I've got an application where I will be using a standalone C programming to read a CAN bus port with a socket. The user interface on this is Qt/QML code. I would like to use a non-blocking approach to call the bin program and either return nothing or return a string of the CAN packet.
The application will be low speed (just monitoring key presses, etc) so speed is not an issue. The current approach involves writing data from the socket program to a file, then having ANOTHER C program take the file and echo the string back to QML. UGH! Seems very messy. A simple Go/NoGo call would be easier. Here's the code I've got so far.
Thanks for any comments.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <net/if.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <linux/can.h>
#include <linux/can/raw.h>
// Returns 0 if no errors, > 0 if errors found
int main(void) {
struct ifreq ifr;
struct can_frame frame;
struct sockaddr_can addr;
int s; // CAN socket descriptor
int nbytes; // Number of bytes read from CAN socket
char run_daemon = 0; // Set to 1 to run as a daemon process
char show_errors = 0; // Set to 1 to print errors
char *ifname = "can0"; // Define the CAN driver for use
if (run_daemon) // Skip the daemon call if not enabled
daemon(1,1);
if ((s = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0) {
if (show_errors)
perror("Error while opening RAW socket");
return 1;
}
strcpy (ifr.ifr_name, ifname);
ioctl(s, SIOCGIFINDEX, &ifr);
addr.can_family = AF_CAN;
addr.can_ifindex = ifr.ifr_ifindex;
if (bind(s, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
if (show_errors)
perror("Error in socket bind");
return 2;
}
// Loop here for daemon process
while (1) {
// Read CAN frame data
nbytes = read(s, &frame, sizeof(struct can_frame));
// If data is ready, process it
if (nbytes > 0) {
// Print all relevent frame data to QML
printf("%d ",frame.can_id);
printf("%d ",frame.can_dlc);
if(frame.can_dlc>0) printf("%d ",frame.data[0]);
if(frame.can_dlc>1) printf("%d ",frame.data[1]);
if(frame.can_dlc>2) printf("%d ",frame.data[2]);
if(frame.can_dlc>3) printf("%d ",frame.data[3]);
if(frame.can_dlc>4) printf("%d ",frame.data[4]);
if(frame.can_dlc>5) printf("%d ",frame.data[5]);
if(frame.can_dlc>6) printf("%d ",frame.data[6]);
if(frame.can_dlc>7) printf("%d ",frame.data[7]);
printf("\n");
}
if (!run_daemon) { // Exit if daemon is not running
close(s); // Close the CAN socket
return 0;
}
}
return 0; // Should never get here !!!
}
Related
In the below code scanf() is working for getting the name from the user but fgets() is not working pls someone help me to understand why it's not working
#include <stdio.h>
#include <stdlib.h>
typedef struct university{
int roll_no;
char name[16];
}uni;
int main()
{
uni *ptr[5],soome;char i,j=0;
for(i=0;i<5;i++)
{
ptr[i]=(uni*)calloc(1,20);
if(ptr[i]==NULL)
{
printf("memory allocation failure");
}
printf("enter the roll no and name \n");
printf("ur going to enter at the address%u \n",ptr[i]);
scanf("%d",&ptr[i]->roll_no);
//scanf("%s",&ptr[i]->name);
fgets(&ptr[i]->name,16,stdin);
}
while(*(ptr+j))
{
printf("%d %s\n",ptr[j]->roll_no,ptr[j]->name);
j++;
}
return 0;
}
First of all, fgets(char *s, int n, FILE *stream) takes three argument: a pointer s to the beginning of a character array, a count n, and an input stream.
In the original application you used the address operator & to get the pointer not to the first element of the name[16] array, but to something else (to use the address operator, you should have referenced the first char in the array: name[0]).
You use a lot of magic numbers in your application (e.g. 20 as the size of the uni struct). In my sample I'm using sizeof as much as possible.
Given that you use calloc, I've used the fact that the first parameter is the number of elements of size equal to the second parameter to preallocate all the five uni struct at once.
