Cgo - How to convert string to C fixed char array
I am trying to instantiate a C struct inside my Go code. The structure is defined as follows (in an external library which I cannot change):
typedef struct {
char field1[256];
} S1
In go, I did this:
func myfunc(mystr string){
// We need to convert mystr from string to char array
cStr := C.CString(mystr)
defer C.free(unsafe.Pointer(cStr)
// Should work now
s1 := &C.S1{field1: cStr}
// Do something with s1...
}
But it doesn't compile because:
cannot use cStr (type * C.char) as type [256] C.char in field value
I tried to force ([256] C.char) (cStr) but it obviously doesn't work either.
Is there a way to achieve what I am trying to do?
source to share
The simplest solution is to change the structure field definition to char -pointer, which is pretty standard for strings in C:
typedef struct {
char *field1;
} S1
A more complex solution would be [1] :
arr := [256]C.char{}
for i := 0; i < len(mystr) && i < 255; i++ { // leave element 256 at zero
arr[i] = C.char(mystr[i])
}
s1 := &C.S1{field1: arr}
[1] The code is untested and cannot compile on this workstation.
source to share
Unfortunately, there are no convenience methods to be treated [size]C.char
as strings in Go (I think I saw a suggestion to add this at some point, though ...)
In my code, instead of referring directly to it, I decided to manually write the string to the structure as needed with some types
func strCopy(dest *[maxTextExtent]C.char, src []byte) {
for i, c := range src {
dest[i] = C.char(c)
}
// This is C, we need to terminate the string!
dest[len(src)] = 0
}
And the way I used it, which is much less secure,
s1 := &C.S1{
field1: *(*[256]C.char)(unsafe.Pointer(cStr)),
}
source to share