When is the init function run

Question

I ve tried to find a precise explanation of what the init   function does in Go  I read what Effective Go says but I was unsure if I understood fully what it said  The exact sentence I am unsure is the following      And finally means finally  init is called after all the variable declarations in the package have evaluated their initializers  and those are evaluated only after all the imported packages have been initialized    What does all the variable declarations in the package have evaluated their initializers mean  Does it mean if you declare  global  variables in a package and its files  init   will not run until all of it is evaluated and then it will run all the init function and then main   when   main file name is ran   I also read Mark Summerfield s go book the following      If a package has one or more init   functions they are automatically executed before the main package s main   function is called    In my understanding  init   is only relevant when you run intend to run main   right  or the Main package  Anyone understands more precisely init   feel free to correct me

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A few more tips    There can only be 1 main   function  but there can be many init   functions  You do not need to explicitly call init   or main    they are called automatically  init   and main   do not accept any parameters and also don t return anything  init   runs before main    If you have many init    they run in the order they are declared

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https   golang org ref mem tmp 4  Program initialization runs in a single goroutine  but that goroutine may create other goroutines  which run concurrently  If a package p imports package q  the completion of q s init functions happens before the start of any of p s  The start of the function main main happens after all init functions have finished

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init will be called everywhere uses its package no matter blank import or import   but only one time   this is a package   package demo  import        some logs     var count int  func init         logs Debug count        Do do func Do         logs Debug  dd       any package main package or any test package  import it as blank       printfcoder com we models demo    or import it using it func    printfcoder com we models demo   func someFunc       demo Do       the init will log 0 only one time  the first package using it  its init func will run before the package s init  So   A calls B  B calls C  all of them have init func  the C s init will be run first before B s  B s before A s

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Yes assuming you have this   var WhatIsThe   AnswerToLife    func AnswerToLife   int       return 42    func init         WhatIsThe   0    func main         if WhatIsThe    0           fmt Println  It s all a lie              AnswerToLife   is guaranteed to run before init   is called  and init   is guaranteed to run before main   is called   Keep in mind that init   is always called  regardless if there s main or not  so if you import a package that has an init function  it will be executed   Additionally  you can have multiple init   functions per package  they will be executed in the order they show up in the file  after all variables are initialized of course   If they span multiple files  they will be executed in lexical file name order  as pointed out by  benc       It seems that init   functions are executed in lexical file name order  The Go spec says  build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler   It seems that go build works this way      A lot of the internal Go packages use init   to initialize tables and such  for example https   github com golang go blob 883bc6 src compress bzip2 bzip2 go L480

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When is the init   function run   With Go 1 16  Q1 2021   you will see precisely when it runs  and for how long  See commit 7c58ef7 from CL  Change List  254659  fixing issue 41378    Runtime  implement GODEBUG inittrace 1 support Setting inittrace 1 causes the runtime to emit a single line to standard error for each package with init work  summarizing the execution time and memory allocation  The emitted debug information for init functions can be used to find bottlenecks or regressions in Go startup performance  Packages with no init function work  user defined or compiler generated  are omitted  Tracing plugin inits is not supported as they can execute concurrently  This would make the implementation of tracing more complex while adding support for a very rare use case  Plugin inits can be traced separately by testing a main package importing the plugins package imports explicitly    GODEBUG inittrace 1 go test init internal bytealg  0 008 ms  0 ms clock  0 bytes  0 allocs init runtime  0 059 ms  0 026 ms clock  0 bytes  0 allocs init math  0 19 ms  0 001 ms clock  0 bytes  0 allocs init errors  0 22 ms  0 004 ms clock  0 bytes  0 allocs init strconv  0 24 ms  0 002 ms clock  32 bytes  2 allocs init sync  0 28 ms  0 003 ms clock  16 bytes  1 allocs init unicode  0 44 ms  0 11 ms clock  23328 bytes  24 allocs      Inspired by stapelberg google com who instrumented doInit in a prototype to measure init times with GDB

