I've written the program which spawns a thread that reads in a loop from stdin in a blocking fashion. I want to make the thread return from blocked read immediately. I've registered my signal handler (with sigaction and without SA_RESTART flag) in the reading thread, send it a signal and expect read to exit with EINTR error. But it doesn't happen. Is it issue or limitation of Cygwin or am I doing something wrong?
Here is the code:
#include <stdio.h>
#include <errno.h>
#include <pthread.h>
pthread_t thread;
volatile int run = 0;
void root_handler(int signum)
{
printf("%s ENTER (thread is %x)\n", __func__, pthread_self());
run = 0;
}
void* thr_func(void*arg)
{ int res;
char buffer[256];
printf("%s ENTER (thread is %x)\n", __func__, pthread_self());
struct sigaction act;
memset (&act, 0, sizeof(act));
act.sa_sigaction = &root_handler;
//act.sa_flags = SA_RESTART;
if (sigaction(SIGUSR1, &act, NULL) < 0) {
perror ("sigaction error");
return 1;
}
while(run)
{
res = read(0,buffer, sizeof(buffer));
if(res == -1)
{
if(errno == EINTR)
{
puts("read was interrupted by signal");
}
}
else
{
printf("got: %s", buffer);
}
}
printf("%s LEAVE (thread is %x)\n", __func__, pthread_self());
}
int main() {
run = 1;
printf("root thread: %x\n", pthread_self());
pthread_create(&thread, NULL, &thr_func, NULL);
printf("thread %x started\n", thread);
sleep(4);
pthread_kill(thread, SIGUSR1 );
//raise(SIGUSR1);
pthread_join(thread, NULL);
return 0;
}
I'm using Cygwin (1.7.32(0.274/5/3)).
I've just tried to do the same on Ubuntu and it works (I needed to include signal.h, though, even though in Cygwin it compiled as it is). It seems to be peculiarity of Cygwin's implementation.
Related
The Linux robust_list mechanism is a tool used by robust mutexes to support automatic unlocking in the event that the lock owner fails to unlock before terminating, maybe due to unexpected death. According to man set_robust_list:
The purpose of the robust futex list is to ensure that if a thread accidentally fails to unlock a futex before terminating or calling execve(2), another thread that is waiting on that futex is notified that the former owner of the futex has died. This notification consists of two pieces: the FUTEX_OWNER_DIED bit is set in the futex word, and the kernel performs a futex(2) FUTEX_WAKE operation on one of the threads waiting on the futex.
This is not the behavior I'm seeing.
I'm seeing the futex replaced with FUTEX_OWNER_DIED, not ored with.
And I'm not getting the FUTEX_WAKE call.
#include <chrono>
#include <thread>
#include <linux/futex.h>
#include <stdint.h>
#include <stdio.h>
#include <syscall.h>
#include <unistd.h>
using ftx_t = uint32_t;
struct mtx_t {
mtx_t* next;
mtx_t* prev;
ftx_t ftx;
};
thread_local robust_list_head robust_head;
void robust_init() {
robust_head.list.next = &robust_head.list;
robust_head.futex_offset = offsetof(mtx_t, ftx);
robust_head.list_op_pending = NULL;
syscall(SYS_set_robust_list, &robust_head.list, sizeof(robust_head));
}
void robust_op_start(mtx_t* mtx) {
robust_head.list_op_pending = (robust_list*)mtx;
__sync_synchronize();
}
void robust_op_end() {
__sync_synchronize();
robust_head.list_op_pending = NULL;
}
void robust_op_add(mtx_t* mtx) {
mtx_t* old_first = (mtx_t*)robust_head.list.next;
mtx->prev = (mtx_t*)&robust_head;
mtx->next = old_first;
__sync_synchronize();
robust_head.list.next = (robust_list*)mtx;
if (old_first != (mtx_t*)&robust_head) {
old_first->prev = mtx;
}
}
int futex(ftx_t* uaddr,
int futex_op,
int val,
uintptr_t timeout_or_val2,
ftx_t* uaddr2,
int val3) {
return syscall(SYS_futex, uaddr, futex_op, val, timeout_or_val2, uaddr2, val3);
}
int ftx_wait(ftx_t* ftx, int confirm_val) {
return futex(ftx, FUTEX_WAIT, confirm_val, 0, NULL, 0);
}
int main() {
mtx_t mtx = {0};
std::thread t0{[&]() {
fprintf(stderr, "t0 start\n");
ftx_wait(&mtx.ftx, 0);
fprintf(stderr, "t0 done\n");
}};
std::this_thread::sleep_for(std::chrono::milliseconds(100));
std::thread t1{[&]() {
fprintf(stderr, "t1 start\n");
robust_init();
robust_op_start(&mtx);
__sync_bool_compare_and_swap(&mtx.ftx, 0, syscall(SYS_gettid));
robust_op_add(&mtx);
robust_op_end();
fprintf(stderr, "t1 ftx: %x\n", mtx.ftx);
fprintf(stderr, "t1 done\n");
}};
t1.join();
std::this_thread::sleep_for(std::chrono::milliseconds(100));
fprintf(stderr, "ftx: %x\n", mtx.ftx);
t0.join();
}
Running
g++ -o ./example ~/example.cpp -lpthread && ./example
prints something like:
t0 start
t1 start
t1 ftx: 12ea65
t1 done
ftx: 40000000
and hangs.
