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Unix presentation.pdf
1. Unix Programming
Module 5
BY:
S Guru Prasad -1BY20CS154
Saeed Ahmed Alojily -1BY20CS159
Sai Harshit B -1BY20CS162
Vigneshwaran P S -1BY20CS215
2. Contents
➔ Setjmp and Longjmp
➔ Sigsetjmp and Siglongjmp
➔ Kill
➔ Alarm
➔ Interval Timers
➔ Posix 1b Timers
These are APIs that are used to create
functions we use in higher level
languages to make code simpler
3. 1. Setjmp and longjmp
● setjmp and longjmp are a pair of C function facilitating cross-procedure transfer of control. Typically
they are used to allow resumption of execution at a known good point after an error.
● Both take as first argument a buffer, which is used to hold the machine state at the jump destination.
When setjmp is called it populates that buffer with the current location state (which includes stack and
frame pointers and the return address for the call to setjmp, and returns zero.
● longjmp takes a buffer previously populated by setjmp. It also takes a (non-zero) second argument,
which will ultimately be the result of the function call. longjmp restores the machine state from the
buffer. It then jumps to the return address it has just restored, passing its second argument as the
result. That return address is the return address from the original call to setjmp, so the effect will be as
if setjmp has just returned with a non-zero argument.
4. 1. Setjmp and longjmp
setjmp and longjmp are typically used in a top level function in the following way.
5. 1. Setjmp and
longjmp
During normal processing if an error is found, the state held in buf can be used to
return control back to the top level using longjmp.
The program will behave as though the original call to setjmp had just returned with
result 1.
6. 2. Sigsetjmp and siglongjmp
● The sigsetjmp and siglongimp APIs have similar functions as their corresponding
setjmp and longjmp APIs.
● The sigsetjmp and siglongjmp APIs are defined in POSIX 1 and on most UNIX systems that
support signal mask.
● The function prototypes of the APIs are:
#include <setjmp.h>
int sigsetjmp (sigjmpbuf env, int save_sigmask );
int siglongimp (sigjmpbuf env, int ret_val ):
7. 2. Sigsetjmp and siglongjmp
● The sigsetjmp and siglongimp are created to support signal mask processing
Specifically, it is implementation dependent on whether a process signal mask is
saved and restored when it invokes the setjmp and longjmp APIs respectively.
● The sigsetjmp API behaves similarly to the setjmp API, except that it has a second
argument, save_sigmask, which allows user to specify whether a calling process
signal mask should be saved to the provided env argument.
● If the save_sigmask argument is nonzero, the caller's signal mask is saved, else
signal mask is not saved.
● The siglongjmp API does all operations as the longimp API, but it also restores a
calling process signal mask if the mask was saved in its env argument.
8. 2. Sigsetjmp and siglongjmp
● The ret_val argument specifies the return value of the corresponding sigsetjmp API
when called by siglongjmp API. Its value should be nonzero number, and if it is zero
the siglongjmp API will reset it to 1.
9. 2. Sigsetjmp and siglongjmp
#include <iostream.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <setjmp.h>
sigjmp_buf env;
void callme(int sig_num)
{
cout<< “catch signal:” <<sig_num <<endl;
siglongjmp(env,2);
}
int main()
{
sigset_t sigmask;
struct sigaction action,old_action;
sigemptyset(&sigmask);
if(sigaddset(&sigmask,SIGTERM)==1)||sigprocmask(SIG_SETM
ASK,&sigmask,0)==-1)
perror(“set signal mask”);
sigemptyset(&action.sa_mask);
sigaddset(&action.sa_mask,SIGSEGV);
action.sa_handler=(void(*)())callme;
action.sa_flags=0;
if(sigaction(SIGINT,&action,&old_action)==-1)
perror(“sigaction”);
if(sigsetjmp(env,1)!=0)
{
cerr<<”return from signal interruption”;
return 0;
}
else
cerr<<”return from first time sigsetjmp is called”;
pause();
}
10. The kill API can be used by a process to send a signal to a related process.
➔ This is a simple means of IPC or control
➔ The sender and recipient processes must be related such that either sender
process real or effective user ID matches that of the recipient process, or the
sender has su privileges
➔ For example, a parent and child process can send signals to each other via
the kill API.
