System Files and Process Environment Password file Group file System identification Time Process environment

Password File doej:x:293:105:John Doe:/home/doej:/bin/sh passwd struct char *pw_name char *pw_passwd uid_t pw_uid gid_t pw_gid char *pw_gecos char *pw_dir char *pw_shell

Password File struct passwd *getpwnam(const char *name); Gets a pointer to a passwd struct based on the login name struct passwd *getpwuid(uid_t uid); Gets a pointer to a passwd struct based on the user ID Both return a pointer if OK and NULL on error Requires

Shadow Passwords Encrypted passwords kept in separate file /etc/shadow Minimally contains the username and encrypted password Password field of /etc/passwd replaced with ‘x’ Only root can open and read the shadow file

Group File Located in /etc/group struct group char *gr_name char *gr_passwd int gr_gid char **gr_mem

Group File struct group *getgrnam(const char *name); Gets a pointer to a group struct based on group name struct group *getgrgid(gid_t gid); Gets a pointer to a group struct based on group ID

System Identification int uname(struct utsname *buf); Stores system information in the structure pointed to by buf char sysname[] char nodename[] char release[] car version[] char machine[]

System Identification int gethostname(char *name, size_t len); Copies hostname into the buffer of size len pointed to by name. If the buffer is not large enough, hostname is truncated and may not be null terminated Returns 0 on success or -1 on error

Date and Time time_t time(time_t *t); Stores an instance of the time_t at the location pointed to by t Holds both time and date Returns a time_t value if ok, -1 on error struct tm *gmtime(const time_t *timep); struct tm *localtime(const time_t *timep); These functions break down a raw time_t into a struct tm representation

Date and Time struct tm tm_sec0-60 tm_min0-59 tm_hour0-23 tm_mday1-31 tm_mon0-11 tm_year tm_wday0-6 tm_yday0-365 tm_isdst 0

Date and Time More functions See man pages for gettimeofday settimeofday mktime asctime ctime strftime See page 174 fig 6.8

Process Environment main function When a C program is started by the kernel, a start up routine is called before main is called. Added in by the linker automatically Start up routine is given as the starting address of the executable program

Process Termination Normal Return from main A call to exit A call to _exit or _Exit Abnormal A call to abort (SIGABRT) Receiving a kill signal (SIGTERM)

Exit Functions _exit and _Exit functions return to Kernel immediately exit function performs cleanup and then returns to Kernel Start up routine returns to the Kernel the value returned from main exit(main(argc, argv));

Environment List Name=value pairs Passed to every process Global variable environ. Pointer to an array of strings Functions char *getenv(const char *name); int putenv(char *string); int setenv(const char *name, const char *value, int overwrite);

Memory Layout of a C Program Text segment – read only machine instructions Initialized data segment Uninitialized data segment Stack Heap See fig 7.6 on page 188 size command can be used to find the sizes of the various sections

Allocate and Free Memory void *calloc(size_t nmemb, size_t size); void *malloc(size_t size); void *realloc(void *ptr, size_t size); void free(void *ptr); calloc clears (sets to zero) the memory it allocates realloc changes the size of the allocated memory (usually to allocate more)