Working with the Unix OS

Processes I
USER=neville
PATH=/userl/staff/joe/bin:/usr/local/bin:/bin:/usr/bin:
main (argc, argv)
int argc; char *argv[];
{
int i;
extern char **environ;
for (i = 0; environ[i] != (char *) 0; i++)
printf("%s\n", environ[i]);
exit(0);
}
main ()
{
char *ptr, *getenv();
if ( (ptr = getenv("HOME")) == (char *) 0)
printf ("HOME is not defined\n ") ;
else
printf("HOME=%s\n", ptr);
exit(0);
}
The argument list, environment pointers and character strings pointed to are in the data space of the process. The
process can modify these but this has no effect on the parent process.
The only value passed by the terminating process to its parent process by the operating system is the 8-bit argument
to the exit function.
The parent and child can exchange information using a disk file or by interprocess communication. A process can
modify its environment to affect any child processes it created.
! Process user context
kernel context
stack
kernel data
heap
uninitialized data
initialized readwrite
data
initialized readonly
data
text
read from program file
when program is executed
user context
text
data
heap
stack
portion of address space accessible to the process while it is running in user mode.
the actual machine instruction that are executed by hardware. Often set readonly
so that process cannot modify its instructions. It is read into memory from disk, unless as
supports shared text and it already is executing.
contains the program's data
- initialized read-only – ro while program executing. e.g. literal strings; not supported on many
OSs
- initialized read-write – modified during execution uninitialized - set to zero before process
starts, advantages – save disk space & time to read data
used to allocate data space dynamically to the process while the process is running.
used dynamically while process is running to contain stack frames that are used by the
programming language. Stack frames contain the calling arguments and return addresses.
kernel context is maintained and accessible only to the kernel. It contains information that the kernel needs to
keep track of the process and to stop and restart the process while other processes are allowed to
execute.
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