➔ The kill API is defined in most UNIX system and is a POSIX.1 standard.
3.Kill
11. The function prototype is as
● The sig_num argument is the integer value of a signal to be sent to one or more processes
designated by pid.
● The possible values of pid and its use by the kill API are:
#include <signal.h>
int kill ( pid_t pid, int signal_num);
Pid value Effects on the Kill API
A Positive Value pid is a process ID. Sends Signal to that process
0 Sends signal to all processes whose process group ID is the same as the calling
process.
-1 Sends signal to all processes whose real user ID is the same as the effective user ID of
the calling process
Negative value Sends signal to all processes whose process group ID matches the absolute value of pid
3.Kill
12. ● The return value of kill is zero if it succeeds or -1 if it fails.
● The following C program illustrates the implementation of the UNIX kill Command.
#include<iostream.h>
#include<stdio.h>
#include<unistd.h>
#include<string.h>
#include<signal.h>
int main(int argc,char** argv)
{
int pid, sig = SIGTERM;
if(argc==3)
{
if(sscanf(argv[1],”%d”,&sig)!=1)
{
cerr<<”invalid number:” << argv[1] << endl;
return -1;
}
argv++,argc--;
}
while(--argc>0)
if(sscanf(*++argv, “%d”, &pid)==1)
{
if(kill(pid,sig)==-1)
perror(“kill”);
}
else
cerr<<”invalid pid:” << argv[0] <<endl;
return 0;
}
3.Kill
$ kill - < signal_num > < pid >
13. 4. Alarm
An API that can be used a process to request the
kernel for an alarm, like setting an alarm clock.
➔ What is it for?
To send the SIGALARM signal after a certain
number of real clock seconds.
➔ Note
Alarm api is a POSIX.1 standard
Syntax:
#include<signal.h>
unsigned int alarm( unsigned int timeInterval);
The arguement timeInterval denotes the number
of CPU seconds elapse time.
14. 4. Alarm
➔ Example:
#include<signal.h>
#include<stdio.h>
#include<unistd.h>
Void wakeup() {} ;
unsigned int sleep(unsigned int timer)
{
struct sigaction action;
action.sa_handler=wakeup;
action.sa_flags=0;
sigemptyset(&action.sa_mask);
if(sigaction(SIGALRM, &action, 0)==-1)
{
perror("sigaction");return -1;
}
(void)alarm(timer);
(void)pause();
return 0;
}
➔ Explanation:
The sleep function defined in the code
suspends calling a process for the
specified number of seconds (timer
variable). The process will be awakened by
either the elapsed time exceeding the
timer or when process is interrupted by a
signal.
The sleep function sets up a signal handler
for the SIGALRM, calls the API to request
the kernel to send the SIGALRM signal after
the interval and and finally suspends its
execution using the pause system call.
The wakeup signal handler function is
called when the SIGALRM signal is sent to
the process. When it returns, the pause
system is aborted and calling process will
return from sleep function.
15. 5. Interval Timers
#include<signal.h>
#include<stdio.h>
#include<unistd.h>
#define INTERVAL 5
void call(int sig_no)
{alarm(INTERVAL);/*Do scheduled actions*/}
int main()
{
struct sigaction action;
sigemptyset(&action.sa_mask);
action.sa_handler=(void(*)())call;
action.sa_flags=SA_RESTART;
if(sigaction(SIGALRM, &action, 0)==-1)
{perror("sigaction");return -1; }
if(alarm(INTERVAL)==-1)
{perror("alarm");}
else while(1)
{/*Do normal Operations*/}
return 0;
}
➔ Explanation:
As seen earlier, we set up the call function
handling function using sigaction for
SIGALRM.
