using select() with pipe - c

I am reading/writing to a pipe created by pipe(pipe_fds). So basically with following code, I am reading from that pipe:
fp = fdopen(pipe_fds[0], "r");
And when ever I get something, I print it out by:
while (fgets(buf, 200, fp)) {
printf("%s", buf);
}
What I want is, when for certain amount of time nothing appears on the pipe to read from, I want to know about it and do:
printf("dummy");
Can this be achieved by select() ? Any pointers on how to do that will be great.

Let's say you wanted to wait 5 seconds and then if nothing was written to the pipe, you print out "dummy."
fd_set set;
struct timeval timeout;
/* Initialize the file descriptor set. */
FD_ZERO(&set);
FD_SET(pipe_fds[0], &set);
/* Initialize the timeout data structure. */
timeout.tv_sec = 5;
timeout.tv_usec = 0;
/* In the interest of brevity, I'm using the constant FD_SETSIZE, but a more
efficient implementation would use the highest fd + 1 instead. In your case
since you only have a single fd, you can replace FD_SETSIZE with
pipe_fds[0] + 1 thereby limiting the number of fds the system has to
iterate over. */
int ret = select(FD_SETSIZE, &set, NULL, NULL, &timeout);
// a return value of 0 means that the time expired
// without any acitivity on the file descriptor
if (ret == 0)
{
printf("dummy");
}
else if (ret < 0)
{
// error occurred
}
else
{
// there was activity on the file descripor
}

IIRC, select has a timeout that you then check with FD_ISSET to tell if it was I/O or not that returned.

Related

pthreads: wait until read() returns a value > 0

I'm working on a client program that will operate as a basic instant messenger. I'm using pthread to to open up a thread dedicated to waiting for a message to be received and the the message to be read. Is using pthread_cond_wait the correct way to go about waiting for read(sockfd, buffer, 256) to be above 0?
void *threadRead() {
while (1) {
bzero(buffer,256);
pthread_cond_wait(&buffer_lock, read(sockfd, buffer, 255) > 0);
n = read(sockfd, buffer, 255);
printf("%s\n",buffer);
}
}
You see I just need to wait until read() comes back with a value above 0 to continue and I can't find the right system to do that. If anyone could link something that would put me on the right track or give me a hint that would be great.
No. pthread_cond_wait() is for waiting on a condition that will be changed by one of your other threads.
If you just want to wait for read() to return something, just call read(). Unless you have specifically marked the socket as non-blocking, it will block the calling thread until there is something to return.
If read() ever returns 0 then it indicates end of file: it means that the socket has been closed on the remote side, so there will never be any more to read.
You should use select() instead, like this
int running;
running = 1;
while (running != 0) /* Just in case you want to end the loop, you can */
{
fd_set rdset;
struct timeval timeout;
timeout.tv_sec = NUMBER_OF_SECONDS_TO_WAIT;
timeout.tv_usec = YOU_CAN_HAVE_MICRO_SECONDS_PRECISION;
FD_ZERO(&rdset);
FD_SET(fd, &rdset);
if (select(fd + 1, &rdset, NULL, NULL, &timeout) == 1)
{
ssize_t length;
char buffer[100];
length = read(fd, buffer, sizeof(buffer));
/* use buffer now */
}
else
{
/* Timed out and still nothing to read */
/* do something meanwhile and retry if */
/* you want to. */
}
running = use_a_function_to_check_this();
}
you can use it in a different thread, but you need to be careful.
Non-blocking IO is difficult, it doesn't matter how you implement it is hard.
One more thing, this
n = read(sockfd, buffer, 255);
printf("%s\n",buffer);
is likely undefined behavior, since apparently buffer is
char buffer[256];
you could
n = read(sockfd, buffer, 255 /* or sizeof(buffer) - 1 */);
buffer[n] = '\0';
printf("%s\n",buffer);
ensuring that buffer is nul terminated.

