ios - How to get value of UnsafeMutableRawPointer? -

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i'm trying address pointed unsafemutablerawpointer, i'm unable so. i'm new swift might missing or doing wrong. preferably cast raw value cchar.

enter image description here

note passers-by: of answer won't make sense, doesn't answer initial question above, rather question(s) arose in chat op.

took me few hours, i've learned assembly, can answer questions.

  1. cchar c char ... literally. represents char type of c. it's typealias int8. it's single byte. can't use pointer type, 8 bytes (on 64 bit machines).

  2. you don't need unsafemutablerawpointer boilerplate, , certainly don''t need access raw value. can pass arrays directly pointers expected.

    when function declared taking unsafepointer argument, can accept of following: ...

    • a [type] value, passed pointer start of array.

    from interacting c apis - pointers.

  3. you experiencing issues mutation of 0x8(%rdi) did not seem reflected on swift side. issue here you're writing @ offset of 8 bytes, print(a.load(as: void_star.self)) reading first byte. you're reading byte never modified.

i did further exploration. here's loot adventure:

exampleswift.swift:

@_silgen_name("incrementbyvalue")     func incrementbyvalue(_: int64)  @_silgen_name("incrementbyreference")     func incrementbyreference(_: inout int64)  @_silgen_name("return1234")     func return1234() -> int64  @_silgen_name("incrementelements")     func incrementelements(of _: unsaferawpointer, count _: int)  var a: int64 = 100 print("\"a\" before \"incrementbyvalue(a)\": \(a)") incrementbyvalue(a) print("\"a\" after \"incrementbyvalue(a)\": \(a)\n")  var b: int64 = 200 print("\"b\" before \"incrementbyvalue(b)\": \(b)") incrementbyreference(&b) print("\"b\" after \"incrementbyvalue(b)\": \(b)\n")  print("return1234() returned: \(return1234())\n")  var array: [int64] = array(0...5)  print("\"array\" before incrementelements(of: array, count: array.count): \n\t\(array)") incrementelements(of: array, count: array.count) print("\"array\" after incrementelements(of: array, count: array.count): \n\t\(array)\n")

exampleasm.s:

.text  .globl _incrementbyvalue .globl _incrementbyreference .globl _return1234 .globl _incrementelements  // test routine demonstrates operating on value _incrementbyvalue:     // %rdi contains copy of argument passed in.     // changes here won't reflected in swift     incq %rdi     ret  // test routine demonstrates operating on reference _incrementbyreference:     // %rdi contains reference tp argument passed in.     // changes reference won't reflected in swift,     // changes referenced memory will.     incq (%rdi)     ret  // test routine demonstrates use of %rax returning value _return1234:     movq $1234, %rax    // return value in rax     ret  //a test routine demonstrates operating on array _incrementelements:     // %rdi: pointer first of n int64 elements     // %rsi: array count, n      movq    %rsi, %rcx  // set loop counter (%rcx) n     aloop:         incq    (%rdi)      // increment value pointer %rdi         add     $8, %rdi    // advance pointer 8 bytes         loop    aloop       // loop aloop if rcx > 0      ret

compile, link , run with:

llvm-g++ -c exampleasm.s && swiftc -c exampleswift.swift && ld exampleasm.o exampleswift.o -o examplebinary -force_load /applications/xcode.app/contents/developer/toolchains/xcodedefault.xctoolchain/usr/lib/arc/libarclite_macosx.a -framework corefoundation -macosx_version_min 10.12.0 -syslibroot /applications/xcode.app/contents/developer/platforms/macosx.platform/developer/sdks/macosx10.12.sdk -lobjc -lsystem -arch x86_64 -l /applications/xcode.app/contents/developer/toolchains/xcodedefault.xctoolchain/usr/lib/swift/macosx -rpath /applications/xcode.app/contents/developer/toolchains/xcodedefault.xctoolchain/usr/lib/swift/macosx -no_objc_category_merging && ./examplebinary

output:

"a" before "incrementbyvalue(a)": 100 "a" after "incrementbyvalue(a)": 100  "b" before "incrementbyvalue(b)": 200 "b" after "incrementbyvalue(b)": 201  return1234() returned: 1234  "array" before incrementelements(of: array, count: array.count):      [0, 1, 2, 3, 4, 5] "array" after incrementelements(of: array, count: array.count):      [1, 2, 3, 4, 5, 6]

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