If you want it to be initialized without the extra allocation and copy from Go, you would need to implement the strings.Repeat
function over a C string:
func emptyString(size int) *C.char {
p := C.malloc(C.size_t(size + 1))
pp := (*[1 << 30]byte)(p)
bp := copy(pp[:], " ")
for bp < size {
copy(pp[bp:], pp[:bp])
bp *= 2
}
pp[size] = 0
return (*C.char)(p)
}
If it doesn't need to be initialized, you can simply malloc/calloc the pointer yourself and pass it to your function.
b := C.malloc(50) // or 51 if the depending on what size your function is expecting
defer C.free(unsafe.Pointer(b))
C.cFunctionCall((*C.char)(b), 50)
Unless this is being called many times and actually poses a performance problem, use what you already have and reduce the amount of C code you have to deal with.