Working with the Unix OS

Processes I
- the kernel assigns the parent process ID field in the child slot, putting the child in the process tree structure,
initialises scheduling parameters such as priority, CPU usage, timing.
- the kernel increments reference counts for files. Both processes manipulate the same file table entries, the
effect of "fork" is similar to that of dup.
- the kernel allocates memory for the
child process u area, regions and page
tables.
- the kernel create a context layer for the
child containing registers and sets the
program counter. The child state is set
to "ready-to-run".
Figure 21. Fork Creating New Process Context
#include
int fdrd, fdwt;
char c;
main(argc, argv)
int argc;
char *argv[];
{
if (argc != 3)
exit(1);
if((fdrd = open(argv[1], O_RDONLY)) == -1)
exit(1);
if((fdwt = creat(argv[2], 0666)) == -1)
exit(1);
fork();
/* both procs execute same code */
rdwrt();
exit(0);
}
rdwrt()
{
for (;;)
{
if (read (fdrd, &c, 1) != 1)
return;
write(fdwt, &c, 1);
}
}
Figure 22. Example of Parent and Child Share File A
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