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<time datetime="2016-12-05T00:00:00+00:00" pubdate="pubdate">
Mon 05 December 2016
</time>
<h1>
<a rel="bookmark"
href="/go-basics.html"
title="Lien permanent vers «Go basics»">
Go basics
</a>
</h1>
<div class="meta">
In «<a href="/category/cheatsheet.html">cheatsheet</a>»
by <a href="/author/rcs.html">RCS</a>
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</header>
<div class="post-content">
<h2><a href="#contents" id="contents">Contents</a></h2>
<ul>
<li><a href="#overview">Overview</a><ul>
<li><a href="#go+git">Go+git</a></li>
<li><a href="#namedtypes">Named types</a></li>
<li><a href="#concurrency">Concurrency</a><ul>
<li><a href="#channels">channels</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#builtin">Builtin</a><ul>
<li><a href="#append">append</a></li>
</ul>
</li>
<li><a href="#packages">Packages</a><ul>
<li><a href="#async">async</a></li>
<li><a href="#sort">sort</a></li>
<li><a href="#signal">signal</a></li>
<li><a href="#testing">testing</a></li>
</ul>
</li>
<li><a href="#modules">Modules</a></li>
<li><a href="#testing">Testing</a></li>
<li><a href="#tool">Tool</a></li>
<li><a href="#gotchas">Gotchas</a></li>
<li><a href="#refs">References</a></li>
</ul>
<h3><a href="#overview" id="overview">Overview</a></h3>
<div class="highlight"><pre><span></span>go build [-i] github.com/user/aclient/cmd/multiget // (-i installs binary - like running go install)
go clean [-i] github.com/user/aclient/cmd/multiget // removes object files and executables (-i removes binaries installed with go install)
</pre></div>
<h4><a href="#go+git" id="go+git">Go+git</a></h4>
<div class="highlight"><pre><span></span>git clone https://github.com/origin/repo.git OR go get https://github.com/origin/repo
git remote add fork https://github.com/fork/repo.git
git push fork
</pre></div>
<p>This is preferred to cloning the fork because it enables pushing changes to your fork without changing import paths. So for instance if there are local import paths like github.com/origin/repo/sub then you don't have to change them all to github.com/fork/repo/sub because you're working on the origin. </p>
<p>Sending pull requests from the fork then becomes easy because the package import paths are same as for the origin. </p>
<p>tldr; don't clone or work on the fork - simply add it as a remote and push changes to it. Anyway in most cases you won't be able to push changes to the origin because you won't have permission. </p>
<h4><a href="#namedtypes" id="namedtypes">Named types</a></h4>
<div class="highlight"><pre><span></span><span class="kd">type</span> <span class="nx">StringSlice</span> <span class="p">[]</span><span class="kt">string</span>
</pre></div>
<p>Methods can be defined for any named type (except a pointer or an interface); the receiver does not have to be a struct. So:</p>
<div class="highlight"><pre><span></span><span class="kd">func</span> <span class="p">(</span><span class="nx">s</span> <span class="o">*</span><span class="nx">StringSlice</span><span class="p">)</span> <span class="nx">Len</span><span class="p">()</span> <span class="kt">int</span> <span class="p">{</span>
<span class="k">return</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
<h4><a href="#concurrency" id="concurrency">Concurrency</a></h4>
<p>Share memory by communicating.</p>
<h5><a href="#channels" id="channels">channels</a></h5>
<ul>
<li>Read channel: </li>
</ul>
<div class="highlight"><pre><span></span><span class="kd">var</span> <span class="nx">ch</span> <span class="o"><-</span><span class="kd">chan</span> <span class="kt">int</span>
<span class="nx">ch</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="o"><-</span><span class="kd">chan</span> <span class="kt">int</span><span class="p">)</span>
</pre></div>
<ul>
<li>Write channel</li>
</ul>
<div class="highlight"><pre><span></span><span class="kd">var</span> <span class="nx">ch</span> <span class="kd">chan</span><span class="o"><-</span> <span class="kt">int</span>
<span class="nx">ch</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span><span class="o"><-</span> <span class="kt">int</span><span class="p">)</span>
</pre></div>
<ul>
<li>Channels are usually declared bidirectional. Read/write channels are useful as arguments / return values from functions. So for instance:</li>
</ul>
<div class="highlight"><pre><span></span><span class="kd">func</span> <span class="nx">returnReadChannel</span><span class="p">()</span> <span class="o"><-</span><span class="kd">chan</span> <span class="p">{</span>