The program then invokes the alarm API to
call the call function to do the scheduled
actions, and then carry on with its usual
routine.
16. 5. Interval Timers
In addition to alarm API, UNIX also has setitimer
API which provides features in addition to alarm API
1. setitimer resolution time is in microseconds,
while in alarm it is in seconds.
2. The alarm API can be used to set up one real
time clock per timer per process, but
setitimer allows defining upto 3 different
types of timers per process, a real time
clock timer, a timer based on user time
spent on process, and timer based on total
user and system time spent on process.
The setitimer API is not available in POSIX as there
is another set of timers specified under POSIX.1b.
There is another related API called getitimer that
users can use to query the timer values set by
setitimer API.
17. 5. Interval Timers
Syntax: (function prototypes)
#include<sys/time.h>
/*setitimer:*/
int setitimer(int which, const struct itimeval *val, struct itimerval *old);
/*getitimer*/
int getitimer(int which, struct itimeval *old);
Here:
which = to specify which timer to process
Possible argument values:
- ITIMER_REAL: Real time clock. Generates SIGALRM signal.
- ITIMER_VIRTUAL: User time spent on process. Generates
SIGVTALRM signal.
- ITIMER_PROF: Total user and system time spent on process.
Generates SIGPROF signal.
18. 5. Interval Timers
Syntax: (struct itimeval)
struct itimeval
{
struct timeval it_interval; /*timer interval*/
struct timeval it_value; /*current value*/
};
Note: setitimer and getitimer return 0 if they succeed and -1 if they fail.
They are not inherited by child processes, but are retained when a process
exec’s a new program.
The next slide shows the code on the first slide of the topic, but using
setitimer API instead of alarm API.
20. 6. Posix 1b Timers
POSIX.1b defines a set of APIs for interval timer
manipulation. The POSIX.1b timers are more
flexible and powerful than UNIX timers.
What makes POSIX.1b timers better?
1. Users may define multiple independent
timers per system clock
2. The timer resolution is in nanoseconds
3. Users may specify, on a per timer basis, the
signal to be raised when a timer expires.
4. The timer interval may be specified as an
either absolute or relative time.
21. 6. Posix.1b Timers
There is a limit on how many POSIX timers can
be created per process. This limit is set by the
TIMER_MAX constant, as defined in the
<limits.h> header.
POSIX timers are not inherited by child
processes, but are retained across the exec
system call, just like in interval timers.
Unlike UNIX timers, POSIX timers do not use
the SIGALRM signal when they expire, it can
be used in the same program with sleep API.
POSIX timers have 5 different functions:
Create, set, get time, get overrun and delete.
They are described in the following slide
22. 6. POSIX.1b Timers
Syntax: (function prototypes)
There are a few functions under POSIX timers, and their prototypes are as follows:-
int timer_create(clockid_t clock, struct sigevent* spec, timer_t* timer_hdrp);
Used to dynamically create a timer and returns its handler. The clock argument value
may be CLOCK_REALTIME for creating a real time clock timer. Other values for the
argument are system dependent.
int timer_settime(timer_t timer_hdr, int flag, struct itimrspec* val, struct itimrspec* old);
int timer_gettime(timer_t timer_hdr, struct itimrspec* old);
int timer_getoverrun(timer_t timer_hdr);
int timer_delete(timer_t timer_hdr);
23. 6. POSIX.1b Timers
Syntax:
There are a few structures and union, listed as follows:-
struct sigevent
{
int sigev_notify;
int sigev_signo;
union sigval sigev_value;
};
union sigval
{
int sival
void *sival_ptr;
};
24. 6. POSIX.1b Timers
Syntax:
There are a few structures and union, listed as follows:-
struct itimerspec
{
struct timespec it_interval;
struct timespec it_value;
};
struct timespec
{
time_t tv_sec;
long tv_nsec;
};