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Take for example a framework or a library you re designing for other users  these users eventually will have a main function in their code in order to execute their app  If the user directly imports a sub-package of your library s project then the init of that sub-package will be called once  first of all  The same for the root package of the library  etc     There are many times when you may want a code block to be executed without the existence of a main func  directly or not    If you  as the developer of the imaginary library  import your library s sub-package that has an init function  it will be called first and once  you don t have a main func but you need to make sure that some variables  or a table  will be initialized before the calls of other functions   A good thing to remember and not to worry about  is that   the init always execute once per application   init execution happens     right before the init function of the  caller  package  before the  optionally  main func  but after the package-level variables  var         or cost             When you import a package it will run all of its init functions  by order   I ll will give a very good example of an init function  It will add mime types to a standard go s library named mime and a package-level function will use the mime standard package directly to get the custom mime types that are already be initialized at its init function   package mime  import        mime       path filepath     var types   map string string        3dm          x-world x-3dmf         3dmf         x-world x-3dmf         7z           application x-7z-compressed         a            application octet-stream         aab          application x-authorware-bin         aam          application x-authorware-map         aas          application x-authorware-seg         abc          text vndabc         ace          application x-ace-compressed         acgi         text html         afl          video animaflex         ai           application postscript         aif          audio aiff         aifc         audio aiff         aiff         audio aiff         aim          application x-aim         aip          text x-audiosoft-intra         alz          application x-alz-compressed         ani          application x-navi-animation         aos          application x-nokia-9000-communicator-add-on-software         aps          application mime         apk          application vnd android package-archive         arc          application x-arc-compressed         arj          application arj         art          image x-jg         asf          video x-ms-asf         asm          text x-asm         asp          text asp         asx          application x-mplayer2         au           audio basic         avi          video x-msvideo         avs          video avs-video         bcpio        application x-bcpio         bin          application mac-binary         bmp          image bmp         boo          application book         book         application book         boz          application x-bzip2         bsh          application x-bsh         bz2          application x-bzip2         bz           application x-bzip         c            text plain         c            text x-c         cab          application vnd ms-cab-compressed         cat          application vndms-pkiseccat         cc           text x-c         ccad         application clariscad         cco          application x-cocoa         cdf          application cdf         cer          application pkix-cert         cha          application x-chat         chat         application x-chat         chrt         application vnd kde kchart         class        application java         com          text plain         conf         text plain         cpio         application x-cpio         cpp          text x-c         cpt          application mac-compactpro         crl          application pkcs-crl         crt          application pkix-cert         crx          application x-chrome-extension         csh          text x-scriptcsh         css          text css         csv          text csv         cxx          text plain         dar          application x-dar         dcr          application x-director         deb          application x-debian-package         deepv        application x-deepv         def          text plain         der          application x-x509-ca-cert         dif          video x-dv         dir          application x-director         divx         video divx         dl           video dl         dmg          application x-apple-diskimage         doc          application msword         dot          application msword         dp           application commonground         drw          application drafting         dump         application octet-stream         dv           video x-dv         dvi          application x-dvi         dwf          drawing x-dwf  old          dwg          application acad         dxf          application dxf         dxr          application x-director         el           text x-scriptelisp         elc          application x-bytecodeelisp  compiled elisp          eml          message rfc822         env          application x-envoy         eps          application postscript         es           application x-esrehber         etx          text x-setext         evy          application envoy         exe          application octet-stream         f77          text x-fortran         f90          text x-fortran         f            text x-fortran         fdf          application vndfdf         fif          application fractals         fli          video fli         flo          image florian         flv          video x-flv         flx          text vndfmiflexstor         fmf          video x-atomic3d-feature         for          text x-fortran         fpx          image vndfpx         frl          application freeloader         funk         audio make         g3           image g3fax         g            text plain         gif          image gif         gl           video gl         gsd          audio x-gsm         gsm          audio x-gsm         gsp          application x-gsp         gss          application x-gss         gtar         