what's the use of return 0 ?

[supercoollee]

i know return 0 means making eax = 0 before return. i see a lot of moduls have return 0 at the end. what is it for ?
currently i don't have any of that in my program but it seems to be working without problem.

[hutch--]

The specification for many Windows messages when used is to return 0. You can do it manually with xor eax, eax, mov eax, 0 or use the macro that picks out if its returning zero or not.

[supercoollee]

sorry, what i meant is not "how to use return 0" . but "why should i use return 0"  :lol

can i cancel the "xor eax,eax" before ret ?  - i mean, in the modules written by myself.
i certainly will not modify the modules written by other people.

[ecube]

supercoollee the purpose is a easy way to signify if the function worked correctly, for example 1 = everythings fine, 0 = it failed. It can be any number just 1 and 0 are common.

[supercoollee]

which means, if i don't need to check whether the function fails, i don't need to rewrite eax , right?
that's good for me.

[hutch--]

No, actually read what has been said, some Windows mesages REQUIRE that 0 is returned in EAX. Some functions that you write can return anything you like in EAX including ZERO. Learn how a system works and you will understand WHY you can return 0.

[jj2007]

Most common use is in subclassing procedures; Windows wants to know, for example, if it should erase a button's background, or if you have done it already yourself.

WM_ERASEBKGND Message

The WM_ERASEBKGND message is sent when the window background must be erased (for example, when a window is resized). The message is sent to prepare an invalidated portion of a window for painting.
...
Return Value

An application should return nonzero if it erases the background; otherwise, it should return zero.

[dedndave]

after learning how the API functions work, i tend to write routines much the same way
nearly all API functions return a value in EAX

for some, a zero return value indicates failure - a non-zero value is the result of the operation
one example of this might be a function that returns a handle in EAX - if the call fails, EAX will be zero, otherwise it is the handle

for some other API calls, a zero value indicates success
in these cases, a non-zero value may indicate the error code - or you may have to call GetLastError to get the error code

i try to emulate these behaviors when i write my own routines
notice, in ASM, you can also return values in ECX and EDX - i might use ECX for a string length and EDX for the pointer, for example
not sure how those registers are used in different compilers, though

[Slugsnack]

http://users.aber.ac.uk/auj/voidmain.shtml

the system uses the return value after your program exits. different return values signify different things. on windows, you generally return 0 for success and anything else for non-success.

another relevant link :
http://stackoverflow.com/questions/1188335/why-default-return-value-of-main-is-0-and-not-exit-success

[supercoollee]

OK, i see   :lol

[jj2007]

on windows, you generally return 0 for success and anything else for non-success.

Careful with that statement. For most API calls, it is actually the other way round.

Then, there are some silly outliers, such as the FileOpen/CreateFile/FindFirstFile family that uses HFILE_ERROR and INVALID_HANDLE_VALUE, both of which are -1. Which implies that a return value of 0 would be a valid handle ::)

GetKeyState is also a nice example: A key is pressed .if sword ptr ax<0, i.e. according to the documentation, eax might be positive but ax must be negative.

The best advice is: Read the documentation individually. Don't rely on your intuition.

[GregL]

For the procedures I write, I almost always return zero for success and non-zero for an error.

You should set eax to zero for success at the end of the program too, for ERRORLEVEL and it's just good practice to do so.

[jj2007]

For the procedures I write, I almost always return zero for success and non-zero for an error.

It could be declared a matter of taste. However, in many cases you want to return something useful in eax, such as a handle or a pointer. That is easier done if you declare eax!=0 a success.

[dedndave]

just like the API functions - it is neither "set in stone" nor "a matter of taste"
it simply depends on the requirement of the function

[GregL]

jj,

Yes, I said "almost always". :lol

Returning zero for success is a C convention, but it is not "set in stone" either.