{"id":4340,"date":"2020-11-13T00:19:44","date_gmt":"2020-11-12T18:49:44","guid":{"rendered":"https:\/\/golangbyexamples.com\/?p=4340"},"modified":"2021-04-29T22:45:47","modified_gmt":"2021-04-29T17:15:47","slug":"order-execution-program-golang","status":"publish","type":"post","link":"https:\/\/golangbyexamples.com\/order-execution-program-golang\/","title":{"rendered":"Order of execution of a Go program"},"content":{"rendered":"\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_17 counter-hierarchy counter-decimal ez-toc-light-blue\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" style=\"display: none;\"><i class=\"ez-toc-glyphicon ez-toc-icon-toggle\"><\/i><\/a><\/span><\/div>\n<nav><ul class=\"ez-toc-list ez-toc-list-level-1\"><li class=\"ez-toc-page-1 ez-toc-heading-level-1\"><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/golangbyexamples.com\/order-execution-program-golang\/#Overview\" title=\"Overview\">Overview<\/a><\/li><li class=\"ez-toc-page-1 ez-toc-heading-level-1\"><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/golangbyexamples.com\/order-execution-program-golang\/#Example\" title=\"Example\">Example<\/a><\/li><\/ul><\/nav><\/div>\n<h1 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Overview\"><\/span><strong>Overview<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h1>\n\n\n\n<p class=\"has-medium-font-size\">Below the order of execution of a go program.<\/p>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>The program starts with the main package.<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>All imported packages in the source files of the main package is initialized. The same thing happens recursively for further imported packages.<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Then global variables declaration in these packages is initialized. The initialization dependency kicks in for the initialization of these variables.  <a href=\"https:\/\/golang.org\/ref\/spec#Order_of_evaluation\" data-type=\"URL\" data-id=\"https:\/\/golang.org\/ref\/spec#Order_of_evaluation\">https:\/\/golang.org\/ref\/spec#Order_of_evaluation<\/a><\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>After this, init() function is run in these packages<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Global variables in the main package are initialized<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>init function in the main package is run if present<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>main function in main package is run.<\/li><\/ul>\n\n\n\n<p class=\"has-medium-font-size\">Note here that package initialization is only done once even if it is imported several times.<\/p>\n\n\n\n<p class=\"has-medium-font-size\">For example, if main package imports package <strong>a<\/strong> and in turn package <strong>a<\/strong> imports package <strong>b<\/strong>, then below will be the order<\/p>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Global variables in package b will be initialized. init function in source files of package b will be run<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Global variables in the package <strong>a<\/strong> will be initialized. init function in source files of the package <strong>a<\/strong> will be run<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>Global variables in main package will be initialized. init function in source files of main package will be run<\/li><\/ul>\n\n\n\n<ul class=\"has-medium-font-size wp-block-list\"><li>main function will start executing.<\/li><\/ul>\n\n\n\n<h1 class=\"wp-block-heading\"><span class=\"ez-toc-section\" id=\"Example\"><\/span><strong>Example<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h1>\n\n\n\n<p class=\"has-medium-font-size\">Let&#8217;s see a program for the same.<\/p>\n\n\n\n<p class=\"has-medium-font-size\"><strong>go.mod<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">module sample\n\ngo 1.14<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>sample\/b\/b1.go<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">package b\n\nimport (\n\t\"fmt\"\n)\n\nfunc init() {\n\tfmt.Println(\"Init: b1\")\n}\n\nfunc TestB() error {\n\treturn nil\n}<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>sample\/b\/b2.go<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">package b\n\nimport (\n\t\"fmt\"\n)\n\nfunc init() {\n\tfmt.Println(\"Init: b2\")\n}<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>sample\/a\/a1.go<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">package a\n\nimport (\n\t\"fmt\"\n\t\"sample\/b\"\n)\n\nfunc init() {\n\tfmt.Println(\"Init: a1\")\n}\n\nfunc TestA() error {\n\treturn b.TestB()\n}<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>sample\/a\/a2.go<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">package a\n\nimport (\n\t\"fmt\"\n)\n\nfunc init() {\n\tfmt.Println(\"Init: a2\")\n}<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>sample\/main.go<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">package main\n\nimport (\n\t\"fmt\"\n\t\"sample\/a\"\n)\n\nfunc init() {\n\tfmt.Println(\"Init: main\")\n}\nfunc main() {\n\tfmt.Println(\"Main Function Executing\")\n\ta.TestA()\n}<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\"><strong>Output<\/strong><\/p>\n\n\n\n<pre class=\"wp-block-prismatic-blocks\"><code class=\"language-go\">Init: b1\nInit: b2\nInit: a1\nInit: a1\nInit: main\nMain Function Executing<\/code><\/pre>\n\n\n\n<p class=\"has-medium-font-size\">Notice in above example that init function in source files of package <strong>b<\/strong> are run first. Then init function in source files of package <strong>a<\/strong> is run and then init function in source file of main package is run. After that main function is run<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Table of Contents OverviewExample Overview Below the order of execution of a go program. The program starts with the main package. All imported packages in the source files of the main package&#8230;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"spay_email":"","footnotes":"","jetpack_publicize_message":"","jetpack_is_tweetstorm":false},"categories":[1],"tags":[3,4],"class_list":["post-4340","post","type-post","status-publish","format-standard","hentry","category-tech","tag-go","tag-golang"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.7 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Order of execution of a Go program - Welcome To Golang By Example<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/golangbyexamples.com\/order-execution-program-golang\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Order of execution of a Go program - Welcome To Golang By Example\" \/>\n<meta property=\"og:description\" content=\"Table of Contents OverviewExample Overview Below the order of execution of a go program. 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