{"id":1371,"date":"2015-05-23T00:01:50","date_gmt":"2015-05-23T07:01:50","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=1371"},"modified":"2016-05-07T16:21:10","modified_gmt":"2016-05-07T23:21:10","slug":"before-you-say-goodbye","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=1371","title":{"rendered":"Before You Say Goodbye . . ."},"content":{"rendered":"

The standard C library contains a lot of interesting and unusual routines. Some can really put the fun<\/em> into function. One of them I’ve rarely used, but which can be extremely handy, is atexit()<\/em>.
\n
\nThe atexit()<\/em> function is one of those weirdo functions that uses another function as its argument. (I’ll explain how that works in a future Lesson<\/a>.) The function that atexit()<\/em> references is run automatically whenever your program exits, either by returning from the main()<\/em> function or at any point by calling exit()<\/em>. Either way, whichever function atexit()<\/em> registered is called before the program terminates.<\/p>\n

Sounds fun, right?<\/p>\n

Here’s a silly example:<\/p>\n

\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nvoid bye(void);\r\n\r\nint main()\r\n{\r\n    atexit(bye);\r\n\r\n    puts(\"Happy Little Program\");\r\n    puts(\"Massive processing completed!\");\r\n\r\n    return(0);\r\n}\r\n\r\nvoid bye(void)\r\n{\r\n    puts(\"So long, sailor!\");\r\n}<\/pre>\n
The atexit()<\/em> function (Line 8) registers the bye()<\/em> function to be run when the program quits. So when the return<\/em> statement is hit at Line 13, the bye()<\/em> function is called. Here’s sample output:<\/p>\n
Happy Little Program\r\nMassive processing completed!\r\nSo long, sailor!<\/code><\/pre>\n
The atexit()<\/em> can be called multiple times to have a spate of functions run when the program quits. The functions are run in the reverse order they’re called; so the first atexit()<\/em> specifies the last function run. Here’s a modification to the sample code that illustrates that behavior:<\/p>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nvoid bye(void);\r\nvoid solong(void);\r\n\r\nint main()\r\n{\r\n    atexit(bye);\r\n    atexit(solong);\r\n\r\n    puts(\"Happy Little Program\");\r\n    puts(\"Massive processing completed!\");\r\n\r\n    return(0);\r\n}\r\n\r\nvoid bye(void)\r\n{\r\n    puts(\"So long, sailor!\");\r\n}\r\n\r\nvoid solong(void)\r\n{\r\n    puts(\"Whew! That was rough.\");\r\n}<\/pre>\n
The bye()<\/em> function is registered first by atexit()<\/em> in Line 9. It’s called last.<\/p>\n
The solong()<\/em> function is registered next, so it’s called before bye()<\/em>.<\/p>\n
Here’s sample output:<\/p>\n
Happy Little Program\r\nMassive processing completed!\r\nWhew! That was rough.\r\nSo long, sailor!<\/code><\/pre>\n
A more practical example of how to use atexit()<\/em> would be in a large program where some necessary cleanup would be performed. Rather than try to snake all the return<\/em> calls and exit()<\/em> routines together, simply register the cleanup functions by using atexit()<\/em>. Then no matter where the program chooses to bail, you’ll know that things are taken care of.<\/p>\n","protected":false},"excerpt":{"rendered":"
The wonders of the atexit()<\/em> function. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-1371","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1371","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1371"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1371\/revisions"}],"predecessor-version":[{"id":1934,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1371\/revisions\/1934"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1371"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1371"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1371"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}