Works fine in C++, but runtime error in C

Question

I have this code here ::

int f(int x, int *py, int **ppz)
{
  int y, z;
  **ppz += 1; 
   z  = **ppz;
  *py += 2;
   y = *py;
   x += 3;
   return x + y + z;
}

int main()
{
   int c, *b, **a;
   c = 4;
   b = &c;
   a = &b; 
   printf( "%d\n", f(c,b,a));
   printf( "%d\n", c);
}

(Please choose the header files accordingly)

When I run this code as a C++ program, it runs smoothly. But, when I run it as a C program, it completes its execution, prints all the values, but on the last line of the code, gives a Runtime Error. I attached a debugger, and at the end of the program(while running this code in C) on clicking Step-Over, it takes me to a min_gw file, which looks something like this::

__mingw_CRTStartup()
__mingw_CRTStartup+208: mov    %eax,%ebx
__mingw_CRTStartup+210: call   0x407fb0 <_cexit>
__mingw_CRTStartup+215: mov    %ebx,(%esp)
__mingw_CRTStartup+218: call   0x408088 <ExitProcess@4>
__mingw_CRTStartup+223: nop

What is this file?? And why does the code fail in C??

Ideone references ::

C :: http://ideone.com/sDbqnI

C++ :: http://ideone.com/Ubb71k

Thanks for any help in advance.. :)

Both programs are fine. Ideone reports runtime error because your `main` reaches the end without returning a value. — Sergey Kalinichenko, Jun 23 '15 at 23:27
@dasblinkenlight no, it's UB to fall off the end (and use the value, which the runtime does). — o11c, Jun 23 '15 at 23:31
@dasblinkenlight https://stackoverflow.com/questions/13545291/can-i-omit-return-from-main-in-c — this, Jun 23 '15 at 23:35
@this: Yeah, just read in the standard. `main()` is a special case (I hate such unnecessary exceptions). Well, my `main()`s are mostly `void` anyway. — too honest for this site, Jun 23 '15 at 23:38
@o11c: gotcha - be careful with absolute statements;-). Read about [freestanding environments](http://port70.net/~nsz/c/c11/n1570.html#5.1.2.1). There is no reqirement for a special `main()` - it could be called whatever you want and have any signature. — too honest for this site, Jun 24 '15 at 00:16

o11c · Accepted Answer · 2015-06-23T23:42:05.663

11

All versions of C++, and C since C99, add an implicit return 0; at the end of main before falling off the end.

C89 (which is the default for most compilers) does not, so it is UB if you actually return from main by falling off the end.

Edit: In practice (assuming you are running without sanitizers), this means that you will return whatever happens to be in $eax already, i.e. the return value of the last function. Since printf usually returns a non-zero value, that is treated as failure.

edited Jun 23 '15 at 23:42

answered Jun 23 '15 at 23:28

o11c

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As I understand, as of the C99 standard, this is added implicitly in C as well. – wjl Jun 23 '15 at 23:30
@wjl 5.1.2.2.3, yeah. – o11c Jun 23 '15 at 23:33
You should add C11 to the list. – too honest for this site Jun 23 '15 at 23:38
Thank you so much.. That was indeed a dumb error. But, I tried inserting an empty `printf` statement at the end of the program, so that it returns `0` but, It still gives the runtime error – user007 Jun 23 '15 at 23:39
1

@user007 Likely `-fsanitize=ubsan` catches it regardless. – o11c Jun 23 '15 at 23:41
1

@user007: Not sure if that helps, but: MSVC does not comply to C99 (not to talk about C11). – too honest for this site Jun 23 '15 at 23:51
@Olaf probably Netbeans doesn't either.. Thanks though :) – user007 Jun 24 '15 at 00:01
@user007: Don't worry, I didn't know either that the standard requires `main()` to properly terminate even without explicit `return`. Just always use it and you are on the safe side. – too honest for this site Jun 24 '15 at 00:11
1

@user007 Netbeans is not a compiler, just an IDE. It usually just calls `gcc`. – o11c Jun 24 '15 at 00:13
1

@o11c: Interesting sidenote: gcc actually _does_ complain when using `-Wall` for a freestanding environment, but not for a hosted - the default for non-crosscompilation. Reason is likely, there is no requirement for a special `main()` by the standard. – too honest for this site Jun 24 '15 at 00:14
1

Note that the current standard is C11. Earlier standards have been withdrawn, so talking about "standard" means C11. gcc for instance does use C11 as default now (5.1); it has been C90/89 before, however. – too honest for this site Jun 24 '15 at 00:47

Andreas DM · Answer 2 · 2015-06-23T23:45:26.820

0

You forgot the return statement in the end of main indicating success.

edited Jun 23 '15 at 23:45

answered Jun 23 '15 at 23:35

Andreas DM

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That would be `return 0`. This is automatically done for `main()`. No actual need for that. – too honest for this site Jun 23 '15 at 23:48

Works fine in C++, but runtime error in C

2 Answers2