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    <p>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.</p> <pre><code>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(&amp;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. } </code></pre>
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