poll is running for infinite time interval.
i want poll to hit when some thing is written into file or when file is updated.
but poll is not able to detect when file is written.
#include <stdio.h>
#include <poll.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/types.h>
#include <unistd.h>
int main() {
char buf[5]="true";
struct pollfd ufds[1];
int rv;
ufds[0].fd = 0;
ufds[0].events = POLLIN;
char *filename="textfile.txt";
ssize_t ret_write,ret_read;
ufds[0].fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 5);
if(ufds[0].fd== -1){
perror("open");
return 3;
}
while(1) {
ret_write= write (ufds[0].fd, &buf, (ssize_t) 5);
if((rv = poll(ufds, 1,-1 )) == -1) perror("select");
else if (rv == 0) printf("Timeout occurred!\n");
else if (ufds[0].revents & POLLIN) {
printf("return hit\n");
read(ufds[0].fd, buf, 5);
}
fflush(stdout);
}
return 0;
}
Your example can't work because the file is not open for reading. Even if the file was opened for reading, the code would not work as intended because poll would return sucessfully on end of file.
What you want is the inotify function. Please try it by yourself first, and ask a question when you have code not working as intended.
Related
I'm trying to use named pipe in Linux, using mkfifo.
Below are codes for sender / reader of named pipe :
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#define MSG_SIZE 100
//SENDER
int main() {
char msg[MSG_SIZE] = "FIFO is for first-in, first-out.";
int fd;
int cnt;
if(fd = open("./hello", O_RDWR) < 0) { //fifo acts as a typical file
perror("Failed to make fifo : ");
printf("Error : %d\n", errno);
exit(1);
}
int len = strlen(msg) + 1;
while(1) {
if(write(fd, msg, len) == -1) {
perror("Failed to write into fifo : ");
exit(1);
}
else{
printf("Succeed to write msg!");
break;
}
}
sleep(1);
return 0;
}
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#define MSG_SIZE 100
//READER
int main() {
char msg_rc[MSG_SIZE];
int fd;
if(access("./hello",F_OK) == 0) {
fd = open("./hello", O_RDWR);
if(fd < 0) {
perror("Failed to open fifo : ");
exit(1);
}
}
else {
if(fd = mkfifo("./hello", O_RDWR) < 0) {
perror("Failed to open fifo : ");
exit(1);
}
}
printf("Waiting for fifo update");
while(1) {
if(read(fd, msg_rc, MSG_SIZE) == -1) {
perror("Failed to read from fifo : ");
exit(1);
}
printf("Read msg : %s\n", msg_rc);
break;
}
return 0;
}
When I used these codes,
% ./fifoSender
FIFO is for first-in, first-out.Succeed to write msg!%
% ./fifoReader
fifoSender shows msg, and fifoReader shows no msg, which are not desirable behavior.
Any advice will be helpful. Thanks in advance
The main problem is these two lines
if(fd = open("./hello", O_RDWR) < 0) {
if(fd = mkfifo("./hello", O_RDWR) < 0) {
They don't do what you think they do.
Firstly, O_RDWR is a wrong argument for mkfifo. It expects Unix file permissions mask, such as S_IRWU"S_IRWG|S_IRWO, or just 0666.
Secondly and most importantly, since the priority of = is lower than that of <, they are parsed like this:
if(fd = (open("./hello", O_RDWR) < 0)) {
Now, when open("./hello", O_RDWR) is less than 0 (which is highly likely, given that it is either never created or created with an incorrect mode), fd becomes 1, which is the standard output file descriptor.
The correct incantation is
if((fd = open("./hello", O_RDWR)) < 0) { // note () around assignment
if((fd = mkfifo("./hello", 0666)) < 0) {
There are several more subtle shortcomings.
Here is the working code,
// sender
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#define PIPE_NAME "./hello"
int main(int argc, char *argv[])
{
const char *message = "FIFO is for first-in, first-out\n";
// Open the pipe for writing
int pipe_fd = open(PIPE_NAME, O_WRONLY);
if (pipe_fd < 0) {
perror("open");
exit(1);
}
// Write data to the pipe
while(1){
int n = write(pipe_fd, message, strlen(message));
if (n < 0) {
perror("write");
exit(1);
}
sleep(1);
}
// Close the pipe
close(pipe_fd);
return 0;
}
//receiver
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#define PIPE_NAME "./hello"
int main(int argc, char *argv[])
{
// Create the named pipe
mknod(PIPE_NAME, S_IFIFO | 0666, 0);
// Open the pipe for reading
int pipe_fd = open(PIPE_NAME, O_RDONLY);
if (pipe_fd < 0) {
perror("open");
exit(1);
}
// Read data from the pipe
char buffer[1024];
while(1){
int n = read(pipe_fd, buffer, sizeof(buffer));
if (n < 0) {
perror("read");
exit(1);
}
// Print the data that was read
printf("Received: %.*s\n", n, buffer);
}
// Close the pipe
close(pipe_fd);
return 0;
}
I wonder if I should open a file using mode O_RDONLY, O_WRONLY or O_RDWR before calling ioctl.