I would expect the final value of the futex to be 4012ea65 and for thread 0 to unblock after thread 1 completes.
What would be your suggestion in order to create a single instance application, so that only one process is allowed to run at a time? File lock, mutex or what?
A good way is:
#include <sys/file.h>
#include <errno.h>
int pid_file = open("/var/run/whatever.pid", O_CREAT | O_RDWR, 0666);
int rc = flock(pid_file, LOCK_EX | LOCK_NB);
if(rc) {
if(EWOULDBLOCK == errno)
; // another instance is running
}
else {
// this is the first instance
}
Note that locking allows you to ignore stale pid files (i.e. you don't have to delete them). When the application terminates for any reason the OS releases the file lock for you.
Pid files are not terribly useful because they can be stale (the file exists but the process does not). Hence, the application executable itself can be locked instead of creating and locking a pid file.
A more advanced method is to create and bind a unix domain socket using a predefined socket name. Bind succeeds for the first instance of your application. Again, the OS unbinds the socket when the application terminates for any reason. When bind() fails another instance of the application can connect() and use this socket to pass its command line arguments to the first instance.
Here is a solution in C++. It uses the socket recommendation of Maxim. I like this solution better than the file based locking solution, because the file based one fails if the process crashes and does not delete the lock file. Another user will not be able to delete the file and lock it. The sockets are automatically deleted when the process exits.
Usage:
int main()
{
SingletonProcess singleton(5555); // pick a port number to use that is specific to this app
if (!singleton())
{
cerr << "process running already. See " << singleton.GetLockFileName() << endl;
return 1;
}
... rest of the app
}
Code:
#include <netinet/in.h>
class SingletonProcess
{
public:
SingletonProcess(uint16_t port0)
: socket_fd(-1)
, rc(1)
, port(port0)
{
}
~SingletonProcess()
{
if (socket_fd != -1)
{
close(socket_fd);
}
}
bool operator()()
{
if (socket_fd == -1 || rc)
{
socket_fd = -1;
rc = 1;
if ((socket_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
{
throw std::runtime_error(std::string("Could not create socket: ") + strerror(errno));
}
else
{
struct sockaddr_in name;
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
rc = bind (socket_fd, (struct sockaddr *) &name, sizeof (name));
}
}
return (socket_fd != -1 && rc == 0);
}
std::string GetLockFileName()
{
return "port " + std::to_string(port);
}
private:
int socket_fd = -1;
int rc;
uint16_t port;
};
For windows, a named kernel object (e.g. CreateEvent, CreateMutex). For unix, a pid-file - create a file and write your process ID to it.
You can create an "anonymous namespace" AF_UNIX socket. This is completely Linux-specific, but has the advantage that no filesystem actually has to exist.
Read the man page for unix(7) for more info.
Avoid file-based locking
It is always good to avoid a file based locking mechanism to implement the singleton instance of an application. The user can always rename the lock file to a different name and run the application again as follows:
mv lockfile.pid lockfile1.pid
Where lockfile.pid is the lock file based on which is checked for existence before running the application.