Final result is:
#include <stdio.h>
#include <stdlib.h>
#define NUM_ITEMS (5)
#define NAME_LENGTH (16)
typedef struct university{
int roll_no;
char name[NAME_LENGTH];
} uni;
int main()
{
uni *ptr;
int i;
ptr = (uni*)calloc(NUM_ITEMS, sizeof(uni));
if(NULL == ptr) {
printf("memory allocation failure");
return -1;
}
for(i=0; i<NUM_ITEMS; i++) {
printf("enter the roll no and name \n");
printf("You're going to enter at the address: 0x%X \n",(unsigned int)&ptr[i]);
scanf("%d",&ptr[i].roll_no);
fgets(ptr[i].name, NAME_LENGTH, stdin);
}
for(i=0; i<NUM_ITEMS; i++) {
printf("%d - %s",ptr[i].roll_no,ptr[i].name);
}
free(ptr);
return 0;
}
Note: I've added a call to free(ptr); to free the memory allocated by calloc at the end of the application and a different return code if it's not possible to allocate the memory.
I need to create three child processes, each of which reads a string from the command line arguments and writes the string to a single pipe. The parent would then read the strings from the pipe and display all three of them on the screen. I tried doing it for two processes to test and it is printing one of the strings twice as opposed to both of them.
#include <stdio.h>
#include <unistd.h>
int main (int argc, char *argv[]) {
char *character1 = argv[1];
char *character2 = argv[2];
char inbuf[100]; //creating an array with a max size of 100
int p[2]; // Pipe descriptor array
pid_t pid1; // defining pid1 of type pid_t
pid_t pid2; // defining pid2 of type pid_t
if (pipe(p) == -1) {
fprintf(stderr, "Pipe Failed"); // pipe fail
}
pid1 = fork(); // fork
if (pid1 < 0) {
fprintf(stderr, "Fork Failed"); // fork fail
}
else if (pid1 == 0){ // if child process 1
close(p[0]); // close the read end
write(p[1], character1, sizeof(&inbuf[0])); // write character 1 to the pipe
}
else { // if parent, create a second child process, child process 2
pid2 = fork();
if (pid2 < 0) {
fprintf(stderr, "Fork Failed"); // fork fail
}
if (pid2 = 0) { // if child process 2
close(p[0]); // close the read end
write(p[1], character2, sizeof(&inbuf[0])); // write character 2 to the pipe
}
else { // if parent process
close(p[1]); // close the write end
read(p[0], inbuf, sizeof(&inbuf[0])); // Read the pipe that both children write to
printf("%s\n", inbuf); // print
read(p[0], inbuf, sizeof(&inbuf[0])); // Read the pipe that both children write to
printf("%s\n", inbuf); // print
}
}
}
Your code doesn't keep looping until there's no more data to read. It does a single read. It also doesn't check the value returned by read(), but it should.
I've abstracted the fork() and write() (and error check) code into a function. This seems to work:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
static void child(int fd, const char *string)
{
pid_t pid = fork();
int len = strlen(string);
if (pid < 0)
{
fprintf(stderr, "%.5d: failed to fork (%d: %s)\n",
(int)getpid(), errno, strerror(errno));
exit(1);
}
else if (pid > 0)
return;
else if (write(fd, string, len) != len)
{
fprintf(stderr, "%.5d: failed to write on pipe %d (%d: %s)\n",
(int)getpid(), fd, errno, strerror(errno));
exit(1);
}
else
exit(0);
}
int main (int argc, char *argv[])
{
char inbuf[100]; //creating an array with a max size of 100
int p[2]; // Pipe descriptor array
if (argc != 4)
{
fprintf(stderr, "Usage: %s str1 str2 str3\n", argv[0]);
return 1;
}
if (pipe(p) == -1)
{
fprintf(stderr, "Pipe Failed"); // pipe fail
return 1;
}
for (int i = 0; i < 3; i++)
child(p[1], argv[i+1]);
int nbytes;
close(p[1]); // close the write end
while ((nbytes = read(p[0], inbuf, sizeof(inbuf))) > 0)
printf("%.*s\n", nbytes, inbuf); // print
return 0;
}
I ran the command multiple times, each time using the command line:
./p3 'message 1' 'the second message' 'a third message for the third process'
On one run, the output was:
the second messagemessage 1
a third message for the third process
On another, I got:
the second messagemessage 1a third message for the third process
And on another, I got:
message 1
the second messagea third message for the third process
(This is on a MacBook Pro with Intel Core i7, running Mac OS X 10.8.3, and using GCC 4.7.1.)
I have a client server program where client writes a command on PIPE for server. While reading the command from Server it reads only first char of command and throws error. Can anyone help me with this?
