Hello,
On 3/29/07, Rajat Jain <rajat.noida.india@xxxxxxxxx> wrote:
Hi,
This has reference to LDD3 chap3 - Char drivers (Pg 60).
The following is my understanding which seems to be incorrect. Can
somebody please point out what am I missing?
LDD3 states that the fops->open() and fops->release() methods are
called only when "struct file" is created / destroyed. It further says
that "struct file" is created ONLY when a process calls open() system
all on a file. To give an example, it also says that if process 1
opens file A and then forks another process B, then A and B will share
the same struct file??
LDD3 is correct. Processes A and B will share the same struct file
in case of fork.
This seems quite strange to me as I had an understanding that two
processes working on the same file will always have different file
structure. Also, this would mean that a read by one of the processes
on the file would affect the file position pointer in the other
process?
It is not strange. It depends on how the two processes work on the same
file. If each one opens the file, then two struct file structures are created
in the kernel. This way, a read by one of the processes does not affect
the file position pointer in the other process. But it does affect in case
of fork.
I am sure I am missing something here. What is it?
Here is a small example to demonstrate the fork situation.
This small program reads data either from stdin or an input file.
In case of a file, it first opens the file for reading. It then forks.
Then both the parent and the child are reading. When you try it out,
you'll see that both affect each others read position.
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
#define SIZE 60
char buf[SIZE];
int fd=0;
int st;
int rd;
if(argc==2)
fd=open(argv[1], O_RDONLY);
if(fork()){
do{
rd=read(fd, buf, SIZE);
write(1, "\nParent:\n", sizeof("Parent: "));
write(1, buf, rd);
sleep(1);
}while(rd);
wait(&st);
}
else{
do{
rd=read(fd, buf, SIZE);
write(1, "\nChild:\n", sizeof("Child: "));
write(1, buf, rd);
sleep(1);
}while(rd);
}
return 0;
}
BlackHole
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