So, it is always preferable to use a locking scheme on object directly visible to only the kernel. So, anything which has to do with a file system is not reliable.
So the best option would be to bind to a inet socket. Note that unix domain sockets reside in the filesystem and are not reliable.
Alternatively, you can also do it using DBUS.
It's seems to not be mentioned - it is possible to create a mutex in shared memory but it needs to be marked as shared by attributes (not tested):
pthread_mutexattr_t attr;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_t *mutex = shmat(SHARED_MEMORY_ID, NULL, 0);
pthread_mutex_init(mutex, &attr);
There is also shared memory semaphores (but I failed to find out how to lock one):
int sem_id = semget(SHARED_MEMORY_KEY, 1, 0);
No one has mentioned it, but sem_open() creates a real named semaphore under modern POSIX-compliant OSes. If you give a semaphore an initial value of 1, it becomes a mutex (as long as it is strictly released only if a lock was successfully obtained).
With several sem_open()-based objects, you can create all of the common equivalent Windows named objects - named mutexes, named semaphores, and named events. Named events with "manual" set to true is a bit more difficult to emulate (it requires four semaphore objects to properly emulate CreateEvent(), SetEvent(), and ResetEvent()). Anyway, I digress.
Alternatively, there is named shared memory. You can initialize a pthread mutex with the "shared process" attribute in named shared memory and then all processes can safely access that mutex object after opening a handle to the shared memory with shm_open()/mmap(). sem_open() is easier if it is available for your platform (if it isn't, it should be for sanity's sake).
Regardless of the method you use, to test for a single instance of your application, use the trylock() variant of the wait function (e.g. sem_trywait()). If the process is the only one running, it will successfully lock the mutex. If it isn't, it will fail immediately.
Don't forget to unlock and close the mutex on application exit.
It will depend on which problem you want to avoid by forcing your application to have only one instance and the scope on which you consider instances.
For a daemon — the usual way is to have a /var/run/app.pid file.
For user application, I've had more problems with applications which prevented me to run them twice than with being able to run twice an application which shouldn't have been run so. So the answer on "why and on which scope" is very important and will probably bring answer specific on the why and the intended scope.
Here is a solution based on sem_open
/*
*compile with :
*gcc single.c -o single -pthread
*/
/*
* run multiple instance on 'single', and check the behavior
*/
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <unistd.h>
#include <errno.h>
#define SEM_NAME "/mysem_911"
int main()
{
sem_t *sem;
int rc;
sem = sem_open(SEM_NAME, O_CREAT, S_IRWXU, 1);
if(sem==SEM_FAILED){
printf("sem_open: failed errno:%d\n", errno);
}
rc=sem_trywait(sem);
if(rc == 0){
printf("Obtained lock !!!\n");
sleep(10);
//sem_post(sem);
sem_unlink(SEM_NAME);
}else{
printf("Lock not obtained\n");
}
}
One of the comments on a different answer says "I found sem_open() rather lacking". I am not sure about the specifics of what's lacking
Based on the hints in maxim's answer here is my POSIX solution of a dual-role daemon (i.e. a single application that can act as daemon and as a client communicating with that daemon). This scheme has the advantage of providing an elegant solution of the problem when the instance started first should be the daemon and all following executions should just load off the work at that daemon. It is a complete example but lacks a lot of stuff a real daemon should do (e.g. using syslog for logging and fork to put itself into background correctly, dropping privileges etc.), but it is already quite long and is fully working as is. I have only tested this on Linux so far but IIRC it should be all POSIX-compatible.
In the example the clients can send integers passed to them as first command line argument and parsed by atoi via the socket to the daemon which prints it to stdout. With this kind of sockets it is also possible to transfer arrays, structs and even file descriptors (see man 7 unix).
