英文:
Cgo: How to return double array from C to Go
问题
我有一个像这样的C函数：
double* c_func(int n_rows) {
double result[n_rows];
for (int i = 0; i < n_rows; ++i) {
result[i] = (double)i;
}
return result;
}
我使用以下Go函数来处理C中的double：
// convert C double pointer to float64 slice ...
func doubleToFloats(in *C.double, length int) []float64 {
out := make([]float64, length, length)
start := unsafe.Pointer(in)
size := unsafe.Sizeof(C.double(0))
for i := 0; i < length; i++ {
val := *(*C.double)(unsafe.Pointer(uintptr(start) + size*uintptr(i)))
out[i] = float64(val)
}
return out
}
有时它可以正常工作，但有时不行。当它不工作时，它会返回类似以下的内容：
[0 1 2 3 4 5 6 7 8 9 10 2.53e-321 3.32018606e-316 4.24664374149e-312 4.24399158193e-312 1.1383e-320 3.31882387e-316 3.71924634e-316 3.31885594e-316 3.71924634e-316 5e-324 0 4.6950308e-316 4.24664374149e-312 3.7175681e-316 3.3200616e-316]
这看起来像是内存问题...
我不确定这是否是正确处理从C返回的double数组的方法。如果是，如何解决这个问题（偶尔出现）？如果不是，正确处理从C返回的double数组的方法是什么？
谢谢。
英文:
I got a C function like this
double* c_func(int n_rows) {
double result[n_rows];
for (int i = 0; i < n_rows; ++i) {
result[i] = (double)i;
}
return result;
}
And I use this Go function to process the C double:
// convert C double pointer to float64 slice ...
func doubleToFloats(in *C.double, length int) []float64 {
out := make([]float64, length, length)
start := unsafe.Pointer(in)
size := unsafe.Sizeof(C.double(0))
for i := 0; i < length; i++ {
val := *(*C.double)(unsafe.Pointer(uintptr(start) + size*uintptr(i)))
out[i] = float64(val)
}
return out
}
This sometimes works but sometimes not. When it not works, it return something like this:
[0 1 2 3 4 5 6 7 8 9 10 2.53e-321 3.32018606e-316 4.24664374149e-312 4.24399158193e-312 1.1383e-320 3.31882387e-316 3.71924634e-316 3.31885594e-316 3.71924634e-316 5e-324 0 4.6950308e-316 4.24664374149e-312 3.7175681e-316 3.3200616e-316]
which looks like somewhat memory issue to me...
I'm not sure if this is the correct way to handle returned double array from C in Go. If yes, how to fix the problem (which occurs ocassionally). If no, what is the correct way to handle returned double array from C?
Thanks.
答案1
得分: 0
在C语言中，你返回的指针将会变得无效。你需要像这样分配内存空间：double *result = calloc(sizeof(double), nrows)，同时也需要有一种方法来释放内存。
英文:
In C, that pointer you return is going to be stale. You would need to allocate the table like double *result = calloc(sizeof(double), nrows) --- which would require also have a way to free the memory.
答案2
得分: -1
好的，以下是翻译好的内容：
好的，所以我找到了一种简单的方法来实现这个。
我们首先使用calloc来为数组分配内存：
double* c_func(int n_rows) {
double* result;
result = calloc(n_rows, sizeof(double));
for (int i = 0; i < n_rows; ++i) {
result[i] = (double)i;
}
return result;
}
之后，我们只需将数据转换为Go中的适当类型。关键是使用C.free来释放从C端分配的内存。
// 将C的double指针转换为float64切片...
func doubleToFloats(in *C.double, size int) []float64 {
defer C.free(unsafe.Pointer(in))
out := (*[1 << 30]float64)(unsafe.Pointer(in))[:size:size]
return out
}
英文:
OK, so I figured out a simple way to achieve this.
We first use calloc to allocate memory for the array:
double* c_func(int n_rows) {
double* result;
result = calloc(n_rows, sizeof(double));
for (int i = 0; i < n_rows; ++i) {
result[i] = (double)i;
}
return result;
}
and after that, we simply convert the data into proper type in Go. The trick is to use C.free to free the memory allocated from C side.
// convert C double pointer to float64 slice ...
func doubleToFloats(in *C.double, size int) []float64 {
defer C.free(unsafe.Pointer(in))
out := (*[1 << 30]float64)(unsafe.Pointer(in))[:size:size]
return out
}
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