<span class="nx">ch</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">int</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
<span class="k">defer</span> <span class="nb">close</span><span class="p">(</span><span class="nx">ch</span><span class="p">)</span>
<span class="c1">//do something else</span>
<span class="p">}</span>
<span class="k">return</span> <span class="nx">ch</span> <span class="c1">// ch gets converted to a read channel when function returns so caller of function can only read from channel</span>
<span class="p">}</span>
</pre></div>
<ul>
<li>Buffered</li>
</ul>
<div class="highlight"><pre><span></span><span class="nx">bch</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span><span class="o"><-</span> <span class="kt">int</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
</pre></div>
<ul>
<li>Range over channel (or read channel) doesn't block if channel is closed. </li>
</ul>
<div class="highlight"><pre><span></span><span class="k">for</span> <span class="nx">x</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">ch</span> <span class="p">{</span> <span class="c1">// x has zero value of ch type</span>
<span class="c1">// </span>
<span class="p">}</span>
<span class="nx">x</span><span class="p">,</span> <span class="nx">_</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">ch</span> <span class="c1">//doesn't block if ch is closed. x has zero value of ch type</span>
</pre></div>
<ul>
<li>Reading from closed channel results in zero value of channel type. Can use the untyped bool value returned to check whether channel closed (false).</li>
</ul>
<div class="highlight"><pre><span></span><span class="k">if</span> <span class="nx">val</span><span class="p">,</span> <span class="nx">ok</span> <span class="o">:=</span> <span class="o"><-</span><span class="nx">ch</span><span class="p">;</span> <span class="nx">ok</span> <span class="p">{</span> <span class="c1">// ok is false if ch is closed</span>
<span class="c1">// do something with val</span>
<span class="p">}</span>
</pre></div>
<ul>
<li>
<p>Sending on closed channel panics.</p>
</li>
<li>
<p>Sending on nil results in fatal error. </p>
</li>
</ul>
<h3><a href="#builtin" id="builtin">Builtin</a></h3>
<h4><a href="#append" id="append">append</a></h4>
<div class="highlight"><pre><span></span><span class="nx">s</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">int</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
<span class="nx">s</span> <span class="p">=</span> <span class="nb">append</span><span class="p">(</span><span class="nx">s</span><span class="p">,</span> <span class="mi">6</span><span class="p">)</span> <span class="c1">// Out: [0,0,0,0,0,6]</span>
</pre></div>
<h4><a href="#sort" id="sort">sort</a></h4>
<p>A type, typically a collection, that satisfies sort.Interface can be sorted by the routines in the sort package. The methods require that the elements of the collection be enumerated by an integer index. </p>
<div class="highlight"><pre><span></span><span class="kd">type</span> <span class="nx">Interface</span> <span class="kd">interface</span> <span class="p">{</span>
<span class="c1">// Len is the number of elements in the collection.</span>
<span class="nx">Len</span><span class="p">()</span> <span class="kt">int</span>
<span class="c1">// Less reports whether the element with</span>
<span class="c1">// index i should sort before the element with index j.</span>
<span class="nx">Less</span><span class="p">(</span><span class="nx">i</span><span class="p">,</span> <span class="nx">j</span> <span class="kt">int</span><span class="p">)</span> <span class="kt">bool</span>
<span class="c1">// Swap swaps the elements with indexes i and j.</span>
<span class="nx">Swap</span><span class="p">(</span><span class="nx">i</span><span class="p">,</span> <span class="nx">j</span> <span class="kt">int</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
<h4><a href="#signal" id="signal">signal</a></h4>
<p>The signal package implements access to incoming signals.</p>
<p>Signals are a form of inter-process communication (IPC) mediated by the kernel and handled by processes/threads. They are asynchronous messages sent to processes or specific threads. Signals are similar to interrupts but differ from interrupts which are mediated by the processor and handled by the kernel. </p>
<p>The following signals cannot be caught by Go programs:
+ SIGKILL
+ SIGSTOP</p>
<p>The following signals are synchronous (triggered by program execution errors) and will get converted to run time panic by Go programs.