#include <stdio.h>
#include <stddef.h>
#include <stdbool.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <signal.h>
#include <sys/socket.h>
#include <sys/un.h>
#define SOCKET_NAME "/tmp/exampled"
static int socket_fd = -1;
static bool isdaemon = false;
static bool run = true;
/* returns
* -1 on errors
* 0 on successful server bindings
* 1 on successful client connects
*/
int singleton_connect(const char *name) {
int len, tmpd;
struct sockaddr_un addr = {0};
if ((tmpd = socket(AF_UNIX, SOCK_DGRAM, 0)) < 0) {
printf("Could not create socket: '%s'.\n", strerror(errno));
return -1;
}
/* fill in socket address structure */
addr.sun_family = AF_UNIX;
strcpy(addr.sun_path, name);
len = offsetof(struct sockaddr_un, sun_path) + strlen(name);
int ret;
unsigned int retries = 1;
do {
/* bind the name to the descriptor */
ret = bind(tmpd, (struct sockaddr *)&addr, len);
/* if this succeeds there was no daemon before */
if (ret == 0) {
socket_fd = tmpd;
isdaemon = true;
return 0;
} else {
if (errno == EADDRINUSE) {
ret = connect(tmpd, (struct sockaddr *) &addr, sizeof(struct sockaddr_un));
if (ret != 0) {
if (errno == ECONNREFUSED) {
printf("Could not connect to socket - assuming daemon died.\n");
unlink(name);
continue;
}
printf("Could not connect to socket: '%s'.\n", strerror(errno));
continue;
}
printf("Daemon is already running.\n");
socket_fd = tmpd;
return 1;
}
printf("Could not bind to socket: '%s'.\n", strerror(errno));
continue;
}
} while (retries-- > 0);
printf("Could neither connect to an existing daemon nor become one.\n");
close(tmpd);
return -1;
}
static void cleanup(void) {
if (socket_fd >= 0) {
if (isdaemon) {
if (unlink(SOCKET_NAME) < 0)
printf("Could not remove FIFO.\n");
} else
close(socket_fd);
}
}
static void handler(int sig) {
run = false;
}
int main(int argc, char **argv) {
switch (singleton_connect(SOCKET_NAME)) {
case 0: { /* Daemon */
struct sigaction sa;
sa.sa_handler = &handler;
sigemptyset(&sa.sa_mask);
if (sigaction(SIGINT, &sa, NULL) != 0 || sigaction(SIGQUIT, &sa, NULL) != 0 || sigaction(SIGTERM, &sa, NULL) != 0) {
printf("Could not set up signal handlers!\n");
cleanup();
return EXIT_FAILURE;
}
struct msghdr msg = {0};
struct iovec iovec;
int client_arg;
iovec.iov_base = &client_arg;
iovec.iov_len = sizeof(client_arg);
msg.msg_iov = &iovec;
msg.msg_iovlen = 1;
while (run) {
int ret = recvmsg(socket_fd, &msg, MSG_DONTWAIT);
if (ret != sizeof(client_arg)) {
if (errno != EAGAIN && errno != EWOULDBLOCK) {
printf("Error while accessing socket: %s\n", strerror(errno));
exit(1);
}
printf("No further client_args in socket.\n");
} else {
printf("received client_arg=%d\n", client_arg);
}
/* do daemon stuff */
sleep(1);
}
printf("Dropped out of daemon loop. Shutting down.\n");
cleanup();
return EXIT_FAILURE;
}
case 1: { /* Client */
if (argc < 2) {
printf("Usage: %s <int>\n", argv[0]);
return EXIT_FAILURE;
}
struct iovec iovec;
struct msghdr msg = {0};
int client_arg = atoi(argv[1]);
iovec.iov_base = &client_arg;
iovec.iov_len = sizeof(client_arg);
msg.msg_iov = &iovec;
msg.msg_iovlen = 1;
int ret = sendmsg(socket_fd, &msg, 0);
if (ret != sizeof(client_arg)) {
if (ret < 0)
printf("Could not send device address to daemon: '%s'!\n", strerror(errno));
else
printf("Could not send device address to daemon completely!\n");
cleanup();
return EXIT_FAILURE;
}
printf("Sent client_arg (%d) to daemon.\n", client_arg);
break;
}
default:
cleanup();
return EXIT_FAILURE;
}
cleanup();
return EXIT_SUCCESS;
}
All credits go to Mark Lakata. I merely did some very minor touch up only.
