cmd/cgo: reject C structs with Go-incompatible field alignment #7560
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that particular C struct mixer can't be represented in Go at all.
complex float in C has 4-byte alignment (because it is treated
as struct { float real; float imag; }; however in Go, complex64
has 8-byte alignment.
If C has bigger alignment than Go, we can always adding padding,
but in this case, that won't help.
Either we support some kind of __attribute__((pack)) in Go (highly
unlikely to happen), or change float64 alignment to 4-byte (but
probably this is not allowed)
Of course, another option is to forbid this kind of struct in cmd/cgo.
What do you think? Ian?
Status changed to Accepted. |
I think you're right. We should make cgo give an error for this case. We don't want to give an error for an unrepresentable struct in the C code, but we do want to give an error for an attempt to use that struct in the Go code. Labels changed: added release-go1.3maybe, removed release-go1.3. |
I may not understand why go aligns the way it does, but I get what is happening. What concerns me is how this will be handled in future versions of Go. If Go will forbid complex64 in C structs by way of compilation error then my application will stop working. If I understand better what this error will be then I can start writing compatible code today. I've attached a more complete example so you can see exactly what I'm doing. The workaround I'm using is to put all the complex64 numbers at the start of the C struct. This Mixer example is only one of dozens that is implemented in this pattern. You may wonder why I use this unusual pattern. The process() function needs to run in real-time on a 700MHz ARM. Using C lets me run this on an OS thread to bypass the range checking and GC in Go. Moving to complex128 is a 20% performance hit on ARM (but insignificant on amd64). Attachments:
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you can use this gcc-ism to force 8-byte alignment for your complex float in C
to be compatible with Go:
__attribute__((align(8)) complex float x;
(you can also use anonymous union to achieve the same effect if you don't want
to use gcc attributes, but you will have to make sure to choose another type that
has a alignment of 8-byte.)
for example, define your mixer struct this way:
typedef struct {
uint32_t unalign;
__attribute__((align(8))) complex float osc;
} mixer;
This will waste 4 byte of memory, so if you control the struct layout, you'd better
just align complex float to 8-byte boundary manually;
I will add something along this line to the cgo article on go-wiki for this workaround.
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Go is not going to forbid complex64 in structs. As I said before, complex64 is not the problem. I think that Go should forbid referring directly to C structs with fields whose alignment can not be represented in Go. There are many ways to handle this case, the most obvious being, don't use a struct. Or, for the particular special case of a misaligned complex64 field, use [2]float32. |
CL https://golang.org/cl/96300045 mentions this issue. |
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This issue was closed by revision 2d1a951. Status changed to Fixed. |
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Issue #8110 has been merged into this issue. |
This change looks like it could introduce unpredictable platform dependencies into cgo code: It looks like this change only hides the struct field when it cannot be represented on the machine you're compiling on right now. It might be entirely possible that other platforms have different alignment constraints so that the field is exposed on one platform as it is aligned but on a different platform it is omitted because that platform has different alignment constraints causing the field to be misaligned. Perhaps a more predictable solution has to be devised. |
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by dturnbull:
go version go1.2.1 darwin/amd64 Apple LLVM version 5.0 (clang-500.2.79) Using this program: package main /* #include "stdlib.h" #include "stdint.h" #include "complex.h" typedef struct { uint32_t unalign; float complex osc; } mixer; mixer* Mixer() { mixer *o = malloc(sizeof(mixer)); o->osc = (3.0 + 5.0*I); return o; } */ import "C" import "fmt" func main() { o := C.Mixer() fmt.Println(o) } I get incorrect output: &{0 (5+0i)} Removing the unalign element or changing to a double complex gives the expected output: &{(3+5i)}The text was updated successfully, but these errors were encountered: