The output isCode:#include <stdio.h> int main(int argc, char ** argv) { printf("hello!!!"+printf("world")); return 0; }
EDIT: I believe it's got something to do with both locations sharing some of the memory... The arg to printf is just a "const char *" and it gets offset by the return from the second printf.Code:world!!!
Last edited by Syscal; 10-08-2013 at 05:54 PM.
EDIT: oogabooga's explanation is so much clearer
Nope, not a race condition. While this does not exhibit undefined behavior like ++i + i++ would, it comes close to the "stupid code you shouldn't bother understanding since nobody does this crap in the real world" line.
That being said, there are a few things you must understand here:
First, printf takes a pointer to const char. It could be lots of things, for example, the start of a string literal or somewhere in the middle of a string literal.
Second, read the docs for printf: printf(3): formatted output conversion - Linux man page. It returns the number of characters printed. Thus printf("world") returns 5 since "world" has 5 letters (it omits the null terminator).
Lastly, realize that adding an int n (like 5) to a pointer is like incrementing that pointer n times. It's also similar to indexing an array, except that you get a pointer back instead of the element pointed to/indexed. Taking your example, say the string literal "hello!!!" had address 0x1000. Doing "hello!!!"+printf("world") is equivalent to "hello!!!" + 5 = 0x1000 + 5 = 0x1005. Since the literal is stored in adjacent bytes in memory, you basically skip the first 5 letters ("hello") and print the remaining ones, "!!!".
So first, you must call printf("world") to know what it returns. Then, you add it to "hello!!!", and printf uses the result as the address to print to. That's why it prints "world" followed by "!!!".
Note that this is a dangerous thing to do, since it's easy to screw up and access invalid memory (what if you changed world to "something longer" -- then you are printing past the end of "hello!!!", accessing memory you don't own and ending up with undefined behavior). In fact, if you crank up warnings on your compiler, you may see something like:
Code:$ make bar gcc -Wall -ggdb3 -std=c99 -O0 -o bar bar.c -lm -lpthread -lrt bar.c: In function ‘main’: bar.c:5:3: warning: format not a string literal and no format arguments [-Wformat-security]
Your explanation is incorrect. (EDIT: "your" here refers to syscal !)
The inner printf is executed first, printing "world".
printf has an int return value, which returns the number of characters printed. In this case, it returns 5.
A string constant evaluates to the address of the first character in the string. The addition adds 5 to the address of the 'h' in "hello!!!". That makes the address point to the first exclamation mark.
So the outer printf prints the exclamation points.
The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss
Yup, I figured as much which is why I went back and edited the post. Too slow and you beat me to the punch though. Had to really sit there and think about that before I realized what was happening. Thanks for confirming! Lots of quirks and odd things can be done with this language!
It does not mean they should be doneOriginally Posted by Syscal
All problems in computer science can be solved by another level of indirection,
except for the problem of too many layers of indirection.
– David J. Wheeler