unix select is not sleeping

I am trying handle 4 different named unix pipes in a single listener process.
I tried to use the select to handle the file descriptor of the pipes.I opened all the named pipes in a non blocking mode
I am having a issue, select is not at all sleeping. Continuously running in a loop.
I dont know where is the problem in my code. I pasted my code below.
Always retaining the last file descriptor in select call eventhough it doesnt have a content in pipe.
Please suggest what is wrong in code?
Code
Pipe Open call(Invoked in Constructor)
diag_fd = open(DIAG_PIPE , O_RDONLY | O_NONBLOCK );
Main Loop
while(1)
{
FD_ZERO(&fds);
FD_SET(cp_fd, &fds);
FD_SET(diag_fd, &fds);
FD_SET(err_fd, &fds);
FD_SET(perf_fd, &fds);
if (select(cp_fd+1, &fds, NULL, NULL, &tv) < 0)
{
perror("select");
return ;
}
for (int fd = diag_fd; fd <= cp_fd; fd++)
{
if (FD_ISSET(fd, &fds))
{
if (fd == diag_fd)
{
ProcessDiagLogs();
}
else if (fd == err_fd)
{
ProcessErrLogs();
}
else if (fd == perf_fd)
{
ProcessPerfLogs();
}
else if (fd == cp_fd)
{
ProcessCPLogs();
}
}
}
Read call in One File Descriptor:
ProcessDiagLogs()
do
{
if ((num = read(diag_fd, s, BUF_LENGTH)) == -1)
perror("read");
else {
s[num] = '\0';
fputs(s, filed);
fflush(filed);
}
} while (num > 0);
select() allows you to monitor one or more file descriptors, waiting until one of them becomes "ready", i.e. data is available for.
The const struct timespec *timeout, tv, in your case specifies the timeout period, or how log he select waits for data until it returns ( this behaves like a sleep ). if the timeout is 0, the select return immediately, if it's NULL it can block indefinitely.
You don't show in your code how you initialized tv, but I'm going to guess it's zero, hence the behavior you are seeing.
Try initializing the timeout before you call select and see if that helps:
tv.tv_sec = 1;//or any other value you see fit
tv.tv_usec= 0;

C using select() to read from two named pipes (FIFO)

I am currently trying to write a program in C which will read from two named pipes and print any data to stdout as it becomes available.
for example: If I open two terminals and ./execute pipe1 pipe2 in one of the terminals (with pipe1 and pipe2 being valid named pipes) and then type echo "Data here." > pipe1 then the name of the pipe (here it is pipe1), the size, and the data should print to stdout-- Here it would look like pipe1 [25]: Data here.
I know I need to open the pipes with the O_RDONLY and O_NONBLOCK flags. I have looked at many examples (quite a few on this forum) of people using select() and I still don't understand what the different parameters being passed to select() are doing. If anyone can provide guidance here it would be hugely helpful. Below is the code I have so far.
int pipeRouter(char[] fifo1, char[] fifo2){
fileDescriptor1 = open(fifo1, O_RDONLY, O_NONBLOCK);
fileDescriptor2 = open(fifo2, O_RDONLY, O_NONBLOCK);
if(fileDescriptor1 < 0){
printf("%s does not exist", fifo1);
}
if(fileDescriptor2 < 0){
printf("%s does not exist", fifo2);
}
}
The select lets you wait for an i/o event instead of waisting CPU cycles on read.
So, in your example, the main loop can look like:
for (;;)
{
int res;
char buf[256];
res = read(fileDescriptor1, buf, sizeof(buf));
if (res > 0)
{
printf("Read %d bytes from channel1\n", res);
}
res = read(fileDescriptor2, buf, sizeof(buf));
if (res > 0)
{
printf("Read %d bytes from channel2\n", res);
}
}
If you add the code and run it, you would notice that:
The program actually does what you want - it reads from both pipes.
CPU utilization is 100% for one core, i.e. program wastes CPU even when there is no data to read.
To solve issue, select and poll APIs are introduced. For select we need to know descriptors (we do), and the maximum out of them.
So let's modify the code a bit:
for (;;)
{
fd_set fds;
int maxfd;
FD_ZERO(&fds); // Clear FD set for select
FD_SET(fileDescriptor1, &fds);
FD_SET(fileDescriptor2, &fds);
maxfd = fileDescriptor1 > fileDescriptor2 ? fileDescriptor1 : fileDescriptor2;
select(maxfd + 1, &fds, NULL, NULL, NULL);
// The minimum information for select: we are asking only about
// read operations, ignoring write and error ones; and not
// defining any time restrictions on wait.
// do reads as in previous example here
}
When running the improved code, the CPU would not be wasted as much, but you will notice, that the read operation is performed even when there is no data for a particular pipe, but there is for another.
To check, which pipe actually has the data, use FD_ISSET after select call:
if (FD_ISSET(fileDescriptor1, &fds))
{
// We can read from fileDescriptor1
}
if (FD_ISSET(fileDescriptor2, &fds))
{
// We can read from fileDescriptor2
}
So, after joining said above, the code would look like:
for (;;)
{
fd_set fds;
int maxfd;
int res;
char buf[256];
FD_ZERO(&fds); // Clear FD set for select
FD_SET(fileDescriptor1, &fds);
FD_SET(fileDescriptor2, &fds);
maxfd = fileDescriptor1 > fileDescriptor2 ? fileDescriptor1 : fileDescriptor2;
select(maxfd + 1, &fds, NULL, NULL, NULL);
if (FD_ISSET(fileDescriptor1, &fds))
{
// We can read from fileDescriptor1
res = read(fileDescriptor1, buf, sizeof(buf));
if (res > 0)
{
printf("Read %d bytes from channel1\n", res);
}
}
if (FD_ISSET(fileDescriptor2, &fds))
{
// We can read from fileDescriptor2
res = read(fileDescriptor2, buf, sizeof(buf));
if (res > 0)
{
printf("Read %d bytes from channel2\n", res);
}
}
}
So, add error handling, and you would be set.

