Working with the Unix OS

Processes I
! Example
int debug = 1; /* initialised read-write variable */
char *progname;
/* uninitialised read-write variable*/
main (argc, argv)
int argc; char *argv[];
{
int i; /* automatic variable stored on stack */
char *ptr; /* automatic variable stored on stack */
char *malloc(); /* space allocated stored on heap */
progname = argv[0];
printf("argc = %d\n", argc); /* read-only data */
for (i = 1; i < argc; i++)
{
ptr = malloc(strlen(argv[i]) + 1);
strcpy(ptr, argv[i]);
if (debug)
printf("%s\n", ptr); /* read-only data */
}
} /* functions main, printf, strlen, strcpy &
malloc are all in the text segment */
! Process ID (PID)
int getpid();
0-30000
PID 1 special process called the init process
PID 0 kernel process called swapper/scheduler
PID 2 kernel process called pagedaemon
! Parent Process ID
int getppid();
! Real User ID
unsigned short getuid();
Each user is assigned an unique ID in /etc/passwd.
! Real Group ID
unsigned short getgid();
Groups of users are assigned an ID in /etc/group.
! Effective User ID
unsigned short geteuid();
Set-user-ID program - file's owner ID is zero
(superuser)
! Effective Group ID
unsigned short getegid();
Set-group-ID program
! Superuser
User ID zero - login name root
Superuser can terminate any other process on system.
! Password File
/etc/passwd
login-name:password:user-ID:group-ID:misc:home:shell
#include
struct passwd *getpwuid(int uid);
struct passwd *getpwnam(char *name);
struct passwd {
char *pw_name; /* login-name */
char *pw-passwd; /* encypted-password */
int pw_uid; /* user-ID */
int pw_gid; /* group-ID */
char *pw_age; /* password age System V */
char *pw_gecos; /* miscellany */
char *pw_dir; /* home directory */
char *pw_shell; /* shell */
};
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