application x-gtar         gz           application x-compressed         gzip         application x-gzip         h            text x-h         hdf          application x-hdf         help         application x-helpfile         hgl          application vndhp-hpgl         hh           text x-h         hlb          text x-script         hlp          application hlp         hpg          application vndhp-hpgl         hpgl         application vndhp-hpgl         hqx          application binhex         hta          application hta         htc          text x-component         htm          text html         html         text html         htmls        text html         htt          text webviewhtml         htx          text html         ice          x-conference x-cooltalk         ico          image x-icon         ics          text calendar         icz          text calendar         idc          text plain         ief          image ief         iefs         image ief         iges         application iges         igs          application iges         ima          application x-ima         imap         application x-httpd-imap         inf          application inf         ins          application x-internett-signup         ip           application x-ip2         isu          video x-isvideo         it           audio it         iv           application x-inventor         ivr          i-world i-vrml         ivy          application x-livescreen         jam          audio x-jam         jav          text x-java-source         java         text x-java-source         jcm          application x-java-commerce         jfif-tbnl    image jpeg         jfif         image jpeg         jnlp         application x-java-jnlp-file         jpe          image jpeg         jpeg         image jpeg         jpg          image jpeg         jps          image x-jps         js           application javascript         json         application json         jut          image jutvision         kar          audio midi         karbon       application vnd kde karbon         kfo          application vnd kde kformula         flw          application vnd kde kivio         kml          application vnd google-earth kml xml         kmz          application vnd google-earth kmz         kon          application vnd kde kontour         kpr          application vnd kde kpresenter         kpt          application vnd kde kpresenter         ksp          application vnd kde kspread         kwd          application vnd kde kword         kwt          application vnd kde kword         ksh          text x-scriptksh         la           audio nspaudio         lam          audio x-liveaudio         latex        application x-latex         lha          application lha         lhx          application octet-stream         list         text plain         lma          audio nspaudio         log          text plain         lsp          text x-scriptlisp         lst          text plain         lsx          text x-la-asf         ltx          application x-latex         lzh          application octet-stream         lzx          application lzx         m1v          video mpeg         m2a          audio mpeg         m2v          video mpeg         m3u          audio x-mpegurl         m            text x-m         man          application x-troff-man         manifest     text cache-manifest         map          application x-navimap         mar          text plain         mbd          application mbedlet         mc           application x-magic-cap-package-10         mcd          application mcad         mcf          text mcf         mcp          application netmc         me           application x-troff-me         mht          message rfc822         mhtml        message rfc822         mid          application x-midi         midi         application x-midi         mif          application x-frame         mime         message rfc822         mjf          audio x-vndaudioexplosionmjuicemediafile         mjpg         video x-motion-jpeg         mm           application base64         mme          application base64         mod          audio mod         moov         video quicktime         mov          video quicktime         movie        video x-sgi-movie         mp2          audio mpeg         mp3          audio mpeg3         mp4          video mp4         mpa          audio mpeg         mpc          application x-project         mpe          video mpeg         mpeg         video mpeg         mpg          video mpeg         mpga         audio mpeg         mpp          application vndms-project         mpt          application x-project         mpv          application x-project         mpx          application x-project         mrc          application marc         ms           application x-troff-ms         mv           video x-sgi-movie         my           audio make         mzz          application x-vndaudioexplosionmzz         nap          image naplps         naplps       image naplps         nc           application x-netcdf         ncm          application vndnokiaconfiguration-message         nif          image x-niff         niff         image x-niff         nix          application x-mix-transfer         nsc          application x-conference         nvd          application x-navidoc         o            application octet-stream         oda          application oda         odb          application vnd oasis opendocument database         odc          application vnd oasis opendocument chart         odf          application vnd oasis opendocument formula         odg          application vnd oasis opendocument graphics         odi          application vnd oasis opendocument image         odm          application vnd oasis opendocument text-master         odp          application vnd oasis opendocument presentation         ods          application vnd oasis opendocument spreadsheet         odt          application vnd oasis opendocument text         oga          audio ogg         ogg          audio ogg         ogv          video ogg         omc          application x-omc         omcd         application x-omcdatamaker         omcr         application x-omcregerator         otc          application vnd oasis opendocument chart-template         otf          