main.cpp
#include "singleton.hpp"
#include <iostream>
using namespace std;
int main()
{
SingletonProcess singleton(5555); // pick a port number to use that is specific to this app
if (!singleton())
{
cerr << "process running already. See " << singleton.GetLockFileName() << endl;
return 1;
}
// ... rest of the app
}
singleton.hpp
#include <netinet/in.h>
#include <unistd.h>
#include <cerrno>
#include <string>
#include <cstring>
#include <stdexcept>
using namespace std;
class SingletonProcess
{
public:
SingletonProcess(uint16_t port0)
: socket_fd(-1)
, rc(1)
, port(port0)
{
}
~SingletonProcess()
{
if (socket_fd != -1)
{
close(socket_fd);
}
}
bool operator()()
{
if (socket_fd == -1 || rc)
{
socket_fd = -1;
rc = 1;
if ((socket_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
{
throw std::runtime_error(std::string("Could not create socket: ") + strerror(errno));
}
else
{
struct sockaddr_in name;
name.sin_family = AF_INET;
name.sin_port = htons (port);
name.sin_addr.s_addr = htonl (INADDR_ANY);
rc = bind (socket_fd, (struct sockaddr *) &name, sizeof (name));
}
}
return (socket_fd != -1 && rc == 0);
}
std::string GetLockFileName()
{
return "port " + std::to_string(port);
}
private:
int socket_fd = -1;
int rc;
uint16_t port;
};
#include <windows.h>
int main(int argc, char *argv[])
{
// ensure only one running instance
HANDLE hMutexH`enter code here`andle = CreateMutex(NULL, TRUE, L"my.mutex.name");
if (GetLastError() == ERROR_ALREADY_EXISTS)
{
return 0;
}
// rest of the program
ReleaseMutex(hMutexHandle);
CloseHandle(hMutexHandle);
return 0;
}
FROM: HERE
On Windows you could also create a shared data segment and use an interlocked function to test for the first occurence, e.g.
#include <Windows.h>
#include <stdio.h>
#include <conio.h>
#pragma data_seg("Shared")
volatile LONG lock = 0;
#pragma data_seg()
#pragma comment(linker, "/SECTION:Shared,RWS")
void main()
{
if (InterlockedExchange(&lock, 1) == 0)
printf("first\n");
else
printf("other\n");
getch();
}
I have just written one, and tested.
#define PID_FILE "/tmp/pidfile"
static void create_pidfile(void) {
int fd = open(PID_FILE, O_RDWR | O_CREAT | O_EXCL, 0);
close(fd);
}
int main(void) {
int fd = open(PID_FILE, O_RDONLY);
if (fd > 0) {
close(fd);
return 0;
}
// make sure only one instance is running
create_pidfile();
}
Just run this code on a seperate thread:
void lock() {
while(1) {
ofstream closer("myapplock.locker", ios::trunc);
closer << "locked";
closer.close();
}
}
Run this as your main code:
int main() {
ifstream reader("myapplock.locker");
string s;
reader >> s;
if (s != "locked") {
//your code
}
return 0;
}
When the following C program is executed, and SIGUSR1 is sent to the running process repeatedly, the pclose() call will sometimes return 13. 13 corresponds to SIGPIPE on my system.
Why does this happen?
I am using while true; do kill -SIGUSR1 <process-id>; done to send SIGUSR1 to the program. The program is executed on Ubuntu 14.04.
#include <pthread.h>
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
void handler(int i) {}
void* task(void*)
{
FILE *s;
char b [BUFSIZ];
while (1) {
if ((s = popen("echo hello", "r")) == NULL) {
printf("popen() failed\n");
}
while (fgets(b, BUFSIZ, s) != NULL) ;
if (int r = pclose(s)) {
printf("pclose() failed (%d)\n", r);
}
}
return 0;
}
int main(int argc, char **argv)
{
struct sigaction action;
action.sa_handler = handler;
sigemptyset(&action.sa_mask);
action.sa_flags = 0;
sigaction(SIGUSR1, &action, NULL);
pthread_t tid;
pthread_create(&tid, 0, task, NULL);
pthread_join(tid, NULL);
}
This happens when fgets gets interrupted by the signal. The program doesn't read the pipe to the end and closes it. The other program then SIGPIPEs.
