Working with the Unix OS

Processes I
Rules about sharing of file pointers:
- the only time a single process table entry contains pointers to the same file table entry is from a dup system call
- if a single process opens the same file more than once, each open returns a file descriptor that points to a
unique file table entry, but all these file table entries point to the same i-node table entry
- a file table entry can only have more than one process table entry pointing to it from a fork operation
- if the parent and child do not coordinate the use of a shared file from a fork, then any changes made by one of
the two processes to the file position affects the other
! Daemon Processes
A daemon is a process that executes in the background waiting for some event to occur, or waiting to perform a
task on a periodic basis.
A standard Unix process named cron performs periodic tasks at given times during
the day from /usr/lib/crontab /* cron table */.
Daemon process startup:
1. started at boot by initialization script /etc/rc
2. from system's /usr/lib/crontab on periodic basis
3. from user's crontab on periodic basis (System V)
4. by executing the at command - schedules a job
5. from user terminal - foreground or background job
Typical system daemons characteristics:
(e.g. a line printer)
- started once, when system is initialized
- lifetime is the entire time system is operating . spend most time waiting from some event to occur
- spawn other processes to handle service requests
Close all Open File Descriptors
All unnecessary file descriptors should be closed.
#include
for (i=0; i