Question

I came across this piece of code on a website.

main(i)
{
  gets(&i);
  puts();
}

This code compiles and runs fine!

It gets a string as input from the user and prints it!!!!

But, my question is, how?

(note that puts() function does not contain any arguments!)

Answer 1

Old versions of C had implicit types for variables and functions, and this code makes use of that and some other stuff. It also was very lax about actually returning values.

main(i) // i is implicitly an integer (the default type for old C), and normally named argc 
// int main(int i) or void main(int i)
{ // The stack (which lives in high memory but grows downward) has any arguments and
  // probably the environmental variables and maybe even other (possibly blank/filler)
  // stuff on it in addition to the return address for whatever called main and possibly
  // the argument i, but at this point that could either be on the stack just under the
  // return address or in a register, depending on the ABI (application binary interface)


// extern int gets(int) or extern void gets(int)
// and sizeof(int) is probably sizeof(char *)
 gets(&i); // By taking the address of i even if it wasn't on the stack it will be pushed to
           // it so that it will have an address (some processors have addressable registers
           // but they are rarely used by C for many reasons that I won't go into).


           // The address of i is either also pushed onto the stack or put into a register
           // that the ABI says should be used for the first argument of a function, and
           // and then a call is made to gets (push next address to stack; jump to gets)

           // The function gets does what it does, but according to the ABI there are
           // some registers that it can do whatever it wants to and some that it must
           // make sure are the same as they were before it was called and possibly one
           // or more where it is supposed to store a return value.
           // If the address of i was passed to it on the stack then it probably would be
           // restricted from changing that, but if it was passed in a register it may
           // have just been luckily left unchanged.
           // Another possiblity is that since gets returns the string address it was
           // passed is that it returns that in the same location as the first argument
           // to functions is passed.  

 puts();   // Since, like gets, puts takes one pointer argument it will be passed this
           // this argument in the same way as gets was passed it's argument.  Since we
           // were somehow lucky enough for gets to not overwrite the argument that we
           // passed to it and since the C compiler doesn't think it has anything new to
           // pass to puts it doesn't change any registers' values or do too much to the
           // stack.  This leaves us in the situation where puts is called with the stack
           // and registers set up in the same way as they would be if it were passed the
           // address of i, just the same as gets.

   // The gets call with the stack variable's address (so an address high on the stack)
   // could have left main's return address intact, but also could have overwritten it
   // with garbage.  Garbage as main's return address would likely result in a jump to
   // a random location (probably not part of your program) and cause the OS to kill the
   // program (possibly with an unhandled SIGSEGV) which may have looked to you like a
   // normal exit.  Since puts appended a '\n' to the string it wrote and stdout is
   // line buffered by default it would have been flushed before returning from puts
   // even if the program did not terminate properly. 
}