The correct pipe reading operation is:
do {
while (fgets(b, BUFSIZ, s) != NULL) ;
} while (errno == EINTR);
Parent receives SIGPIPE sending chars to aborted child process through FIFO pipe.
I am trying to avoid this, using select() function. In the attached sample code,
select() retruns OK even after the child at the other end of pipe having been terminated.
Tested in
RedHat EL5 （Linux 2.6.18-194.32.1.el5）
GNU C Library stable release version 2.5
Any help appreciated. Thnak you.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/stat.h>
#include <unistd.h>
static void sigpipe_fct();
main()
{
struct stat st;
int i, fd_out, fd_in, child;
char buf[1024];
#define p_out "/tmp/pout"
signal(SIGPIPE, sigpipe_fct);
if (stat(p_out, &st) != 0) {
mknod(p_out, S_IFIFO, 0);
chmod(p_out, 0666);
}
/* start receiving process */
if ((child = fork()) == 0) {
if ((fd_in = open(p_out, O_RDONLY)) < 0) {
perror(p_out);
exit(1);
}
while(1) {
i = read(fd_in, buf, sizeof(buf));
fprintf(stderr, "child %d read %.*s\n", getpid(), i, buf);
lseek(fd_in, 0, 0);
}
}
else {
fprintf(stderr,
"reading from %s - exec \"kill -9 %d\" to test\n", p_out, child);
if ((fd_out = open(p_out, O_WRONLY + O_NDELAY)) < 0) { /* output */
perror(p_out);
exit(1);
}
while(1) {
if (SelectChkWrite(fd_out) == fd_out) {
fprintf(stderr, "SelectChkWrite() success write abc\n");
write(fd_out, "abc", 3);
}
else
fprintf(stderr, "SelectChkWrite() failed\n");
sleep(3);
}
}
}
static void sigpipe_fct()
{
fprintf(stderr, "SIGPIPE received\n");
exit(-1);
}
SelectChkWrite(ch)
int ch;
{
#include <sys/select.h>
fd_set writefds;
int i;
FD_ZERO(&writefds);
FD_SET (ch, &writefds);
i = select(ch + 1, NULL, &writefds, NULL, NULL);
if (i == -1)
return(-1);
else if (FD_ISSET(ch, &writefds))
return(ch);
else
return(-1);
}
From the Linux select(3) man page:
A descriptor shall be considered ready for writing when a call to an
output function with O_NONBLOCK clear would not block, whether or not
the function would transfer data successfully.
When the pipe is closed, it won't block, so it is considered "ready" by select.
BTW, having #include <sys/select.h> inside your SelectChkWrite() function is extremely bad form.
Although select() and poll() are both in the POSIX standard, select() is much older and more limited than poll(). In general, I recommend people use poll() by default and only use select() if they have a good reason. (See here for one example.)
This is my first Program....ctrlcsignal.c
enter code here
#include<stdio.h>
#include<unistd.h>
#include<signal.h>
void signal_handler(int sigNo)
{
//if Ctrl+c signal
if(sigNo==SIGINT){
printf("value of SIGINT:-%d\t",SIGINT);
printf("received SIGINT\n");
}
// if some other signal , but this part wont get executed
// as the signal_handler function is registered with SIGINT only
else
{
printf("Some other signal found");
printf("value of other signal:-%d",sigNo);
}
}
int main(void)
{
//registering the signal handler function with a signal
kill(19574,SIGUSR1);
if(signal(SIGINT,signal_handler)==SIG_ERR)
{
printf("\n can't catch SIGINT\n");
}
while(1) //infinite loop
sleep(1); // 1s ,so that the CPU is not busy
return 0;
}
and this my second program....userdefinedsignals.c
enter code here
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
void signal_handler(int sigNo)
{
printf("function entered...");
// check for userdefined Signal SIGUSR1
if (sigNo == SIGUSR1)
{
printf("received SIGUSR1 with value :- %d",SIGUSR1);
}
//checking for KILL Signal
else if (sigNo == SIGKILL)
{
printf("received SIGKILL with value :- %d",SIGKILL);
}
//checking for STOP Signal
else if (sigNo == SIGSTOP)
{
printf("received SIGSTOP with value :- %d",SIGSTOP);
}
// if some other signal , but this part wont get executed
// as the signal_handler function is registered with SIGINT only
else
{
printf("Some other signal found");
printf("value of other signal:-%d",sigNo);
}
}
int main(void)
{
int pid_t;
printf("process id is %d",getpid());
//registering the signal handler function with a signal
if(signal(SIGUSR1,signal_handler) == SIG_ERR)
{
printf("\n can't catch SIGSIGUSR1\n");
}
if(signal(SIGKILL,signal_handler)==SIG_ERR)
{
printf("\n can't catch SIGKILL\n");
}
if(signal(SIGSTOP,signal_handler)==SIG_ERR)
{
printf("\n can't catch SIGSTOP\n");
}
while(1) //infinite loop
sleep(1); // 1s ,so that the CPU is not busy
return 0;
}
I get the pid of the second process ... suppose xxxx
then i use the following command...
