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diff --git a/_content/doc/gdb.html b/_content/doc/gdb.html new file mode 100644 index 00000000..115b63fb --- /dev/null +++ b/_content/doc/gdb.html @@ -0,0 +1,553 @@ +<!--{ + "Title": "Debugging Go Code with GDB" +}--> + +<!-- +NOTE: In this document and others in this directory, the convention is to +set fixed-width phrases with non-fixed-width spaces, as in +<code>hello</code> <code>world</code>. +Do not send CLs removing the interior tags from such phrases. +--> + +<i> +<p> +The following instructions apply to the standard toolchain +(the <code>gc</code> Go compiler and tools). +Gccgo has native gdb support. +</p> +<p> +Note that +<a href="https://github.com/go-delve/delve">Delve</a> is a better +alternative to GDB when debugging Go programs built with the standard +toolchain. It understands the Go runtime, data structures, and +expressions better than GDB. Delve currently supports Linux, OSX, +and Windows on <code>amd64</code>. +For the most up-to-date list of supported platforms, please see +<a href="https://github.com/go-delve/delve/tree/master/Documentation/installation"> + the Delve documentation</a>. +</p> +</i> + +<p> +GDB does not understand Go programs well. +The stack management, threading, and runtime contain aspects that differ +enough from the execution model GDB expects that they can confuse +the debugger and cause incorrect results even when the program is +compiled with gccgo. +As a consequence, although GDB can be useful in some situations (e.g., +debugging Cgo code, or debugging the runtime itself), it is not +a reliable debugger for Go programs, particularly heavily concurrent +ones. Moreover, it is not a priority for the Go project to address +these issues, which are difficult. +</p> + +<p> +In short, the instructions below should be taken only as a guide to how +to use GDB when it works, not as a guarantee of success. + +Besides this overview you might want to consult the +<a href="https://sourceware.org/gdb/current/onlinedocs/gdb/">GDB manual</a>. +</p> + +<p> +</p> + +<h2 id="Introduction">Introduction</h2> + +<p> +When you compile and link your Go programs with the <code>gc</code> toolchain +on Linux, macOS, FreeBSD or NetBSD, the resulting binaries contain DWARFv4 +debugging information that recent versions (≥7.5) of the GDB debugger can +use to inspect a live process or a core dump. +</p> + +<p> +Pass the <code>'-w'</code> flag to the linker to omit the debug information +(for example, <code>go</code> <code>build</code> <code>-ldflags=-w</code> <code>prog.go</code>). +</p> + +<p> +The code generated by the <code>gc</code> compiler includes inlining of +function invocations and registerization of variables. These optimizations +can sometimes make debugging with <code>gdb</code> harder. +If you find that you need to disable these optimizations, +build your program using <code>go</code> <code>build</code> <code>-gcflags=all="-N -l"</code>. +</p> + +<p> +If you want to use gdb to inspect a core dump, you can trigger a dump +on a program crash, on systems that permit it, by setting +<code>GOTRACEBACK=crash</code> in the environment (see the +<a href="/pkg/runtime/#hdr-Environment_Variables"> runtime package +documentation</a> for more info). +</p> + +<h3 id="Common_Operations">Common Operations</h3> + +<ul> +<li> +Show file and line number for code, set breakpoints and disassemble: +<pre>(gdb) <b>list</b> +(gdb) <b>list <i>line</i></b> +(gdb) <b>list <i>file.go</i>:<i>line</i></b> +(gdb) <b>break <i>line</i></b> +(gdb) <b>break <i>file.go</i>:<i>line</i></b> +(gdb) <b>disas</b></pre> +</li> +<li> +Show backtraces and unwind stack frames: +<pre>(gdb) <b>bt</b> +(gdb) <b>frame <i>n</i></b></pre> +</li> +<li> +Show the name, type and location on the stack frame of local variables, +arguments and return values: +<pre>(gdb) <b>info locals</b> +(gdb) <b>info args</b> +(gdb) <b>p variable</b> +(gdb) <b>whatis variable</b></pre> +</li> +<li> +Show the name, type and location of global variables: +<pre>(gdb) <b>info variables <i>regexp</i></b></pre> +</li> +</ul> + + +<h3 id="Go_Extensions">Go Extensions</h3> + +<p> +A recent extension mechanism to GDB allows it to load extension scripts for a +given binary. The toolchain uses this to extend GDB with a handful of +commands to inspect internals of the runtime code (such as goroutines) and to +pretty print the built-in map, slice and channel types. +</p> + +<ul> +<li> +Pretty printing a string, slice, map, channel or interface: +<pre>(gdb) <b>p <i>var</i></b></pre> +</li> +<li> +A $len() and $cap() function for strings, slices and maps: +<pre>(gdb) <b>p $len(<i>var</i>)</b></pre> +</li> +<li> +A function to cast interfaces to their dynamic types: +<pre>(gdb) <b>p $dtype(<i>var</i>)</b> +(gdb) <b>iface <i>var</i></b></pre> +<p class="detail"><b>Known issue:</b> GDB can’t automatically find the dynamic +type of an interface value if its long name differs from its short name +(annoying when printing stacktraces, the pretty printer falls back to printing +the short type name and a pointer).</p> +</li> +<li> +Inspecting goroutines: +<pre>(gdb) <b>info goroutines</b> +(gdb) <b>goroutine <i>n</i> <i>cmd</i></b> +(gdb) <b>help goroutine</b></pre> +For example: +<pre>(gdb) <b>goroutine 12 bt</b></pre> +You can inspect all goroutines by passing <code>all</code> instead of a specific goroutine's ID. +For example: +<pre>(gdb) <b>goroutine all bt</b></pre> +</li> +</ul> + +<p> +If you'd like to see how this works, or want to extend it, take a look at <a +href="/src/runtime/runtime-gdb.py">src/runtime/runtime-gdb.py</a> in +the Go source distribution. It depends on some special magic types +(<code>hash<T,U></code>) and variables (<code>runtime.m</code> and +<code>runtime.g</code>) that the linker +(<a href="/src/cmd/link/internal/ld/dwarf.go">src/cmd/link/internal/ld/dwarf.go</a>) ensures are described in +the DWARF code. +</p> + +<p> +If you're interested in what the debugging information looks like, run +<code>objdump</code> <code>-W</code> <code>a.out</code> and browse through the <code>.debug_*</code> +sections. +</p> + + +<h3 id="Known_Issues">Known Issues</h3> + +<ol> +<li>String pretty printing only triggers for type string, not for types derived +from it.</li> +<li>Type information is missing for the C parts of the runtime library.</li> +<li>GDB does not understand Go’s name qualifications and treats +<code>"fmt.Print"</code> as an unstructured literal with a <code>"."</code> +that needs to be quoted. It objects even more strongly to method names of +the form <code>pkg.