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <string.h>
#include <sys/wait.h>
#include <mqueue.h>
#include <sys/stat.h>
#include "Functions.h"
#define MSGBUFFER_SIZE 50000
pid_t serverPid;
pid_t clientPid;
typedef struct msgbuf {
int messageLength;
int messageType;
char messageText[MSGBUFFER_SIZE];
} Message_buf;
int writePIPE(int fd, Message_buf *inputMessage){
printf("\n In write pipe message length :%d",inputMessage->messageLength);
printf("\n In write pipe message Data :%s",inputMessage->messageText);
ssize_t n=write(fd,inputMessage,inputMessage->messageLength);
printf("\n Size :%d", n);
return n;
}
ssize_t readPIPE(int fd, Message_buf *outputMessage)
{
ssize_t len;
ssize_t n;
if((n=read(fd,outputMessage,sizeof(outputMessage)))==0)
{
printf("\n Error");
return 0;
}
if((len=outputMessage->messageLength)>0)
{
printf("\n Length ---->:%d",len);
if((n=read(fd,outputMessage->messageText,strlen(outputMessage->messageText)))!=len)
printf("\n ERRRRROR expected %d got %d",len,n);
}
//printf("\n In Read PIPE: %s",outputMessage->messageText);
return len;
}
void Server(int readfd,int writefd)
{
Message_buf server_MessageBuf;
ssize_t length;
if((length=readPIPE(readfd,&server_MessageBuf))==0)
{
printf("\n End of file while reading pathname");
}
//server_MessageBuf.messageText[length]='\0';
printf("\n LENGTH :%d",server_MessageBuf.messageLength);
printf("\n Printing in server: %s\n",server_MessageBuf.messageText);
}
void Client(int readfd,int writefd)
{
char inputFileName[MAX_SIZE];
char inputOperation[MAX_SIZE];
char *cCommandInput = NULL;
char *fileOperation = NULL;
char *operation = (char *) malloc(MAX_SIZE);
int commandValidateStatus = 0;
int commandInterpretationStatus=0;
Message_buf client_MessageBuf;
for(;;)
{
while(1)
{
cCommandInput = acceptInput();
fileOperation = (char *) malloc(sizeof(cCommandInput));
strcpy(fileOperation,cCommandInput);
/**Function call to determine operation read/delete/exit/invalid choice and filename*****/
commandInterpretationStatus = interpretCommand(cCommandInput,
inputOperation, inputFileName);
operation = inputOperation;
/**Function call to validate the input command******/
commandValidateStatus = validateCommand(
commandInterpretationStatus, inputOperation, inputFileName);
if(commandValidateStatus==-1)
{
printf("\n Invalid Operation");
}
/*Exit command entered***/
if (commandValidateStatus == 1)
{
/*Code to clear resources */
kill(serverPid,SIGKILL);
kill(clientPid,SIGKILL);
exit(0);
}
/***Read or Delete****/
if (commandValidateStatus == 2 || commandValidateStatus == 3)
{
printf("\n Read or Delete\n");
strcpy(client_MessageBuf.messageText,fileOperation);
client_MessageBuf.messageLength=strlen(fileOperation);
client_MessageBuf.messageType=1;
if((writePIPE(writefd,&client_MessageBuf))<0)
{
printf("\n Error writing on client side ");
}
//read(readfd,*client_MessageBuf,sizeof(client_MessageBuf));
//printf("\n Reding server responsed");
//printf("%s",client_MessageBuf.messageText);
}
}
}
}
int main()
{
int pipe1[2],pipe2[2];
pipe(pipe1);
pipe(pipe2);
pid_t pid;
pid=fork();
serverPid=pid;
if(pid==0)
{
/*Call Server*/
close(pipe1[1]);
close(pipe2[0]);
Server(pipe1[0], pipe2[1]);
}
else
{
close(pipe1[0]);
close(pipe2[1]);
Client(pipe2[0],pipe1[1]);
}
return 0;
}
It looks like the code writes and reads struct msgbuf incorrectly:
typedef struct msgbuf {
int messageLength;
int messageType;
char messageText[MSGBUFFER_SIZE];
} Message_buf;
// ...
strcpy(client_MessageBuf.messageText,fileOperation);
client_MessageBuf.messageLength=strlen(fileOperation);
client_MessageBuf.messageType=1;
if((writePIPE(writefd,&client_MessageBuf))<0)
// ....
int writePIPE(int fd, Message_buf *inputMessage){
printf("\n In write pipe message length :%d",inputMessage->messageLength);
printf("\n In write pipe message Data :%s",inputMessage->messageText);
ssize_t n=write(fd,inputMessage,inputMessage->messageLength);
printf("\n Size :%d", n);
return n;
}
The above pieces that write struct msgbuf only write the first messageLength bytes of the structure which doesn't include the length of messageLength and messageType members, i.e. it truncates the object.