enter code here
kill -USR1 xxxx
but it shows nothing ....
also then i tried by calling the following function int the first program...but no use..
enter code herekill(xxxx,SIGUSR1);
HELP ME..!!!!
Works here.
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <stdarg.h> /* vsnprintf() */
#include <signal.h> /* signal */
void myprintf(FILE *fp, char *fmt, ...)
{
char buff[512];
int rc,fd;
va_list argh;
va_start (argh, fmt);
rc = vsnprintf(buff, sizeof buff, fmt, argh);
if (rc < 0 || rc >= sizeof buff) {
rc = sprintf(buff, "Argh!: %d:\n", rc);
}
if (!fp) fp = stderr;
fd = fileno(fp);
if (fd < 0) return;
if (rc > 0) write(fd, buff, rc);
return;
}
void signal_handler(int sigNo)
{
switch (sigNo ) {
case SIGUSR1:
myprintf(NULL, "received SIGUSR1 with value :- %d\n", SIGUSR1);
break;
case SIGKILL:
myprintf(NULL, "received SIGKILL with value :- %d\n", SIGKILL);
break;
case SIGSTOP:
myprintf(NULL, "received SIGSTOP with value :- %d\n", SIGSTOP);
break;
default:
myprintf(NULL, "Some other signal occured: %d\n", sigNo);
break;
}
return;
}
int main(void)
{
pid_t mypid;
mypid = getpid();
printf("process id is %d\n", (int) mypid);
if(signal(SIGUSR1,signal_handler) == SIG_ERR)
{ printf("\n can't catch SIGSIGUSR1\n"); }
if(signal(SIGKILL,signal_handler)==SIG_ERR)
{ printf("\n can't catch SIGKILL\n"); }
if(signal(SIGSTOP,signal_handler)==SIG_ERR)
{ printf("\n can't catch SIGSTOP\n"); }
if(signal(SIGCONT,signal_handler)==SIG_ERR)
{ printf("\n can't catch SIGCONT\n"); }
while(1) {
sleep(1);
}
return 0;
}
You're catching the signal all right, but not seeing the message because you don't terminate lines properly, and the standard output stream on your system is line buffered (assuming your program runs in a terminal).
Standard C defines three levels of buffering for output streams:
unbuffered, where output is transmitted immediately
line buffered, where output is transmitted when a newline character is encountered
fully buffered, where output is transmitted when an internal buffer fills
(This is a simplification - see a C reference or the Standard for details).
Consider:
#include <stdio.h>
#include <unistd.h>
int main(void)
{
printf("Hello");
pause();
}
This produces no output in a terminal. Fix it by terminating the line:
printf("Hello\n");
This will produce the expected output in a terminal.
If stdout is not connected to a terminal - for example, you redirect to a file - then the stream becomes fully buffered. This:
./a.out > foo
Ctrl-C
cat foo
produces no output, even with the newline character added. Here you need an explicit flush to transmit the output before the buffer is full.
#include <stdio.h>
#include <unistd.h>
int main(void)
{
printf("Hello\n");
fflush(stdout);
pause();
}
This produces output even when redirected to a file.