There are reading and writing ioctls. For example _IOC_DIR(VIDIOC_QUERYCAP) will return _IOC_READ.
So I thought when I am calling this ioctl I have to open the file O_RDONLY.
But surprisingly I can open the file with mode=0 and it's still working:
#include <stdio.h> // for printf()
#include <linux/videodev2.h> // for struct v4l2_capability
#include <fcntl.h> // for open()
#include <sys/ioctl.h> // for ioctl()
#include <unistd.h> // for close()
int main(int argc, char** argv)
{
int fd = open("/dev/video0", 0); // mode set to 0
if (fd == -1)
{
printf("open failed\n");
return 1;
}
struct v4l2_capability cap;
if (ioctl(fd, VIDIOC_QUERYCAP, &cap) == -1)
{
printf("ioctl failed\n");
close(fd);
return 1;
}
printf("%s\n", cap.card);
return 0;
}
So my question is how to set the mode properly. Maybe someone got a link to an official documentation.
On the Introduction to Operative Systems course we're asked to build a client-server model using FIFOs. As client we send a string to the server, the server gets this string and if a file with this name exists it sends back the first line on this file. If the file doesn't exist or it does exist but it happens to be empty, it sends an empty string back.
The problem is that in only works once, i send file1 for example, the server sends back the first line and when I send again file1 or another file's name in the same "session" the printf("First line of the file %s: \n%s\n", name, recived); doesn't happen, it enters into if (read(fifo_serv_client, recived, sizeof(recived)) == -1) { printf("An error occurred.\n"); }.
Any idea on why is this? I tried doing it two times with the same file, so it exists 100% but I still get the same result.
Thank you so much!
Here is the code for the client:
#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#define BUFSIZE 512
int main()
{
int fifo_client_serv;
char *fifo1 = "fifo_client_serv";
int fifo_serv_client;
char *fifo2 = "fifo_serv_client";
char name[BUFSIZE];
while(1) {
printf("Write the file's name: ");
scanf("%s", name);
/* write str to the FIFO */
fifo_client_serv = open(fifo1, O_WRONLY);
fifo_serv_client = open(fifo2, O_RDONLY);
write(fifo_client_serv, name, sizeof(name));
char recived[BUFSIZE];
if (read(fifo_serv_client, recived, sizeof(recived)) == -1) {
printf("An error occurred.\n");
} else {
printf("First line of the file %s: \n%s\n", name, recived);
close(fifo_client_serv);
close(fifo_serv_client);
}
}
return 0;
}
And here's the code for the server:
#include <fcntl.h>
#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#define BUFSIZE 512
int main()
{
int fifo_client_serv;
char *fifo1 = "fifo_client_serv";
int fifo_serv_client;
char *fifo2 = "fifo_serv_client";
char buf[BUFSIZE];
char line[BUFSIZE];
FILE *file;
/* create the FIFO (named pipe) */
mkfifo(fifo1, 0777);
mkfifo(fifo2, 0777);
printf("Server runnning...\n");
while (1)
{
fifo_client_serv = open(fifo1, O_RDONLY);
fifo_serv_client = open(fifo2, O_WRONLY);
read(fifo_client_serv, buf, BUFSIZE);
if((file = fopen(buf, "r")) == NULL) {
write(fifo_serv_client, "", BUFSIZE);
} else {
fgets(line, BUFSIZE, file);
write(fifo_serv_client, line, BUFSIZE);
}
/* clear buffer and line */
memset(buf, 0, sizeof(buf));
memset(line, 0, sizeof(buf));
close(fifo_client_serv);
close(fifo_serv_client);
unlink(fifo1);
unlink(fifo2);
}
return 0;
}
UPDATE I found why this happens, if I create the fifo inside the while it works fine! I just put mkfifo(fifo1, 0777); mkfifo(fifo2, 0777); first in the while. My question is, is it really necessary to create the FIFO each time I send a text from the client? Can't I just create once the FIFO, do the communication from it and close once I finish?
Can you try to close the file after read/writes in the server file ?
if((file = fopen(buf, "r")) == NULL) {
write(fifo_serv_client, "", BUFSIZE);
fclose(file);
} else {
fgets(line, BUFSIZE, file);
write(fifo_serv_client, line, BUFSIZE);
fclose(file);
}
By the way. Its worth to do error checking on your writes.
I am trying to read from a UART device in a Linux environment using a C program, but I experience different results respect to communicating to the UART using screen.
