write a word to file by using lseek and mmap

**SoFarAway** · 03-28-2005

Hi all,

I have a file "myfile", i want to write a 4 bytes word to a particular place of offset 20 from the start of the file by using mmap. Can i do like this:

Code:

int fd, n;
unsigned char *mf;

fd = open("myfile", O_WRONLY);
n = lseek(fd, 20, 0);
mf = mmap(0, 40, PROT_WRITE, MAP_SHARED, fd, n);

strcpy(mf, "care");

msync(mf, 40, MS_SYNC);

But i always get segmentation fault.

Can anyone figure out what is wrong? And how to do it?

**Hammer** · 03-28-2005

Do some error checking, add a few things like so:

Code:

  if ((fd = open("myfile", O_WRONLY)) == -1)
  {
    perror(0);
    return 1;
  }

... 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

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