what does 0xdeadbeef in address 0x0 mean? [duplicate]

Question

I just saw in a code the following line :

#define ERR_FATAL( str, a, b, c ) {while(1) {*(unsigned int *)0 = 0xdeadbeef;} }

I know that 0xdeadbeef means error, but what putting this value mean when it's in address 0 ? What address 0 represents ?

Answer 1

The address 0x0 is recognized by the compiler as a NULL pointer and is going to be an implementation defined invalid memory address, on some systems is quite literally at the first addressable location in the system memory but on others it's just a placeholder in the code for some generic invalid memory address. From the point of view of the C code we don't know what address that will be, only that it's invalid to access it. Essentially what this code snippet is trying to do is to write to an "illegal" memory address, with the value 0xdeadbeef. The value itself is some hex that spells out "dead beef" hence indicating that the program is dead beef (ie. a problem), if you aren't a native english speaker I can see how this might not be so clear :). The idea is that this will trigger a segmentation fault or similar, with the intention of informing you that there is a problem by immediately terminating the program with no cleanup or other operations performed in the interim (the macro name ERR_FATAL hints at that). Most operating systems don't give programs direct access to all the memory in the system and the code presumes that the operating system won't let you directly access memory that's located at address 0x0. Given that you tagged this question with the linux tag this is the behavior you will see (because linux will not allow an access to that memory address). Note that if you are working on something like an embedded system where there's no such guarantee then this could cause a bunch of problems as you might be overwriting something important.

Note that there's going to be better ways out there to report problems than this that don't depend on certain types of undefined behaviors causing certain side effects. Using things like assert is going to likely be a better choice. If you want to terminate the program using abort() is a better choice as it in the standard library and does exactly what you want. See the answer from ComicSansMS for more about why this is preferable.