How to loop select() to poll for data ad infinitum

#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
int main ()
{
char name[20];
fd_set input_set;
struct timeval timeout;
int ready_for_reading = 0;
int read_bytes = 0;
/* Empty the FD Set */
FD_ZERO(&input_set );
/* Listen to the input descriptor */
FD_SET(0, &input_set);
/* Waiting for some seconds */
timeout.tv_sec = 10; // 10 seconds
timeout.tv_usec = 0; // 0 milliseconds
/* Invitation for the user to write something */
printf("Enter Username: (in 15 seconds)\n");
printf("Time start now!!!\n");
/* Listening for input stream for any activity */
ready_for_reading = select(1, &input_set, NULL, NULL, &timeout);
/* Here, first parameter is value of the socket descriptor + 1 (STDIN descriptor is 0, so
* 0 +1 = 1)
* in the set, second is our FD set for reading,
* third is the FD set in which any write activity needs to updated, which is not required
* in this case. Fourth is timeout
*/
if (ready_for_reading == -1) {
/* Some error has occured in input */
printf("Unable to read your input\n");
return -1;
} else {
if (ready_for_reading) {
read_bytes = read(0, name, 19);
printf("Read, %d bytes from input : %s \n", read_bytes, name);
} else {
printf(" 10 Seconds are over - no data input \n");
}
}
return 0;
}
How to do the same, but not just once, but in infinite loop which breaks after encountering 'quit' string (for example). Every way I tried - failed.
So if no data has been inputed after 10 seconds program just prints "10 secs are over - no data input" and then starts waiting again. Same after input - just begins again and behave the same every time in infinite loop.
Am little desperate already, please - help.
Thanks.
I don't really see the problem here. Basically just put everything you want in the loop, and let it run. Did you try this?
int main ()
{
/* Declarations and stuff */
/* ... */
/* The loop */
int break_condition = 0;
while (!break_condition)
{
/* Selection */
FD_ZERO(&input_set ); /* Empty the FD Set */
FD_SET(0, &input_set); /* Listen to the input descriptor */
ready_for_reading = select(1, &input_set, NULL, NULL, &timeout);
/* Selection handling */
if (ready_for_reading)
{
/* Do something clever with the input */
}
else
{
/* Handle the error */
}
/* Test the breaking condition */
break_condition = some_calculation();
}
return 0;
}
Note that you have to have keep resetting the selection inside the loop so that it will respond again in the next iteration.
The select() function can be told to block indefinitely by setting timeout to NULL. See select(2) man page:
timeout is an upper bound on the amount of time elapsed before select() returns. If both fields of the timeval stucture are zero, then select() returns immediately. (This is useful for polling.) If timeout is NULL (no timeout), select() can block indefinitely.
So what you want is:
...
ready_for_reading = select(1, &input_set, NULL, NULL, NULL);
...

