[doc][debug] small article about using lldb/gdb with kotlin native
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Vasily Levchenko
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vvlevchenko
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## Debugging
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Currently Kotlin native compiler produces debug info compatible with DWARF 2 specification, so modern debugger tools could
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perform following operations:
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- breakpoints
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- stepping
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- inspection of type information
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### Producing binaries with debug info with Kotlin/Native compiler
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To produce binaries with the Kotlin/Native compiler it's sufficient to use ``-g`` option on the command line.<br/>
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_Example:_
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```
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0:b-debugger-fixes:minamoto@unit-703(0)# cat - > hello.kt
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fun main(args: Array<String>) {
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println("Hello world");
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println("I need your clothes, boots and your motocycle");
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}
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0:b-debugger-fixes:minamoto@unit-703(0)# dist/bin/konanc -g hello.kt -o terminator.kexe
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KtFile: hello.kt
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0:b-debugger-fixes:minamoto@unit-703(0)# lldb terminator.kexe
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(lldb) target create "terminator.kexe"
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Current executable set to 'terminator.kexe' (x86_64).
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(lldb) b kfun:main(kotlin.Array<kotlin.String>)
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Breakpoint 1: where = terminator.kexe`kfun:main(kotlin.Array<kotlin.String>) + 4 at hello.kt:2, address = 0x00000001000012e4
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(lldb) r
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Process 28473 launched: '/Users/minamoto/ws/.git-trees/debugger-fixes/terminator.kexe' (x86_64)
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Process 28473 stopped
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* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
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frame #0: 0x00000001000012e4 terminator.kexe`kfun:main(kotlin.Array<kotlin.String>) at hello.kt:2
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1 fun main(args: Array<String>) {
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-> 2 println("Hello world");
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3 println("I need your clothes, boots and your motocycle");
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4 }
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(lldb) n
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Hello world
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Process 28473 stopped
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* thread #1, queue = 'com.apple.main-thread', stop reason = step over
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frame #0: 0x00000001000012f0 terminator.kexe`kfun:main(kotlin.Array<kotlin.String>) at hello.kt:3
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1 fun main(args: Array<String>) {
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2 println("Hello world");
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-> 3 println("I need your clothes, boots and your motocycle");
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4 }
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(lldb)
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```
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### Breakpoints
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Modern debuggers provide several ways to set breakpoint, see below for per-tool breakdown:
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#### lldb
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- by name
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````
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(lldb) b -n kfun:main(kotlin.Array<kotlin.String>)
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Breakpoint 4: where = terminator.kexe`kfun:main(kotlin.Array<kotlin.String>) + 4 at hello.kt:2, address = 0x00000001000012e4
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````
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_``-n`` is optional, this flag is applied by default_
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- by location (filename, line number)
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````
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(lldb) b -f hello.kt -l 1
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Breakpoint 1: where = terminator.kexe`kfun:main(kotlin.Array<kotlin.String>) + 4 at hello.kt:2, address = 0x00000001000012e4
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````
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- by address
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````
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(lldb) b -a 0x00000001000012e4
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Breakpoint 2: address = 0x00000001000012e4
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````
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- by regex, ones might find it useful for debugging generated artifacts, like lambda etc (where used ``#`` symbol in name).
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````
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3: regex = 'main\(', locations = 1
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3.1: where = terminator.kexe`kfun:main(kotlin.Array<kotlin.String>) + 4 at hello.kt:2, address = terminator.kexe[0x00000001000012e4], unresolved, hit count = 0
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````
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#### gdb
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- by regex
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````
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(gdb) rbreak main(
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Breakpoint 1 at 0x1000109b4
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struct ktype:kotlin.Unit &kfun:main(kotlin.Array<kotlin.String>);
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````
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- by name __unusable__, because ``:`` is separator for breakpoint by location
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``
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(gdb) b kfun:main(kotlin.Array<kotlin.String>)
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No source file named kfun.
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Make breakpoint pending on future shared library load? (y or [n]) y
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Breakpoint 1 (kfun:main(kotlin.Array<kotlin.String>)) pending
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``
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- by location
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````
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(gdb) b hello.kt:1
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Breakpoint 2 at 0x100001704: file /Users/minamoto/ws/.git-trees/hello.kt, line 1.
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````
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- by address
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````
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(gdb) b *0x100001704
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Note: breakpoint 2 also set at pc 0x100001704.
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Breakpoint 3 at 0x100001704: file /Users/minamoto/ws/.git-trees/hello.kt, line 2.
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````
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### Stepping
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Stepping functions mostly the same way as for C/C++ programs
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### Type info
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Some details about type information functionality in modern debuggers:
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#### lldb
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````
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0:b-debugger-fixes:minamoto@unit-703(1)# lldb build/bin/Tetris.kexe
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(lldb) target create "build/bin/Tetris.kexe"
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Current executable set to 'build/bin/Tetris.kexe' (x86_64).
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(lldb) b kfun:main(kotlin.Array<kotlin.String>)
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Breakpoint 1: where = Tetris.kexe`kfun:main(kotlin.Array<kotlin.String>) + 21 at Tetris.kt:955, address = 0x00000001000109c5
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(lldb) r
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Process 41740 launched: '/Users/minamoto/ws/.git-trees/debugger-fixes/samples/tetris/build/bin/Tetris.kexe' (x86_64)
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Process 41740 stopped
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* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
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frame #0: 0x00000001000109c5 Tetris.kexe`kfun:main(kotlin.Array<kotlin.String>) at Tetris.kt:955
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952 }
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953 }
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954
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-> 955 fun main(args: Array<String>) {
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956 var startLevel = 0
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957 var width = 10
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958 var height = 20
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(lldb) type lookup ktype:Point
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struct ktype:Point {
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int kprop:Point.x;
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int kprop:Point.y;
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}
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````
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#### gdb
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Unfortunately ``ptype`` is affected with ``:`` and unusable.
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### Known issues
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- stepping in imported inline functions
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- inspection is not implemented
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_Note:_ Support DWARF 2 specification means that debugger tool recognize Kotlin as C89, because till DWARF 5 specification, there is no
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identifier for Kotlin language type in specification.
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