mmap() error

[SoFarAway]

I have a file "b.txt", which is 16 bytes long:

b.txt:
00 12 32 a5 6f ff 11 67 00 00 00 11 fa 90 44 66

I use the following code to change the first 2 bytes of b.txt, and it works.

int fd, i;
char *mf;
char *buf ;

fd = open("b.txt", O_RDWR);

mf = mmap(0, 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

for(i = 0; i < strlen(buf); i++)
    mf[i] = buf[i];

msync(mf, 2, MS_SYNC);

however, if i use

mf = mmap(0, 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 2);

to change the 3rd and 4th (i.e. 32 and a5) of b.txt, segmentation fault occurs.

Can anyone tell me how to map the middle part of a file into memory and do changes to it?

[Salem]

Well do you get any errors when you mmap?

[Hammer]

Did you not read my reply to your last post:
http://cboard.cprogramming.com/showthread.php?t=63574

.. And double check you're using the correct value as the last parameter on mmap().
http://www.ecst.csuchico.edu/~beej/guide/ipc/mmap.html

To save your lazy ass from following the link , here's an extract from the page I refer to:

void *mmap(void *addr, size_t len, int prot, int flags, int fildes, off_t off);

off
This is the offset in the file that you want to start mapping from. A restriction: this must be a multiple of the virtual memory page size. This page size can be obtained with a call to getpagesize().

If you look at that page, you'll find sample code showing you how to access bytes at a specific offset.

[SoFarAway]

Thanks...

I fixed the problem already..