How to copy the file description when forking instead of sharing?

Simple12 · 04-03-2013, 06:00 AM

Hi all,
When a process forks, the child will share with its parent the file description(that includes the offset and the file status flag) that was open before the forking.

Is there a way to make the child have its own copy of the file description ?

I need that since I don't want both parent and child to share the same offset of the file; if one process has done a read, I don't want the offset of the file for the second process to be changed.

Thank you

mina86 · 04-03-2013, 06:32 AM

pread() function may be of interest to you.

Simple12 · 04-03-2013, 07:08 AM

Quote:

Originally Posted by mina86

pread() function may be of interest to you.

pread() does not change the offset after the read. I want the offset to be changed independently in each process.

linosaurusroot · 04-03-2013, 08:03 AM

Some ideas here
http://stackoverflow.com/questions/1...another-access

Simple12 · 04-03-2013, 08:23 AM

Quote:

Originally Posted by linosaurusroot

Some ideas here
http://stackoverflow.com/questions/1...another-access

I couldn't get how this would help. basically, When the parent forks, I want the child to get the same copy of the open file entry of its parent. meaning if the parent before the fork has done some reads, I want the offset in the child to be the same as just before the fork. after the fork I want each processes to have its own copy of the offset.

mina86 · 04-03-2013, 08:30 AM

Quote:

Originally Posted by Simple12

pread() does not change the offset after the read. I want the offset to be changed independently in each process.

Depending what you are doing, you can maintain the offset by yourself.

Quote:

Originally Posted by Simple12

I couldn't get how this would help. basically, When the parent forks, I want the child to get the same copy of the open file entry of its parent. meaning if the parent before the fork has done some reads, I want the offset in the child to be the same as just before the fork. after the fork I want each processes to have its own copy of the offset.

And your problem is?

Code:

pos = lseek(fd, 0, SEEK_CUR);
reopenFdUsingStackOverflowCode(fd);
lseek(fd, pos, SEEK_SET);

Simple12 · 04-03-2013, 09:08 AM

Quote:

Originally Posted by mina86

Depending what you are doing, you can maintain the offset by yourself.

And your problem is?

Code:

pos = lseek(fd, 0, SEEK_CUR);
reopenFdUsingStackOverflowCode(fd);
lseek(fd, pos, SEEK_SET);

Thank you, I got the idea