如何分配空的CString？

The cFunctionCall populates b and I am able to get content of string into GO string. However, I think that my memory allocation (line #1) is not efficient.

b := C.CString(strings.Repeat(" ", 50))
defer C.free(unsafe.Pointer(b))
C.cFunctionCall(b, 50)
rs := C.GoString(b)
log.Printf("rs: '%v'
", rs)

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.