What is the best way to write a struct to file? - ios

Question

I have this two structs:
struct pcap_hdr_s {
UInt32 magic_number;
UInt16 version_major;
UInt16 version_minor;
int32_t thiszone;
UInt32 sigfigs;
UInt32 snaplen;
UInt32 network;
};
//packet header
struct pcaprec_hdr_s {
UInt32 ts_sec;
UInt32 ts_usec;
UInt32 incl_len;
UInt32 orig_len;
};
which are initialised as follows(for example):
let pcapHeader : pcap_hdr_s = pcap_hdr_s(magic_number: 0xa1b2c3d4,
version_major: 2,
version_minor: 4,
thiszone: 0,
sigfigs: 0,
snaplen:
pcap_record_size,
network: LINKTYPE_ETHERNET)
let pcapRecHeader : pcaprec_hdr_s = pcaprec_hdr_s(ts_sec: UInt32(ts.tv_sec),
ts_usec: UInt32(ts.tv_nsec),
incl_len: plen,
orig_len: length)
I tried to create Data/NSData objects of the structs like this:
//write pcap header
let pcapHeaderData : NSData = NSData(bytes: pcapHeader, length: sizeofValue(pcapHeader))
//write pcaprec header
let pcapRecHeaderData : NSData = NSData(bytes: pcapRecHeader, length: sizeofValue(pcapRecHeader))
but I always get this error for each line:
"Connot convert value if type 'pcap_hdr_s' to expected arguemnt type 'UsafeRawPointer?'"
I had a look at the documentation of UnsafeRawPointers in Swift, but I don't get it enough as for now, to create the NSData object from the structs.
Am I on the right way or is there a better one to accomplish my intend?
If this Data initialisation would work, my next steps would be
Append pcapRecHeaderData to pcapHeaderData
write pcapHeaderData atomically to file/url with the provided function of Data/NSData
EDIT:
//packet ethernet header
struct ethernet_hdr_s {
let dhost : [UInt8]
let shost : [UInt8]
let type : UInt16
};
let src_mac : [UInt8] = [0x66, 0x77, 0x88, 0x99, 0xAA, 0xBB]
let dest_mac : [UInt8] = [0x00, 0x11, 0x22, 0x33, 0x44, 0x55]
let ethernetHeader : ethernet_hdr_s = ethernet_hdr_s(dhost: dest_mac, shost: src_mac, type: 0x0800)
EDIT 2:
let payloadSize = packet.payload.count
let plen = (payloadSize < Int(pcap_record_size) ? payloadSize : Int(pcap_record_size));
bytesWritten = withUnsafePointer(to: &(packet.payload)) {
$0.withMemoryRebound(to: UInt8.self, capacity: Int(plen)) {
ostream.write($0, maxLength: Int(plen))
}
}
if bytesWritten != (Int(plen)) {
// Could not write all bytes, report error ...
NSLog("error in Writting packet payload, not all Bytes written: bytesWritten: %d|plen: %d", bytesWritten, Int(plen))
}

Answer 1

You can write arbitrary data to an InputStream without creating a
(NS)Data object first. The "challenge" is how to convert the pointer to
the struct to an UInt8 pointer as expected by the write method:
let ostream = OutputStream(url: url, append: false)! // Add error checking here!
ostream.open()
var pcapHeader = pcap_hdr_s(...)
let headerSize = MemoryLayout.size(ofValue: pcapHeader)
let bytesWritten = withUnsafePointer(to: &pcapHeader) {
$0.withMemoryRebound(to: UInt8.self, capacity: headerSize) {
ostream.write($0, maxLength: headerSize)
}
}
if bytesWritten != headerSize {
// Could not write all bytes, report error ...
}
In the same way you can read data from in InputStream:
let istream = InputStream(url: url)! // Add error checking here!
istream.open()
let bytesRead = withUnsafeMutablePointer(to: &pcapHeader) {
$0.withMemoryRebound(to: UInt8.self, capacity: headerSize) {
istream.read($0, maxLength: headerSize)
}
}
if bytesRead != headerSize {
// Could not read all bytes, report error ...
}
If the file was possibly created on a different platform with a
different byte order then you can check the "magic" and swap bytes
if necessary (as described on https://wiki.wireshark.org/Development/LibpcapFileFormat):
switch pcapHeader.magic_number {
case 0xa1b2c3d4:
break // Already in host byte order
case 0xd4c3b2a1:
pcapHeader.version_major = pcapHeader.version_major.byteSwapped
pcapHeader.version_minor = pcapHeader.version_minor.byteSwapped
// ...
default:
// Unknown magic, report error ...
}
To simplify the task of writing and reading structs one can define
custom extension methods, e.g.
extension OutputStream {
enum ValueWriteError: Error {
case incompleteWrite
case unknownError
}
func write<T>(value: T) throws {
var value = value
let size = MemoryLayout.size(ofValue: value)
let bytesWritten = withUnsafePointer(to: &value) {
$0.withMemoryRebound(to: UInt8.self, capacity: size) {
write($0, maxLength: size)
}
}
if bytesWritten == -1 {
throw streamError ?? ValueWriteError.unknownError
} else if bytesWritten != size {
throw ValueWriteError.incompleteWrite
}
}
}
extension InputStream {
enum ValueReadError: Error {
case incompleteRead
case unknownError
}
func read<T>(value: inout T) throws {
let size = MemoryLayout.size(ofValue: value)
let bytesRead = withUnsafeMutablePointer(to: &value) {
$0.withMemoryRebound(to: UInt8.self, capacity: size) {
read($0, maxLength: size)
}
}
if bytesRead == -1 {
throw streamError ?? ValueReadError.unknownError
} else if bytesRead != size {
throw ValueReadError.incompleteRead
}
}
}
Now you can write and read simply with
try ostream.write(value: pcapHeader)
try istream.read(value: &pcapHeader)
Of course this works only with "self-contained" structs like your
pcap_hdr_s and pcaprec_hdr_s.

Answer 2

You can convert pcap_hdr_s to Data and vice versa in Swift 3 with
pcap_hdr_s -> Data
var pcapHeader : pcap_hdr_s = pcap_hdr_s(magic_number ...
let data = withUnsafePointer(to: &pcapHeader) {
Data(bytes: UnsafePointer($0), count: MemoryLayout.size(ofValue: pcapHeader))
}
Data -> pcap_hdr_s
let header: pcap_hdr_s = data.withUnsafeBytes { $0.pointee }
Reference: round trip Swift number types to/from Data