(*MyType).Meth</code>. +<li>As of Go 1.11, debug information is compressed by default. +Older versions of gdb, such as the one available by default on MacOS, +do not understand the compression. +You can generate uncompressed debug information by using <code>go +build -ldflags=-compressdwarf=false</code>. +(For convenience you can put the <code>-ldflags</code> option in +the <a href="/cmd/go/#hdr-Environment_variables"><code>GOFLAGS</code> +environment variable</a> so that you don't have to specify it each time.) +</li> +</ol> + +<h2 id="Tutorial">Tutorial</h2> + +<p> +In this tutorial we will inspect the binary of the +<a href="/pkg/regexp/">regexp</a> package's unit tests. To build the binary, +change to <code>$GOROOT/src/regexp</code> and run <code>go</code> <code>test</code> <code>-c</code>. +This should produce an executable file named <code>regexp.test</code>. +</p> + + +<h3 id="Getting_Started">Getting Started</h3> + +<p> +Launch GDB, debugging <code>regexp.test</code>: +</p> + +<pre> +$ <b>gdb regexp.test</b> +GNU gdb (GDB) 7.2-gg8 +Copyright (C) 2010 Free Software Foundation, Inc. +License GPLv 3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> +Type "show copying" and "show warranty" for licensing/warranty details. +This GDB was configured as "x86_64-linux". + +Reading symbols from /home/user/go/src/regexp/regexp.test... +done. +Loading Go Runtime support. +(gdb) +</pre> + +<p> +The message "Loading Go Runtime support" means that GDB loaded the +extension from <code>$GOROOT/src/runtime/runtime-gdb.py</code>. +</p> + +<p> +To help GDB find the Go runtime sources and the accompanying support script, +pass your <code>$GOROOT</code> with the <code>'-d'</code> flag: +</p> + +<pre> +$ <b>gdb regexp.test -d $GOROOT</b> +</pre> + +<p> +If for some reason GDB still can't find that directory or that script, you can load +it by hand by telling gdb (assuming you have the go sources in +<code>~/go/</code>): +</p> + +<pre> +(gdb) <b>source ~/go/src/runtime/runtime-gdb.py</b> +Loading Go Runtime support. +</pre> + +<h3 id="Inspecting_the_source">Inspecting the source</h3> + +<p> +Use the <code>"l"</code> or <code>"list"</code> command to inspect source code. +</p> + +<pre> +(gdb) <b>l</b> +</pre> + +<p> +List a specific part of the source parameterizing <code>"list"</code> with a +function name (it must be qualified with its package name). +</p> + +<pre> +(gdb) <b>l main.main</b> +</pre> + +<p> +List a specific file and line number: +</p> + +<pre> +(gdb) <b>l regexp.go:1</b> +(gdb) <i># Hit enter to repeat last command. Here, this lists next 10 lines.</i> +</pre> + + +<h3 id="Naming">Naming</h3> + +<p> +Variable and function names must be qualified with the name of the packages +they belong to. The <code>Compile</code> function from the <code>regexp</code> +package is known to GDB as <code>'regexp.Compile'</code>. +</p> + +<p> +Methods must be qualified with the name of their receiver types. For example, +the <code>*Regexp</code> type’s <code>String</code> method is known as +<code>'regexp.(*Regexp).String'</code>. +</p> + +<p> +Variables that shadow other variables are magically suffixed with a number in the debug info. +Variables referenced by closures will appear as pointers magically prefixed with '&'. +</p> + +<h3 id="Setting_breakpoints">Setting breakpoints</h3> + +<p> +Set a breakpoint at the <code>TestFind</code> function: +</p> + +<pre> +(gdb) <b>b 'regexp.TestFind'</b> +Breakpoint 1 at 0x424908: file /home/user/go/src/regexp/find_test.go, line 148. +</pre> + +<p> +Run the program: +</p> + +<pre> +(gdb) <b>run</b> +Starting program: /home/user/go/src/regexp/regexp.test + +Breakpoint 1, regexp.TestFind (t=0xf8404a89c0) at /home/user/go/src/regexp/find_test.go:148 +148 func TestFind(t *testing.T) { +</pre> + +<p> +Execution has paused at the breakpoint. +See which goroutines are running, and what they're doing: +</p> + +<pre> +(gdb) <b>info goroutines</b> + 1 waiting runtime.gosched +* 13 running