The C code I use to test the UART communication is the following:
#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>
#include <unistd.h>
#include <string.h>
#include <strings.h>
#include <getopt.h>
#include <stdbool.h>
#include <sys/stat.h>
#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/file.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/select.h>
#include <signal.h>
#include <ctype.h>
bool loop;
void sigHandler(int32_t sig)
{
if(sig == SIGINT)
{
printf("Catched SIGINT");
loop = false;
}
}
int main(int argc, char *argv[])
{
char *devname = argv[1];
int fd = -1;
int nread = -1;
int nwrite = -1;
int ret;
struct termios t_new = {0};
struct termios t_old = {0};
signal(SIGINT, sigHandler);
fd = open(devname, O_RDWR | O_NOCTTY |O_NONBLOCK);
if(fd > 0)
{
printf("TTY open ! Configuring TTY");
}
else
{
fd = -1;
return 1;
}
ret = tcgetattr(fd, &t_old);
if(ret < 0)
{
perror("tcgetattr ");
close(fd);
fd = -1;
return 1;
}
t_new = t_old;
t_new.c_cflag = (B9600 | CS8 | CREAD );
t_new.c_oflag = 0;
t_new.c_iflag = 0;
t_new.c_lflag = 0;
ret = tcsetattr(fd, TCSANOW, &t_new);
loop = true;
while(loop)
{
char s[] = "at+gmi=?\r\n";
nwrite = write(fd, s, strlen(s));
if(nwrite == strlen(s))
{
fd_set rfd;
struct timeval tm = {.tv_sec = 0, .tv_usec = 500000};
FD_ZERO(&rfd);
FD_SET(fd, &rfd);
char buffer[64] = {0};
if(select(fd + 1, &rfd, NULL, NULL, &tm) > 0)
nread = read(fd, buffer, sizeof(buffer));
if(nread > 0)
printf("Reply is: %s\n", buffer);
}
usleep(500000);
}
}
But when I read the reply, it always includes the string I have sent.
I don't experience this problem using screen.
What is the best way to read from an UART in C using Linux ?
Could the multiplexed way (using select) causing the problems ?
EDIT
For completeness, the output is:
Reply is: at+gmi=?
OK
Also, sometimes I don't read anything.
But when I read the reply, it always includes the string I have sent.
Since your termios configuration obliterated the local echo attributes and you're sending an AT modem command, you should try sending an ATE0 command to disable echoing by the modem.
I don't experience this problem using screen.
This observation confirms that the connected modem has its echoing enabled.
The AT command is echoed (by the modem) as you type, but you don't object to this received data in this situation (because you want to see what you type).
If the modem did not have echoing enabled, then you would be complaining that what you type in screen was not visible.
IOW echo is desired when using a terminal emulator program (such as screen), but echoing needs to be disabled when sending data by a program.
What is the best way to read from an UART in C using Linux ?
(Technically you are not reading from a "UART", but rather from a serial terminal that fully buffers all input and output.)
Code that conforms to POSIX standard as described in Setting Terminal Modes Properly
and Serial Programming Guide for POSIX Operating Systems would be far better that what you have now.
I'm surprised that it works at all (e.g. CREAD is not enabled).
Could the multiplexed way (using select) causing the problems ?
Not the echo "problem".
Your program does not do anything that requires using select() and nonblocking mode.
Also, sometimes I don't read anything.
When you write code that is not POSIX compliant, you should not expect reliable program behavior.
I try to use setsockopt with the flag IPT_SO_SET_REPLACE but i keep getting the wired error from errno Protocol not available this is my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sched.h>
#include <linux/sched.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <netinet/in.h>
#include <net/if.h>
#include <linux/netfilter_ipv4/ip_tables.h>
#include <fcntl.h>
int main(void) {
int sock;
int ret;
void *data;
size_t size;
struct ipt_replace *repl;
sock = socket(PF_INET, SOCK_RAW, IPPROTO_RAW);
if (sock == -1) {
perror("socket");
return -1;
}
size = sizeof(struct ipt_replace);
data = malloc(size); Protocol not available
if (data == NULL) {
perror("malloc");
return -1;
}
memset(data, 0, size);
repl = (struct ipt_replace *) data;
repl->num_counters = 0x1;
repl->size = 0xffffffff;
repl->valid_hooks = 0x1;
repl->num_entries = 0x1;
ret = setsockopt(sock, SOL_IP, IPT_SO_SET_REPLACE, (void *) data, size);
printf("\ndone %d\n", ret);
perror("error: ");
return 0;
}
this is the output :
sock:3
data:
size:92
done -1
error: : Protocol not available
Looking briefly at the kernel code, this would seem to indicate that the IP tables module isn't available (i.e. the kernel wasn't built with it configured, or it can't be found or loaded).
It appears to me that for a socket of the kind you created, the code flow is:
enter raw_setsockopt: level != SOL_RAW so...
call ip_setsockopt: level == SOL_IP but option isn't any of the IP_xxx options so...
call nf_setsockopt: Search loaded netfilter modules for one that has registered IPT_SO_SET_REPLACE.
I think the last must have failed, so you get ENOPROTOOPT back (== Protocol not available)