cgo - How to convert string to C fixed char array

Question

I'm trying to instantiate a C struct inside my Go code. The struct is defined like this, (in an external library that I cannot modify):

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've tried forcing ([256]C.char)(cStr) but it obviously doesn't work either.

Is there a way to achieve what I'm trying to do?

Answer 1

The simplest solution is to change your struct's field definition to a char-pointer which is pretty standard for strings in C:

typedef struct {
    char *field1;
} S1

The 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] Code untested, cannot compile on this workstation.

Answer 2

Unfortunately there aren't any convenience methods for handling [size]C.char as a string in Go (I think I saw a proposal to add this at one point though...)

In my code instead of handling it directly, I opted to manually write the string into the struct when needed with some like

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 to handle it, which is much less safe is

s1 := &C.S1{
    field1: *(*[256]C.char)(unsafe.Pointer(cStr)),
}