Working with the Unix OS

Processes I
Process
An instance of program being executed by operating system. A new process is created by issuing the fork
system call. A program may be executed by many processes at same time.
System Calls
The Unix kernel provides a limited number (60-200) of direct entry points for services from the kernel.
The standard Unix C library provides a C interface to each system call or function.
Most system calls return -1 if an error occurs, or a value >= 0
A global integer variable errno is provided by the C interface. The header file contains the names
and values of these error numbers.
Some system calls return a pointer to a structure of information, e.g. stat and fstat system calls.
! C Start-up Function
main()
{
printf ("hello world\n") ;
exit(0);
/* flush standard I/O buffers
& terminate */
}
user functions
or library
call | | return
user' s main
function
user
call | | return
C startoff
routine
exit
C exit
routine
exit
C exit
routine
program invocation: exec
system call
kernel
! Argument List
Whenever a program is executed, a variable-length argument process. The argument list is passed to the process.
The argument list is an array of pointers to character strings (maximum size of 5120 bytes).
echo hello world
main (argc, argv)
int argc; char *argv[];
{…}
argv ------ argv[0] ------ echo\0
------ argv[1] ------ hello\0
------ argv[2] ------ world\0
! Environment List
main (argc, argv, envp)
int argc; char *argv[]; char *envp[];
{
int i;
for (i = 0; envp [i] != (char *) 0; i++)
printf("%s\n", envp[i]);
exit(0) ;
}
HOME=/user1/staff/neville
SHELL=/bin/ksh
TERM=vtl00
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