Hi..fellows, am new to linux so please bare with me.

Iam trying to use mmap() to perform IO and to have the file updated...but I cannot seem to make write to the file.
heres my code..
---------------

//to create(if not there) or open file and truncate)
int file = open("input",O_RDWR|O_TRUNC);
if (file < 0)
{
fprintf(stderr,"FAiled");
return -1;
}
else
{
//to map file
char* buff = (char*)mmap(0,page_size,PROT_READ|PROT_WRITE,MAP_SHARED,file,0);
buff = (char*)"ABC";
printf("%s",buff); //it outputs to screen ABC
if (msync(0,page_size,MS_SYNC) == -1)
printf("%s",str_error[errno]);//it gives a message CANNOT ALLOCATE MEMORY
munmap(buff,page_size);
close(file);

---------------------
i seem to be able to write to buff but it does not update the file

You are not writing to buff, just replacing the location it points to and loosing the mmaped area in doing that.

instead of
buff = (char*)"ABC";
now i have used
buff[0] = 'A';
printf("%c",buff[0]); //NO OUTPUT TO SCREEN
now still get the CANNOT ALLOCATE MEMORY error with msync() but the file is still not updated (still 0 bytes)

BTW thanks a lot for replying

You give msync a null pointer instead of buff, the reason why you got an error.

Your printf doesn't reach your screen because you do not flush stdout (either by calling fflush or with a \n in the format string).