CODEWITHSUNDEEP
cstackprogram-entry-pointactivation-record
user2177515
user2177515

Reputation:

Does main have a return address, dynamic link or return value in C?

According to our book, each function has an activation record in the run-time stack in C. Each of these activation records has a return address, dynamic link, and return value. Does main have these also?

Upvotes: 6

Views: 3983

Answers (4)

dionyziz
dionyziz

Reputation: 2452

While this depends on the implementation, it is worthy looking at a C program compiled with gcc. If you run objdump -d executable, you will see it disassembled and you can see how main() behaves. Here's an example:

08048680 <_start>: ... 8048689: 54 push %esp 804868a: 52 push %edx 804868b: 68 a0 8b 04 08 push $0x8048ba0 8048690: 68 30 8b 04 08 push $0x8048b30 8048695: 51 push %ecx 8048696: 56 push %esi 8048697: 68 f1 88 04 08 push $0x80488f1 804869c: e8 9f ff ff ff call 8048640 <__libc_start_main@plt> 80486a1: f4 hlt ... 080488f1 <main>: 80488f1: 55 push %ebp 80488f2: 89 e5 mov %esp,%ebp 80488f4: 57 push %edi 80488f5: 56 push %esi 80488f6: 53 push %ebx ... 8048b2b: 5b pop %ebx 8048b2c: 5e pop %esi 8048b2d: 5f pop %edi 8048b2e: 5d pop %ebp 8048b2f: c3 ret

You can see that main behaves similarly to a regular function in that it returns normally. In fact, if you look at the linux base documentation, you'll see that the call to __libc_start_main that we see from _start actually requires main to behave like a regular function.

Upvotes: 2

vonbrand
vonbrand

Reputation: 11791

In C/C++, main() is written just like a function, but isn't one. For example, it isn't allowed to call main(), it has several possible prototypes (can't do that in C!). Whatever is returned from it gets passed to the operating system (and the program ends).

Individual C implementations might handle main() like a function called from "outside" for uniformity, but nobody forces them to do so (or disallow switching to some other form of doing it without telling anybody). There are traditional ways of implementing C, but nobody is forced to do it that way. It is just the simplest way on our typical architectures.

Upvotes: 0

user2180519
user2180519

Reputation:

If you disassemble functions you will realize that most of the time the stack doesn't even contain the return value - often times the EAX register does (intel x86). You can also look up "calling conventions" - it all pretty much depends on the compiler. C is a language, how it's interpreted into machine code is not 'its' business.

Upvotes: 2

templatetypedef
templatetypedef

Reputation: 372814

All of these terms are purely implementation details - C has no notion of "return addresses" or "dynamic links." It doesn't even have a notion of a "stack" at all. Most implementations of C have these objects in them, and in those implementations it is possible that they exist for main. However, there is no requirement that this happen.

Hope this helps!

Upvotes: 6

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