application vnd oasis opendocument formula-template         otg          application vnd oasis opendocument graphics-template         oth          application vnd oasis opendocument text-web         oti          application vnd oasis opendocument image-template         otm          application vnd oasis opendocument text-master         otp          application vnd oasis opendocument presentation-template         ots          application vnd oasis opendocument spreadsheet-template         ott          application vnd oasis opendocument text-template         p10          application pkcs10         p12          application pkcs-12         p7a          application x-pkcs7-signature         p7c          application pkcs7-mime         p7m          application pkcs7-mime         p7r          application x-pkcs7-certreqresp         p7s          application pkcs7-signature         p            text x-pascal         part         application pro eng         pas          text pascal         pbm          image x-portable-bitmap         pcl          application vndhp-pcl         pct          image x-pict         pcx          image x-pcx         pdb          chemical x-pdb         pdf          application pdf         pfunk        audio make         pgm          image x-portable-graymap         pic          image pict         pict         image pict         pkg          application x-newton-compatible-pkg         pko          application vndms-pkipko         pl           text x-scriptperl         plx          application x-pixclscript         pm4          application x-pagemaker         pm5          application x-pagemaker         pm           text x-scriptperl-module         png          image png         pnm          application x-portable-anymap         pot          application mspowerpoint         pov          model x-pov         ppa          application vndms-powerpoint         ppm          image x-portable-pixmap         pps          application mspowerpoint         ppt          application mspowerpoint         ppz          application mspowerpoint         pre          application x-freelance         prt          application pro eng         ps           application postscript         psd          application octet-stream         pvu          paleovu x-pv         pwz          application vndms-powerpoint         py           text x-scriptphyton         pyc          application x-bytecodepython         qcp          audio vndqcelp         qd3          x-world x-3dmf         qd3d         x-world x-3dmf         qif          image x-quicktime         qt           video quicktime         qtc          video x-qtc         qti          image x-quicktime         qtif         image x-quicktime         ra           audio x-pn-realaudio         ram          audio x-pn-realaudio         rar          application x-rar-compressed         ras          application x-cmu-raster         rast         image cmu-raster         rexx         text x-scriptrexx         rf           image vndrn-realflash         rgb          image x-rgb         rm           application vndrn-realmedia         rmi          audio mid         rmm          audio x-pn-realaudio         rmp          audio x-pn-realaudio         rng          application ringing-tones         rnx          application vndrn-realplayer         roff         application x-troff         rp           image vndrn-realpix         rpm          audio x-pn-realaudio-plugin         rt           text vndrn-realtext         rtf          text richtext         rtx          text richtext         rv           video vndrn-realvideo         s            text x-asm         s3m          audio s3m         s7z          application x-7z-compressed         saveme       application octet-stream         sbk          application x-tbook         scm          text x-scriptscheme         sdml         text plain         sdp          application sdp         sdr          application sounder         sea          application sea         set          application set         sgm          text x-sgml         sgml         text x-sgml         sh           text x-scriptsh         shar         application x-bsh         shtml        text x-server-parsed-html         sid          audio x-psid         skd          application x-koan         skm          application x-koan         skp          application x-koan         skt          application x-koan         sit          application x-stuffit         sitx         application x-stuffitx         sl           application x-seelogo         smi          application smil         smil         application smil         snd          audio basic         sol          application solids         spc          text x-speech         spl          application futuresplash         spr          application x-sprite         sprite       application x-sprite         spx          audio ogg         src          application x-wais-source         ssi          text x-server-parsed-html         ssm          application streamingmedia         sst          application vndms-pkicertstore         step         application step         stl          application sla         stp          application step         sv4cpio      application x-sv4cpio         sv4crc       application x-sv4crc         svf          image vnddwg         svg          image svg xml         svr          application x-world         swf          application x-shockwave-flash         t            application x-troff         talk         text x-speech         tar          application x-tar         tbk          application toolbook         tcl          text x-scripttcl         tcsh         text x-scripttcsh         tex          application x-tex         texi         application x-texinfo         texinfo      application x-texinfo         text         text plain         tgz          application gnutar         tif          image tiff         tiff         image tiff         tr           application x-troff         tsi          audio tsp-audio         tsp          application dsptype         tsv          text tab-separated-values         turbot       image florian         txt          text plain         uil          text x-uil         uni          text uri-list         unis         text uri-list         unv          application