a->p4, a->p5, (void*)a->p6, (void*)a->p7, (void*)a->p8);
  97.     a = (void*)((char*)a + (_cgo_topofstack() - stktop));
  98.     a->r = r;
  99. }

这个错误在编译的某些细节上是随机发生或不发生的，这些细节不应该影响语义 - 例如，在调用C函数之前添加一个打印&_outptr_7的行会导致它正确返回。对代码进行其他更改会使其恢复，等等。

我做错了什么吗？还是cgo的一个错误？

英文:

I'm calling a C function via cgo thusly:

  1. var _outptr_7 C.double 
  2. var _outptr_8 C.double  
  3. var kk uintptr = uintptr(unsafe.Pointer(&amp;_outptr_7))
  4. gogsl.InitializeGslFunction(f)
  5. _result := int32(C.gsl_integration_qags((*C.gsl_function)(unsafe.Pointer(f.CPtr())), C.double(a), C.double(b), C.double(epsabs), C.double(epsrel), C.size_t(limit), (*C.gsl_integration_workspace)(unsafe.Pointer(workspace.Ptr())), (*C.double)(&amp;_outptr_7), (*C.double)(&amp;_outptr_8)))
  6. fmt.Printf(&quot;%10.10X\n&quot;,kk)
  7. return _result, *(*float64)(unsafe.Pointer(&amp;_outptr_7)), *(*float64)(unsafe.Pointer(&amp;_outptr_8))

Now, the kk is here for debugging purposes. I've modified the C function gsl_integration_qags to output the 8th argument it receives (that's &amp;_outptr_7, the _&lt;n&gt; count from 0!)

(The InitializeGslFunction can be ignored for the purpose here...)

The two values - kk and the C function output - match exactly.

However, I get a different number out than the C function returns - it sets the 8th argument via indirection. I'm sure this is correct, I've watched it work in gdb.

It looks like this: (omitting lines that just read (gdb) where I hit enter to repeat command)

  1. Breakpoint 3, qags (f=0xc20800a260, a=0, b=1, epsabs=0, epsrel=9.9999999999999995e-08, limit=1000, workspace=0x782ec0, result=0xc208031e28, abserr=0xc208031e20, q=0x7ffff79ad12c &lt;gsl_integration_qk21&gt;)
  2.  at qags.c:479
  3. 479	  *result = res_ext;
  4. (gdb) n
  5. 480	  *abserr = err_ext;
  6. (gdb) p *result
  7. $1 = -4.0000000000000853
  8. (gdb) p result
  9. $2 = (double *) 0xc208031e28
  10. (gdb) n
  11. 482	  if (err_ext == GSL_DBL_MAX)
  12. 485	  if (error_type || error_type2)
  13. 513	    double max_area = GSL_MAX_DBL (fabs (res_ext), fabs (area)); 
  14. 515	    if (!positive_integrand &amp;&amp; max_area &lt; 0.01 * resabs0)
  15. 520	    double ratio = res_ext / area;
  16. 522	    if (ratio &lt; 0.01 || ratio &gt; 100.0 || errsum &gt; fabs (area))
  17. 526	  goto return_error;
  18. 535	  if (error_type &gt; 2) 
  19. 540	  if (error_type == 0) 
  20. 542	      return GSL_SUCCESS;
  21. 573	}
  22. gsl_integration_qags (f=0xc20800a260, a=0, b=1, epsabs=0, epsrel=9.9999999999999995e-08, limit=1000, workspace=0x782ec0, result=0xc208031e28, abserr=0xc208031e20) at qags.c:53
  23. 53	  return status ;
  24. 54	}

// Correct value (-4.0) is being returned from C function.