When reading:
ssize_t readPIPE(int fd, Message_buf *outputMessage)
{
// ...
if((n=read(fd,outputMessage,sizeof(outputMessage)))==0)
It reads only sizeof(outputMessage) bytes, which is the size of the pointer, not the object. Even if you fix it by changing it to sizeof(*outputMessage) that won't do enough, since that would expect to read the complete object whereas the writing piece truncates the object.
A good start to fix it would be to split the message into two parts: header and payload. The header is a structure of a fixed size, e.g.:
typedef struct {
int messageType;
int payloadLength;
} MessageHeader;
The payload is a variable-length part following the header. This way it would first write the entire header object into the pipe followed by payloadLength bytes of payload. When reading it would first read again the entire header and then read exactly payloadLength bytes.
Also note, that read() and write() calls may read or write less then asked, so that case needs to be explicitly handled by keeping reading or writing until the exact number of bytes has been processed.
I have JBoss running on Sun Java 1.5.
From time to time, it unexpectedly shuts down (in orderly fashion).
I suspect some other process is sending it kill or CTRL+C signals.
Is there a way on Unix/Solaris to trace who sent the signal?
On Solaris, you can use a simple dtrace script to find who is killing your process (assuming its name is java, adjust it otherwise):
dtrace -qn '
proc:::signal-send
/ args[1]->pr_fname == "java" /
{
printf("Process %d (%s by UID %d) sending signal %d to java (pid=%d)\n",
pid,execname,uid,arg1,args[1]->pr_pid);
}'
You can use sigaction to determine the source of the signal. pid may be zero as the signal was send from the kernel or via some user interaction (pressing ctrl+c)
#include <signal.h>
#include <string.h>
#include <stdio.h>
static void signal_handler(int sig, siginfo_t *info, void *data) {
printf ("signal: [%d], pid: [%d], uid: [%d]\n", sig,
info->si_pid,
info->si_uid );
}
int main(int argc, char *argv[]) {
struct sigaction sa;
memset ( &sa, '\0', sizeof ( struct sigaction ) );
sa.sa_sigaction = &signal_handler;
sa.sa_flags |= SA_SIGINFO;
sigemptyset ( &sa.sa_mask );
sigaction(SIGINT, &sa, NULL);
sigaction(SIGTERM, &sa, NULL);
sigaction(SIGQUIT, &sa, NULL);
while ( 1 ) {
sleep (1);
}
return 0;
}
I've written two programs: the first, the "writer", creates a FIFO and writes data into it. The second one, the "reader" runs in background and looks for data in the FIFO. Once data is there, the reader reads it out.
If I start e.g. two writers and two readers, they all can write/read into/from the same FIFO. How can I restrict it for 3rd and 4th readers/writers to use the FIFO and allow only one writer and one reader to use the FIFO?
My code:
FIFO Writer:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#define BUFFERSIZE 50
#define CHMOD 0777
int main(int argc, char **argv)
{
char outbuf[BUFFERSIZE]; // outbuffer
int fifo, j, anzahl;
// fifo - pipe file deskriptor, j - counter, anzahl - Parameter.