i-deas         uri          text uri-list         uris         text uri-list         ustar        application x-ustar         uu           text x-uuencode         uue          text x-uuencode         vcd          application x-cdlink         vcf          text x-vcard         vcard        text x-vcard         vcs          text x-vcalendar         vda          application vda         vdo          video vdo         vew          application groupwise         viv          video vivo         vivo         video vivo         vmd          application vocaltec-media-desc         vmf          application vocaltec-media-file         voc          audio voc         vos          video vosaic         vox          audio voxware         vqe          audio x-twinvq-plugin         vqf          audio x-twinvq         vql          audio x-twinvq-plugin         vrml         application x-vrml         vrt          x-world x-vrt         vsd          application x-visio         vst          application x-visio         vsw          application x-visio         w60          application wordperfect60         w61          application wordperfect61         w6w          application msword         wav          audio wav         wb1          application x-qpro         wbmp         image vnd wap wbmp         web          application vndxara         wiz          application msword         wk1          application x-123         wmf          windows metafile         wml          text vnd wap wml         wmlc         application vnd wap wmlc         wmls         text vnd wap wmlscript         wmlsc        application vnd wap wmlscriptc         word         application msword         wp5          application wordperfect         wp6          application wordperfect         wp           application wordperfect         wpd          application wordperfect         wq1          application x-lotus         wri          application mswrite         wrl          application x-world         wrz          model vrml         wsc          text scriplet         wsrc         application x-wais-source         wtk          application x-wintalk         x-png        image png         xbm          image x-xbitmap         xdr          video x-amt-demorun         xgz          xgl drawing         xif          image vndxiff         xl           application excel         xla          application excel         xlb          application excel         xlc          application excel         xld          application excel         xlk          application excel         xll          application excel         xlm          application excel         xls          application excel         xlt          application excel         xlv          application excel         xlw          application excel         xm           audio xm         xml          text xml         xmz          xgl movie         xpix         application x-vndls-xpix         xpm          image x-xpixmap         xsr          video x-amt-showrun         xwd          image x-xwd         xyz          chemical x-pdb         z            application x-compress         zip          application zip         zoo          application octet-stream         zsh          text x-scriptzsh         docx         application vnd openxmlformats-officedocument wordprocessingml document         docm         application vnd ms-word document macroEnabled 12         dotx         application vnd openxmlformats-officedocument wordprocessingml template         dotm         application vnd ms-word template macroEnabled 12         xlsx         application vnd openxmlformats-officedocument spreadsheetml sheet         xlsm         application vnd ms-excel sheet macroEnabled 12         xltx         application vnd openxmlformats-officedocument spreadsheetml template         xltm         application vnd ms-excel template macroEnabled 12         xlsb         application vnd ms-excel sheet binary macroEnabled 12         xlam         application vnd ms-excel addin macroEnabled 12         pptx         application vnd openxmlformats-officedocument presentationml presentation         pptm         application vnd ms-powerpoint presentation macroEnabled 12         ppsx         application vnd openxmlformats-officedocument presentationml slideshow         ppsm         application vnd ms-powerpoint slideshow macroEnabled 12         potx         application vnd openxmlformats-officedocument presentationml template         potm         application vnd ms-powerpoint template macroEnabled 12         ppam         application vnd ms-powerpoint addin macroEnabled 12         sldx         application vnd openxmlformats-officedocument presentationml slide         sldm         application vnd ms-powerpoint slide macroEnabled 12         thmx         application vnd ms-officetheme         onetoc       application onenote         onetoc2      application onenote         onetmp       application onenote         onepkg       application onenote         xpi          application x-xpinstall      func init         for ext  typ    range types              skip errors         mime AddExtensionType ext  typ              typeByExtension returns the MIME type associated with the file extension ext     The extension ext should begin with a leading dot  as in   html      When ext has no associated type  typeByExtension returns           Extensions are looked up first case-sensitively  then case-insensitively        The built-in table is small but on unix it is augmented by the local    system s mime types file s  if available under one or more of these    names           etc mime types       etc apache2 mime types       etc apache mime types       On Windows  MIME types are extracted from the registry        Text types have the charset parameter set to  utf-8  by default  func TypeByExtension fullfilename string  string       ext    filepath Ext fullfilename      typ    mime TypeByExtension ext          mime TypeByExtension returns as text plain    charset utf-8 the static  js  not always      if ext      js   amp  amp   typ     text plain     typ     text plain  charset utf-8              if ext      js                typ    application javascript                      return typ     Hope that helped you and other users  don t hesitate to post again if you have more questions

