为什么mmap在这里不起作用

Question

I get segmentation fault when I run the following piece of code...

int * x = mmap( 0, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE, 0, 0 );

x[0] = 42; // <--- Segmentation fault happens due to this

What is wrong here?

Answer 1

You've specified the incorrect flags and file descriptor. It looks like what you want is an anonymous (not backed by a file) mapping. If that's the case, the correct call would be:

x = mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

You need to use the MAP_ANONYMOUS flag to tell Linux there is no file. And you should pass -1 for the file descriptor, not 0.

Answer 2

OK, I got it. I forgot to place MAP_ANONYMOUS , so it should had been like this...

int * x = mmap( 0, 4096, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, 
  0, 0 );

Its working this way.

Answer 3

man mmap says:

On success, mmap() returns a pointer to the mapped area. On error, the value MAP_FAILED (that is, (void *) -1) is returned, and errno is set appropriately

Check, whether x == MAP_FAILED or not. May be this is the case.

Answer 4

And you should always check that the result of mmap is not MAP_FAILED (that is, (void *) -1 ) and use errno to get the error code in that case.

Your mmap could fail (eg because of resource limits set with setrlimit , or because the swap space is full).