if(argc!=2) // Check if parameter is ok
{
printf("Ungültiger Parameter! Bsp.: ./fifow 10\n");
return 1;
}
anzahl=atoi(argv[1]); // convert paramter to integer
mkfifo("namedpipe4", CHMOD); // make FIFO "namedpipe4"
fifo = open("namedpipe4",O_WRONLY); // open FIFO
//
for(j=0;j<anzahl;j++)
{
printf("Writer PID: %d writes record nr. %6d\n", getpid(), j+1);
sprintf(outbuf, "Writer PID: %d writes record nr. %6d\n", getpid(), j+1);
write(fifo, outbuf, BUFFERSIZE);
remove("namedpipe4"); // removing the fifo
sleep(1); // Wait 1 sec
}
close(fifo); //
exit(0);
}
FIFO Reader:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#define BUFFERSIZE 50
int main(void)
{
char inbuf[BUFFERSIZE]; // inbuffer
int fifo, var;
printf("\n Waiting for a Pipe....\n");
while((fifo = open("namedpipe4",O_RDONLY)) == -1) // while "there is no such pipe"
{
remove("namedpipe4");
sleep(1);
}
while((var = read(fifo, inbuf, BUFFERSIZE)) > 0) // while "i can read"
{
printf("Reader PID: %d reads record: %s\n", getpid(), inbuf);
sleep(1);
}
close(fifo); //
printf("\n EOF..\n");
exit(0);
}
Given the code you posted in a separate answer, here is a modified version that fixes the problems you were having. See the comments for details, but in a nutshell:
The writer checks the return value of mkfifo is checked to see if another writer already created the pipe.
The reader gets an exclusive advisory lock on the pipe (via flock) after opening it, to avoid the race condition where a second reader could have opened the pipe before the first reader deleted it.
Writer:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/stat.h> /* needed for mkfifo */
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#define BUFFERSIZE 50
#define CHMOD 0777
int
main (int argc, char **argv)
{
char outbuf[BUFFERSIZE];
int fifo, j, anzahl;
if (argc != 2)
{
printf("Ungültiger Parameter! Bsp.: ./fifow 10\n");
return 1;
}
anzahl=atoi(argv[1]);
/* mkfifo fails if the file already exists, which means there's a
* writer waiting for a reader. This assures that only one writer
* will write to the pipe, since it only opens the pipe if it was
* the one who created it.
*/
if (mkfifo("namedpipe4", CHMOD) == -1)
{
printf("namedpipe4 already exists\n");
return 1;
}
fifo = open("namedpipe4", O_WRONLY);
for (j = 0; j < anzahl; j++)
{
printf("Writer PID: %d writes record nr. %6d\n", getpid(), j + 1);
sprintf(outbuf, "Writer PID: %d writes record nr. %6d\n", getpid(), j + 1);
write(fifo, outbuf, BUFFERSIZE);
remove("namedpipe4");
sleep(1);
}
close(fifo);
exit(0);
}
Reader:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/file.h> /* for flock */
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#define BUFFERSIZE 50
int
main (int argc, char **argv)
{
char inbuf[BUFFERSIZE];
int fifo, var;
printf("\n Waiting for a Pipe....\n");
/* There are *two* ways the open can fail: the pipe doesn't exist
* yet, *or* it succeeded, but a different writer already opened
* it but didn't yet remove it.
*/
while (1)
{
while ((fifo = open("namedpipe4", O_RDONLY)) == -1)
{
/* Since you didn't specify O_CREAT in the call to open, there
* is no way that namedpipe4 would have been created by the
* reader. If there *is* now a namedpipe4, a remove here
* would delete the one the writer created!
*/
sleep(1);
}
/* Get an exclusive lock on the file, failing if we can't get
* it immediately. Only one reader will succeed.
*/
if (flock (fifo, LOCK_EX | LOCK_NB) == 0)
break;
/* We lost the race to another reader. Give up and wait for
* the next writer.
*/
close (fifo);
}
/* We are definitely the only reader.
*/
/* *Here* we delete the pipe, now that we've locked it and thus
* know that we "own" the pipe. If we delete before locking,
* there's a race where after we opened the pipe, a different
* reader also opened, deleted, and locked the file, and a new
* writer created a new pipe; in that case, we'd be deleting the
* wrong pipe.
*/
remove("namedpipe4");
while ((var = read(fifo, inbuf, BUFFERSIZE)) > 0)
{
printf("Reader PID: %d reads record: %s\n", getpid(), inbuf);
/* No need to sleep; we'll consume input as it becomes
* available.
*/
}
close(fifo);
printf("\n EOF..\n");
exit(0);
}
Create the FIFO using pipe(2), and only give the file descriptors for each end of the FIFO to the appropriate process when they get forked from the parent process. (Alternatively, have the reader call pipe(2) and fork the writer, or vice versa.) Since the FIFO never lives on the filesystem, it's impossible for any other process to access it.
If you must use a named FIFO, delete the FIFO after the reader and writer have opened it. The underlying FIFO will still exist as long as the reader and writer have it open, but no new processes will be able to open it. However, there will be a race condition where a second reader or writer could open the FIFO before you've deleted it.