4_curl_request.md

4 - Curl based HTTP requests

Task

Send an HTTP request to obtain your IP (e.g. GET https://api.ipify.org). The application should print out the IP to standard output.

The solution can be based on C code example for handling HTTP requests using libcurl from here.

Majority of the code is already provided. Let's take a look on how we have linked Curl in com.tapad.curl.CCurl.

To accomplish the task you must implement function writing the body of response into memory buffer in class com.tapad.curl.CurlHttp.

Note on interoperability with C

Basic memory management

Zone { implicit z =>
  val buffer = alloc[Byte](n)
}

or well-known stdlib and libc functions:

def malloc(size: CSize): Ptr[Byte]

def calloc(num: CSize, size: CSize): Ptr[Byte]

def realloc(ptr: Ptr[Byte], newSize: CSize): Ptr[Byte]

def free(ptr: Ptr[Byte]): Unit

def memcpy(dst: RawPtr, src: RawPtr, count: CSize): RawPtr

---

Handling Strings

def toCString(str: String)(implicit z: Zone): CString

def fromCString(cstr: CString,
                charset: Charset = Charset.defaultCharset()): String

val msg: CString = c"Hello, world!"

---

Extern objects and linking libraries

@native.link("mylib")
@native.extern
object mylib {
  def f(): Unit = native.extern
}

---

Structs

type MyStructWith2Fields = CStruct2[CString, CString]

---

Pointers:

Operation	C syntax	Scala Syntax
As function argument	char*	Ptr[CChar]
Load value	*ptr	!ptr
Store value	*ptr = value	!ptr = value
Load at index	ptr[i]	ptr(i)
Store at index	ptr[i] = value	ptr(i) = value
Load a field	ptr->name	!ptr._N
Store a field	ptr->name = value	!ptr._N = value

Function pointers

The following signature in C:

void foo(void (* f)(char *));

can be declared as following in Scala:

def foo(f: CFunctionPtr1[CString, Unit]): Unit = native.extern

To pass a Scala function to CFunctionPtr1, you need to use the conversion function CFunctionPtr.fromFunction1():

def f(s: CString): Unit = ???
foo(CFunctionPtr.fromFunction1(f))

For more details, go to the documentation.