runtime.goexit +</pre> + +<p> +the one marked with the <code>*</code> is the current goroutine. +</p> + +<h3 id="Inspecting_the_stack">Inspecting the stack</h3> + +<p> +Look at the stack trace for where we’ve paused the program: +</p> + +<pre> +(gdb) <b>bt</b> <i># backtrace</i> +#0 regexp.TestFind (t=0xf8404a89c0) at /home/user/go/src/regexp/find_test.go:148 +#1 0x000000000042f60b in testing.tRunner (t=0xf8404a89c0, test=0x573720) at /home/user/go/src/testing/testing.go:156 +#2 0x000000000040df64 in runtime.initdone () at /home/user/go/src/runtime/proc.c:242 +#3 0x000000f8404a89c0 in ?? () +#4 0x0000000000573720 in ?? () +#5 0x0000000000000000 in ?? () +</pre> + +<p> +The other goroutine, number 1, is stuck in <code>runtime.gosched</code>, blocked on a channel receive: +</p> + +<pre> +(gdb) <b>goroutine 1 bt</b> +#0 0x000000000040facb in runtime.gosched () at /home/user/go/src/runtime/proc.c:873 +#1 0x00000000004031c9 in runtime.chanrecv (c=void, ep=void, selected=void, received=void) + at /home/user/go/src/runtime/chan.c:342 +#2 0x0000000000403299 in runtime.chanrecv1 (t=void, c=void) at/home/user/go/src/runtime/chan.c:423 +#3 0x000000000043075b in testing.RunTests (matchString={void (struct string, struct string, bool *, error *)} + 0x7ffff7f9ef60, tests= []testing.InternalTest = {...}) at /home/user/go/src/testing/testing.go:201 +#4 0x00000000004302b1 in testing.Main (matchString={void (struct string, struct string, bool *, error *)} + 0x7ffff7f9ef80, tests= []testing.InternalTest = {...}, benchmarks= []testing.InternalBenchmark = {...}) +at /home/user/go/src/testing/testing.go:168 +#5 0x0000000000400dc1 in main.main () at /home/user/go/src/regexp/_testmain.go:98 +#6 0x00000000004022e7 in runtime.mainstart () at /home/user/go/src/runtime/amd64/asm.s:78 +#7 0x000000000040ea6f in runtime.initdone () at /home/user/go/src/runtime/proc.c:243 +#8 0x0000000000000000 in ?? () +</pre> + +<p> +The stack frame shows we’re currently executing the <code>regexp.TestFind</code> function, as expected. +</p> + +<pre> +(gdb) <b>info frame</b> +Stack level 0, frame at 0x7ffff7f9ff88: + rip = 0x425530 in regexp.TestFind (/home/user/go/src/regexp/find_test.go:148); + saved rip 0x430233 + called by frame at 0x7ffff7f9ffa8 + source language minimal. + Arglist at 0x7ffff7f9ff78, args: t=0xf840688b60 + Locals at 0x7ffff7f9ff78, Previous frame's sp is 0x7ffff7f9ff88 + Saved registers: + rip at 0x7ffff7f9ff80 +</pre> + +<p> +The command <code>info</code> <code>locals</code> lists all variables local to the function and their values, but is a bit +dangerous to use, since it will also try to print uninitialized variables. Uninitialized slices may cause gdb to try +to print arbitrary large arrays. +</p> + +<p> +The function’s arguments: +</p> + +<pre> +(gdb) <b>info args</b> +t = 0xf840688b60 +</pre> + +<p> +When printing the argument, notice that it’s a pointer to a +<code>Regexp</code> value. Note that GDB has incorrectly put the <code>*</code> +on the right-hand side of the type name and made up a 'struct' keyword, in traditional C style. +</p> + +<pre> +(gdb) <b>p re</b> +(gdb) p t +$1 = (struct testing.T *) 0xf840688b60 +(gdb) p t +$1 = (struct testing.T *) 0xf840688b60 +(gdb) p *t +$2 = {errors = "", failed = false, ch = 0xf8406f5690} +(gdb) p *t->ch +$3 = struct hchan<*testing.T> +</pre> + +<p> +That <code>struct</code> <code>hchan<*testing.T></code> is the +runtime-internal representation of a channel. It is currently empty, +or gdb would have pretty-printed its contents. +</p> + +<p> +Stepping