How can I implement timeout for read() when reading from a serial port (C/C++)

I am reading bytes from a serial port in C++ using a file descriptor and the posix/unix read() function. In this example, I am reading 1 byte from the serial port (baud rate settings and similiar are omitted for clarity):
#include <termios.h>
#include <fcntl.h>
#include <unistd.h>
int main(void)
{
int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY);
char buf[1];
int bytesRead = read(fd, buf, 1);
close(fd);
return 0;
}
If the device connected to /dev/ttyS0 does not send any information, the program will hang. How can I set a timeout?
I have tried setting a time out like this:
struct termios options;
tcgetattr(fd, &options);
options.c_cc[VMIN] = 0;
options.c_cc[VTIME] = 10;
tcsetattr(fd, TCSANOW, &options);
I thought it was supposed to give 1 second timeout, but it makes no difference. I think I have misunderstood VMIN and VTIME. What is VMIN and VTIME used for?
Then I searched the web and found somebody talking about the select() function. Is that the solution and if so, how would one apply that to the program above to make 1 second timeout?
Any help is appreciated. Thanks in advance :-)
Yes, use select(2). Pass in a file descriptor set containing just your fd in the read set and empty write/exception sets, and pass in an appropriate timeout. For example:
int fd = open(...);
// Initialize file descriptor sets
fd_set read_fds, write_fds, except_fds;
FD_ZERO(&read_fds);
FD_ZERO(&write_fds);
FD_ZERO(&except_fds);
FD_SET(fd, &read_fds);
// Set timeout to 1.0 seconds
struct timeval timeout;
timeout.tv_sec = 1;
timeout.tv_usec = 0;
// Wait for input to become ready or until the time out; the first parameter is
// 1 more than the largest file descriptor in any of the sets
if (select(fd + 1, &read_fds, &write_fds, &except_fds, &timeout) == 1)
{
// fd is ready for reading
}
else
{
// timeout or error
}
What is VMIN and VTIME used for?
If MIN > 0 and TIME = 0, MIN sets the number of characters to receive
before the read is satisfied. As TIME is zero, the timer is not used.
If MIN = 0 and TIME > 0, TIME serves as a timeout value. The read will
be satisfied if a single character is read, or TIME is exceeded (t =
TIME *0.1 s). If TIME is exceeded, no character will be returned.
If MIN > 0 and TIME > 0, TIME serves as an inter-character timer. The
read will be satisfied if MIN characters are received, or the time
between two characters exceeds TIME. The timer is restarted every time
a character is received and only becomes active after the first
character has been received.
If MIN = 0 and TIME = 0, read will be satisfied immediately. The
number of characters currently available, or the number of characters
requested will be returned. According to Antonino (see contributions),
you could issue a fcntl(fd, F_SETFL, FNDELAY); before reading to get
the same result.
Source : http://tldp.org/HOWTO/Serial-Programming-HOWTO/x115.html
You can attempt capture signal to stop read operation. use alarm(1) before read, and if read function did not returned, alarm will send SIGALRM signal, then you can create signal processing function to capture this signal, like this:
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <setjmp.h>
static jmp_buf env_alarm;
static void sig_alarm(int signo)
{
longjmp(env_alarm, 1);
}
int main(void)
{
int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY);
char buf[1];
if (signal(SIGALRM, sig_alarm) == SIG_ERR)
{
exit(0);
}
if (setjmp(env_alarm) != 0)
{
close(fd);
printf("Timeout Or Error\n");
exit(0);
}
alarm(1);
int bytesRead = read(fd, buf, 1);
alarm(0);
close(fd);
return 0;
}
But use select or poll or epoll will be better if your program is big.
select() is the way I would solve this problem.
There are several pages on the internet that will give info on how to use select(), such as http://www.unixguide.net/unix/programming/2.1.1.shtml
There are several possible approaches. If the program will eventually be timing more than one i/o operation, select() is the clear choice.
However, if the only input is from this i/o, then selecting non-blocking i/o and timing is a straightforward method. I have expanded it from single character i/o to multi-character to make it a more generally complete example:
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <sys/time.h>
int main(void)
{
int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY); // sometimes "O_NONBLOCK"
char buf[10];
int done = 0, inbuf = 0;
struct timeval start, now;
gettimeofday (&start, NULL);
while (!done)
{
int bytesRead = read(fd, &buf[inbuf], sizeof buf - inbuf);
if (bytesRead < 0)
{
error_processing_here();
continue;
}
if (bytesRead == 0) // no data read to read
{
gettimeofday (&now, NULL);
if ((now.tv.sec - start.tv_sec) * 1000000 +
now.tv.usec - start.tv_usec > timeout_value_in_microsecs)
{
done = 2; // timeout
continue;
}
sleep(1); // not timed out yet, sleep a second
continue;
}
inbuf += bytesRead;
if (we have read all we want)
done = 1;
}
if (done == 2)
timeout_condition_handling();
close(fd);
return 0;
}

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