User · Answer

Something to add to this  which I would ve added as a comment but the time of writing this post I d not yet enough reputation   Having multiple inits in the same package I ve not yet found any guaranteed way to know what order in which they will be run  For example I have   package config     - config go     - router go   Both config go and router go contain init   functions  but when running router go s function ran first  which caused my app to panic    If you re in a situation where you have multiple files  each with its own init   function be very aware that you aren t guaranteed to get one before the other  It is better to use a variable assignment as OneToOne shows in his example  Best part is  This variable declaration will happen before ALL init   functions in the package   For example  config go   var ConfigSuccess   configureApplication    func init         doSomething      func configureApplication   bool       l4g Info  Configuring application          if valid    loadCommandLineFlags     valid           l4g Critical  Failed to load Command Line Flags           return false           return true     router go   func init         var           rwd string         tmp string         ok  bool           if metapath  ok    Config  fs    metapath    string   ok           var err error         Conn  err   services NewConnection metapath     metadata db           if err    nil               panic err                      regardless of whether var ConfigSuccess   configureApplication   exists in router go or config go  it will be run before EITHER init   is run

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mutil init function in one package execute order    const and variable defined file init   function execute init function execute order by the filename asc

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The init func runs first and then main  It s used for setting something first  before your program runs  for example   Accessing a template    Running the program using all cores   Checking the Goos and arch etc

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Here is another example - https   play golang org p 9P-LmSkUMKY package main  import        quot fmt quot     func callOut   int       fmt Println  quot Outside is beinge executed quot       return 1    var test   callOut    func init         fmt Println  quot Init3 is being executed quot      func init         fmt Println  quot Init is being executed quot      func init         fmt Println  quot Init2 is being executed quot      func main         fmt Println  quot Do your thing   quot      Output of the above program   go run init init go Outside is being executed Init3 is being executed Init is being executed Init2 is being executed Do your thing

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See this picture      import -- gt  const -- gt  var -- gt  init       If a package imports other packages  the imported packages are initialized first  Current package s constant initialized then  Current package s variables are initialized then  Finally  init   function of current package is called       A package can have multiple init functions  either in a single file or distributed across multiple files  and they are called in the order in which they are presented to the compiler         A package will be initialised only once even if it is imported from multiple packages

[go] When is the init() function run?

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Examples related to init