forward: +</p> + +<pre> +(gdb) <b>n</b> <i># execute next line</i> +149 for _, test := range findTests { +(gdb) <i># enter is repeat</i> +150 re := MustCompile(test.pat) +(gdb) <b>p test.pat</b> +$4 = "" +(gdb) <b>p re</b> +$5 = (struct regexp.Regexp *) 0xf84068d070 +(gdb) <b>p *re</b> +$6 = {expr = "", prog = 0xf840688b80, prefix = "", prefixBytes = []uint8, prefixComplete = true, + prefixRune = 0, cond = 0 '\000', numSubexp = 0, longest = false, mu = {state = 0, sema = 0}, + machine = []*regexp.machine} +(gdb) <b>p *re->prog</b> +$7 = {Inst = []regexp/syntax.Inst = {{Op = 5 '\005', Out = 0, Arg = 0, Rune = []int}, {Op = + 6 '\006', Out = 2, Arg = 0, Rune = []int}, {Op = 4 '\004', Out = 0, Arg = 0, Rune = []int}}, + Start = 1, NumCap = 2} +</pre> + + +<p> +We can step into the <code>String</code>function call with <code>"s"</code>: +</p> + +<pre> +(gdb) <b>s</b> +regexp.(*Regexp).String (re=0xf84068d070, noname=void) at /home/user/go/src/regexp/regexp.go:97 +97 func (re *Regexp) String() string { +</pre> + +<p> +Get a stack trace to see where we are: +</p> + +<pre> +(gdb) <b>bt</b> +#0 regexp.(*Regexp).String (re=0xf84068d070, noname=void) + at /home/user/go/src/regexp/regexp.go:97 +#1 0x0000000000425615 in regexp.TestFind (t=0xf840688b60) + at /home/user/go/src/regexp/find_test.go:151 +#2 0x0000000000430233 in testing.tRunner (t=0xf840688b60, test=0x5747b8) + at /home/user/go/src/testing/testing.go:156 +#3 0x000000000040ea6f in runtime.initdone () at /home/user/go/src/runtime/proc.c:243 +.... +</pre> + +<p> +Look at the source code: +</p> + +<pre> +(gdb) <b>l</b> +92 mu sync.Mutex +93 machine []*machine +94 } +95 +96 // String returns the source text used to compile the regular expression. +97 func (re *Regexp) String() string { +98 return re.expr +99 } +100 +101 // Compile parses a regular expression and returns, if successful, +</pre> + +<h3 id="Pretty_Printing">Pretty Printing</h3> + +<p> +GDB's pretty printing mechanism is triggered by regexp matches on type names. An example for slices: +</p> + +<pre> +(gdb) <b>p utf</b> +$22 = []uint8 = {0 '\000', 0 '\000', 0 '\000', 0 '\000'} +</pre> + +<p> +Since slices, arrays and strings are not C pointers, GDB can't interpret the subscripting operation for you, but +you can look inside the runtime representation to do that (tab completion helps here): +</p> +<pre> + +(gdb) <b>p slc</b> +$11 = []int = {0, 0} +(gdb) <b>p slc-></b><i><TAB></i> +array slc len +(gdb) <b>p slc->array</b> +$12 = (int *) 0xf84057af00 +(gdb) <b>p slc->array[1]</b> +$13 = 0</pre> + + + +<p> +The extension functions $len and $cap work on strings, arrays and slices: +</p> + +<pre> +(gdb) <b>p $len(utf)</b> +$23 = 4 +(gdb) <b>p $cap(utf)</b> +$24 = 4 +</pre> + +<p> +Channels and maps are 'reference' types, which gdb shows as pointers to C++-like types <code>hash<int,string>*</code>. Dereferencing will trigger prettyprinting +</p> + +<p> +Interfaces are represented in the runtime as a pointer to a type descriptor and a pointer to a value. The Go GDB runtime extension decodes this and automatically triggers pretty printing for the runtime type. The extension function <code>$dtype</code> decodes the dynamic type for you (examples are taken from a breakpoint at <code>regexp.go</code> line 293.) +</p> + +<pre> +(gdb) <b>p i</b> +$4 = {str = "cbb"} +(gdb) <b>whatis i</b> +type = regexp.input +(gdb) <b>p $dtype(i)</b> +$26 = (struct regexp.inputBytes *) 0xf8400b4930 +(gdb) <b>iface i</b> +regexp.input: struct regexp.inputBytes * +</pre> |