What is the behavior of printing NULL with printf's %s specifier?

Question

Came across an interesting interview question:

test 1: printf("test %s\n", NULL); printf("test %s\n", NULL);  prints: test (null) test (null)  test 2: printf("%s\n", NULL); printf("%s\n", NULL); prints Segmentation fault (core dumped) 

Though this might run fine on some systems, atleast mine is throwing a segmentation fault. What would be the best explanation of this behavior? Above code is in C.

Following is my gcc info:

deep@deep:~$ gcc --version gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3 

Answer 1

First things first: printf is expecting a valid (i.e. non-NULL) pointer for its %s argument so passing it a NULL is officially undefined. It may print "(null)" or it may delete all files on your hard drive--either is correct behavior as far as ANSI is concerned (at least, that's what Harbison and Steele tells me.)

That being said, yeah, this is really wierd behavior. It turns out that what's happening is that when you do a simple printf like this:

printf("%s\n", NULL); 

gcc is (ahem) smart enough to deconstruct this into a call to puts. The first printf, this:

printf("test %s\n", NULL); 

is complicated enough that gcc will instead emit a call to real printf.

(Notice that gcc emits warnings about your invalid printf argument when you compile. That's because it long ago developed the ability to parse *printf format strings.)

You can see this yourself by compiling with the -save-temps option and then looking through the resulting .s file.

When I compiled the first example, I got:

movl    $.LC0, %eax movl    $0, %esi movq    %rax, %rdi movl    $0, %eax call    printf      ; <-- Actually calls printf! 

(Comments were added by me.)

But the second one produced this code:

movl    $0, %edi    ; Stores NULL in the puts argument list call    puts        ; Calls puts 

The wierd thing is that it doesn't print the following newline. It's as though it's figured out that this is going to cause a segfault so it doesn't bother. (Which it has--it warned me when I compiled it.)

Answer 2

As far as the C language is concerned, the reason is that you're invoking undefined behavior and anything can happen.

As for the mechanics of why this is happening, modern gcc optimizes printf("%s\n", x) to puts(x), and puts does not have the silly code to print (null) when it sees a null pointer, whereas common implementations of printf have this special case. Since gcc can't optimize (in general) non-trivial format strings like this, printf actually gets called when the format string has other text present in it.