+ SIGBUS : process causes bus error, i.e. process attempts to access non-existent physical address.
+ SIGFPE : process attempted erroneous arithmetic operation. FPE (floating point exception) is misnomer because also sent if process attempts integer divide by zero on x86 CPU.
+ SIGSEGV : process attempted to access memory address outside of it's virtual memory space.</p>
<p>The following signals are asynchronous (sent by kernel or another program).
+ SIGHUP : when programs loses it's controlling terminal
+ SIGINT : Control-C (interrupt character) sent - program exits
+ SIGTERM : Control-\ sent - program exits with stack dump</p>
<p>os.signal.Signal.Notify(chan<- os.Signal, sig ...os.Signal) can be used to over-ride default behaviour and deliver signals over registered channels. Notify must use a buffered channel or risk missing the signal if program isn't ready to receive when the signal is sent because Notify doesn't block and simply drops the signal.</p>
<div class="highlight"><pre><span></span> <span class="o">//</span> send but do not block <span class="kr">for</span> it
select <span class="p">{</span>
case <span class="kt">c</span> <span class="o"><-</span> sig<span class="o">:</span>
default<span class="o">:</span>
<span class="p">}</span>
</pre></div>
<p>A signal mask will block certain types of signals and prevent a Go program from responding to these signals. Some signals cannot be blocked using masks:
+ SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF, SIGCANCEL, SIGSETXID (last 2 used internally by glibc). </p>
<p>When Go programs write to a broken pipe, kernel will raise signal:
+ SIGPIPE</p>
<p>If no channel has been registered to be notified of SIGPIPE, the program will exit if the write is to a file descriptor of type 1 or 2. Write to all other types will cause write to fail with EPIPE error.</p>
<p>There are precautions while handling signals when calling non-Go programs from Go programs and vice-versa. Check <a href="https://golang.org/pkg/os/signal">os.signal</a> for details. </p>
<h3><a href="#modules" id="modules">Modules</a></h3>
<div class="highlight"><pre><span></span><span class="x">go list -f '</span><span class="cp">{{</span> <span class="nv">.Dir</span> <span class="cp">}}</span><span class="x">' -m all</span>
<span class="x">go list -f '</span><span class="cp">{{</span> <span class="nv">.Dir</span> <span class="cp">}}</span><span class="x">' -m github.com/golang/protobuf // /home/sridhar/dev/mygo/pkg/mod/github.com/golang/protobuf@v1.2.0</span>
</pre></div>
<div class="highlight"><pre><span></span>cd ~/dev/modgo/src/github.com/agnivade/funnel
go build -tags "disableelasticsearch disableinfluxdb disablekafka disableredis disables3 disablenats" ./cmd/funnel // outputs funnel binary in pwd
go install ./cmd/funnel // installs funnel binary in $GOPATH/bin
</pre></div>
<h3><a href="#testing" id="testing">Testing</a></h3>
<div class="highlight"><pre><span></span>go test -run Foo github.com/user/project/
</pre></div>
<h3><a href="#tool" id="tool">Tool</a></h3>
<div class="highlight"><pre><span></span>go tool # lists all available tools
</pre></div>
<ul>
<li>addr2line</li>
<li>asm</li>
<li>buildid</li>
<li>cgo</li>
<li>compile</li>
<li>cover</li>
<li>dist</li>
<li>doc<ul>
<li>go doc pkg # shows documentation for pkg</li>
<li>go doc net/http Server.Close # shows documentation for method Close() of Server in net/http</li>
<li>go doc cmd/<tool> # shows documentation for specific tool</li>
</ul>
</li>
<li>fix</li>
<li>link</li>
<li>nm</li>
<li>objdump</li>
<li>pack</li>
<li>pprof</li>
<li>test2json</li>
<li>tour</li>
<li>trace</li>
<li>vet</li>
</ul>
<h3><a href="#gotchas" id="gotchas">Gotchas</a></h3>
<h3><a href="#refs" id="refs">References</a></h3>
<ul>
<li><a href="https://research.swtch.com/">research!rsc</a></li>
<li><a href="https://golang.org/pkg/os/signal">os.signal</a></li>
<li><a href="https://golang.org/doc/effective_go.html">Effective Go</a></li>
<li><a href="https://golang.org/ref/spec">Language Specification</a> </li>
</ul>
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