mmap MAP_SHARED not working

Question

I'm trying to open a file with mmap: It just works fine when using MAP_PRIVATE, but MAP_SHARED causes a invalid argument error: The file for mmap ist read/write

int size;
struct stat s;
const char * file_name = argv[1];
int fd = open (argv[1], O_RDWR);
int pagesize = sysconf(_SC_PAGE_SIZE);

/* Get the size of the file. */
int status = fstat (fd, & s);
size = s.st_size;
size += pagesize-(size%pagesize);

//mmap memory
d = mmap (0, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
//error handeling
if(d == -1)
{
    perror("mmap");
    printf("Error opening file %s\n",argv[1]);
    return -1;
}

What am I doing wrong?

@AzzUr1 Is there any difference if you do it the proper way: make `d` a pointer, e.g. `void *d;` and compare it to (void*)-1 ? `if (d == (void*)-1)`, if not, tell a bit more about your setup, e.g. which OS is this on, any compiler warnings you get, how you invoke the program and what output you get. And what specific `size` are you passing to mmap() ? (print it out for verification) — nos, Dec 14 '13 at 02:07
You should definitely error check the `open()`, and you should probably error check the `fstat()` call. If either fails, you know you've got problems. Also, as already suggested, you should print out key values — such as `fd`, `pagesize`, `status`, `size`, even `argv[1]`. — Jonathan Leffler, Dec 14 '13 at 02:53
the return check for mmap should be if(d==MAP_FAILED) or if(d==(VOID *)-1). As everyone else points out: check return codes. — jim mcnamara, Dec 14 '13 at 02:57
According to strace everything works well except mmap which says invalid argument — arnoapp, Dec 14 '13 at 11:25

score 1 · Accepted Answer · edited May 23 '17 at 12:19

1

I found out that the cause for the error was that I was using Ubuntu Linux in a VM (Parallels) when running the code on my native system everything worked fine. Seems that Parallels doesn't implement this kind of memory modifications in it's filesystem drivers...

This question helped me a lot: Invalid argument for read-write mmap?

edited May 23 '17 at 12:19

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answered Dec 14 '13 at 11:57

arnoapp

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mmap MAP_SHARED not working

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