Subj : mmap with MAP_ANONYMOUS gives EINVAL
To   : comp.os.linux.questions,comp.os.linux
From : joga2052
Date : Fri Jul 09 2004 09:29 am

Hello *,
I'm experimenting with stack allocation for user-space threads using
mmap. I'm using MAP_ANONYMOUS (so that i won't need a file descriptor)

	void *stack;

        stack = mmap(0, 8192, PROT_READ | PROT_WRITE | PROT_EXEC,
MAP_ANONYMOUS, -1, 0);
        if (stack == MAP_FAILED) {
                perror("mmap failed");
                exit(1);
        }

The call to mmap always fails (errno = EINVAL).

I digged into the source and found the following:

Linux kernel 2.4.18

"mm/mmap.c" [readonly] 1370 lines --41%--                     568,3-17
     41%

	unsigned long do_mmap_pgoff(struct file * file, unsigned long addr,
unsigned long len,
        unsigned long prot, unsigned long flags, unsigned long pgoff)
	{
	//...
	} else {
                vm_flags |= VM_SHARED | VM_MAYSHARE;
                switch (flags & MAP_TYPE) {
                default:
                        return -EINVAL;
                case MAP_PRIVATE:
                        vm_flags &= ~(VM_SHARED | VM_MAYSHARE);
                        /* fall through */
                case MAP_SHARED:
                        break;
                }
        }
	//...
	}

I observed that the MAP_ANONYMOUS flag is filtered in the default case
and the call returns with EINVAL. I tried "MAP_ANONYMOUS | MAP_SHARED"
(to avoid the default case) and things were working fine.

Here did i step on a bug, or am i committing a blunder somewhere?

.