Earlier versions of gcc didn't support C99 or later at all. Making reaching the closing "}" do an implicit "return 0;" was a new language feature, and it was ti

Earlier versions of gcc didn't support C99 or later at all. Making reaching the closing "}" do an implicit "return 0;" was a new language feature, and it was tied to the (non-default) "-std=c9x" or later "-std=c99" option. Some existing code might have depended (foolishly IMHO) on the arbitrary non-zero value returned from main. Even if that's not the case, if you're programming in C90, having your program indicate that it failed is a good reminder to add the "return 0;". I've never bothered to check exactly how "gcc -std=c90" handles this, but having it intentionally return a non-zero value in C90 mode would have been a good choice, since it would make the bug of omitting the return or exit more visible. gcc can also warn about not returning a value from a non-void function. Whatever it returns in C90 mode, it's not a bug, and so the gcc maintainers have probably decided that they have more important things to do than change the behavior. They might have made a different choice if they were implementing "-std=c90" from scratch today.