  1. _cgo_3ce45c051e63_Cfunc_gsl_integration_qags (v=0xc208031db8) at /home/dtrombley/go/src/github.com/dtromb/gogsl/numint/numint.go:241
  2. asmcgocall () at /usr/lib/golang/src/runtime/asm_amd64.s:669
  3. 669		MOVQ	48(SP), DI
  4. 670		MOVQ	(g_stack+stack_hi)(DI), SI
  5. 671		SUBQ	40(SP), SI
  6. 672		MOVQ	DI, g(CX)
  7. 673		MOVQ	SI, SP
  8. asmcgocall () at /usr/lib/golang/src/runtime/asm_amd64.s:674
  9. 674		RET
  10. runtime.asmcgocall_errno () at /usr/lib/golang/src/runtime/asm_amd64.s:627
  11. 627		MOVL	AX, ret+16(FP)
  12. runtime.asmcgocall_errno () at /usr/lib/golang/src/runtime/asm_amd64.s:628
  13. 628		RET
  14. runtime.cgocall_errno (fn=0x405210 &lt;_cgo_3ce45c051e63_Cfunc_gsl_integration_qags&gt;, arg=0xc20805fdb8, ~r2=0) at /usr/lib/golang/src/runtime/cgocall.go:132
  15. 132		exitsyscall()
  16. 134		return errno

// cgo C glue is about to return from call into Go caller

  1. github.com/dtromb/gogsl/numint._Cfunc_gsl_integration_qags (p0=0xc20800a260, p1=0, p2=1, p3=0, p4=9.9999999999999995e-08, p5=1000, p6=0x782ec0, p7=0xc20805fe28, p8=0xc20805fe20, r1=0)
  2. at /home/dtrombley/go/src/github.com/dtromb/gogsl/numint/:92
  3. 92	/home/dtrombley/go/src/github.com/dtromb/gogsl/numint/: No such file or directory.

// Bang, it's broken. p7 value in return val struct is /different than the value that went in/... the location of _outptr_7 has changed.

  1. (gdb) p p7
  2. $3 = (github.com/dtromb/gogsl/numint._Ctype_double *) 0xc20805fe28
  3. (gdb) p *p7
  4. $4 = 0
  5. (gdb) n
  6. github.com/dtromb/gogsl/numint.Qags (f=0xc20805ff58, a=0, b=1, epsabs=0, epsrel=9.9999999999999995e-08, limit=1000, workspace=0xc20800a1d0, ~r7=7753968, ~r8=0, ~r9=0)
  7. at /home/dtrombley/go/src/github.com/dtromb/gogsl/numint/numint.go:75
  8. 75	   fmt.Printf(&quot;%10.10X\n&quot;,kk)
  9. C208031E28
  10. 76	   return _result, *(*float64)(unsafe.Pointer(&amp;_outptr_7)), *(*float64)(unsafe.Pointer(&amp;_outptr_8))
  11. (gdb) p _outptr_7
  12. $5 = 0
  13. (gdb) p &amp;_outptr_7
  14. $6 = (float64 *) 0xc20805fe28

// But the correct value is still hanging out there.

  1.  (gdb) p $1
  2.  $7 = -4.0000000000000853

So, what gives? What is going on here? How to fix/workaround?

Here is the cgo binding showing the struct mapping, for completeness:

  1. void
  2. _cgo_a9ebceabba03_Cfunc_gsl_integration_qags(void *v)
  3. {
  4.     struct {
  5.             gsl_function* p0;
  6.             double p1;
  7.             double p2;
  8.             double p3;
  9.             double p4;
  10.             size_t p5;
  11.             gsl_integration_workspace* p6;
  12.             double* p7;
  13.             double* p8;
  14.             int r;
  15.             char __pad76[4];
  16.     } __attribute__((__packed__, __gcc_struct__)) *a = v;
  17.     char *stktop = _cgo_topofstack();
  18.     __typeof__(a-&gt;r) r = gsl_integration_qags((void*)a-&gt;p0, a-&gt;p1, a-&gt;p2, a-&gt;p3, a-&gt;p4, a-&gt;p5, (void*)a-&gt;p6, (void*)a-&gt;p7, (void*)a-&gt;p8);
  19.     a = (void*)((char*)a + (_cgo_topofstack() - stktop));
  20.     a-&gt;r = r;
  21. }

The bug occurs or does not occur randomly depending on some details of the compilation which should not affect semantics - adding a line which prints out &_outptr_7 before calling the C function causes it to return correctly, for example. Making other changes to the code reverts it back, etc etc.

Something I'm doing wrong? Or bug in cgo?

答案1

得分: 0

这是由于Go 1.4中的一个错误导致的。在1.5中已经修复。

https://golang.org/issue/10303

英文:

This is due to a bug in Go 1.4. Fixed in 1.5.

https://golang.org/issue/10303

huangapple
  • 本文由 发表于 2015年8月11日 02:49:49
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