Communication with Arduino using Linux - c++

Question

This is the first time I'm communicating with Arduino using my computer. I use Ubuntu 14.04. This is the C program for writing to the file. The Arduino shows up ttyACM0.
While compiling using gcc the compiler shows an error saying:
Segmentation fault(core dumped)
How do I rectify this error.
#include<unistd.h>
#include<stdio.h>
int main() {
char data[] = {'f','b','r'}; //Random data we want to send
FILE *file;
file = fopen("/dev/ttyACM0","w"); //Opening device file
int i = 0;
for(i = 0 ; i < 3 ; i++) {
fprintf(file,"%c",data[i]); //Writing to the file
fprintf(file,"%c",','); //To separate digits
sleep(1);
}
fclose(file);
}
Pardon my ignorance. I tried researching on it. Couldn't make it work. Thanks in advance for your help.

Answer 1

You're getting a NULL return from the fopen() that NULL is being passed to fprintf() which is expecting a valid FILE* and messing up causing the SEGV.
If you use fopen you should check what it returns so you can give the user a something more useful than "segmentation fault".
The probable cause of the fopen() failure is you don't have permission to play with the serial port.
Normally you need the group dialout to be able to access the serial port.
As root do:
usermod -a -G dialoutyourusername
Then log out and back in so you get the new group.
Consider using minicom or microcom (on any of the several other serial terminal programs) to access the serial port instead of writing your own.
I also suggest you have the Arduino send a hello message when it boots up so you can be sure you have the right baud rate etc...

Answer 2

You did not put any success check on the return value of fopen("/dev/ttyACM0","w");. In case fopen() fails, using file further is undefined behavior, causing segmentation fault. Do something like
file = fopen("/dev/ttyACM0","w"); //Opening device file
if (file)
{
//do something with file
}
else
return 0;
Also, add a return 0 before ending main().

Answer 3

// the following code:
// compiles cleanly
// performs appropriate error checking
// has proper return statement
#include <unistd.h> // sleep()
#include <stdio.h> // fopen(), fclose(), fprintf(), perror()
#include <stdlib.h> // exit() and EXIT_FAILURE
int main()
{
char data[] = {'f','b','r'}; //Random data we want to send
FILE *file;
if( NULL == (file = fopen("/dev/ttyACM0","w") ) ) //Opening device file
{ // then fopen failed
perror("fopen failed for ttyACM0" );
exit( EXIT_FAILURE );
}
// implied else, fopen successful
int i = 0;
for(i = 0 ; i < 3 ; i++)
{
if( 0 >= fprintf(file,"%c",data[i]) ) //Writing to the file
{ // fprintf failed
perror("fprintf data failed" );
exit( EXIT_FAILURE );
}
// implied else, fprintf successful for data
if( 0 >= fprintf(file,"%c",',') ) //To separate digits
{ // then, fprintf failed
perror( "fprintf for comma failed");
exit( EXIT_FAILURE );
}
// implied else, fprintf successful for comma
sleep(1);
} // end for
fclose(file);
return(0);
} // end function: main

Answer 4

On failure fopen returns NULL, so you are potentially dereferencing a NULL pointer, the correct way of doing that, is checking the result of fopen. I would however suggest low level IO for this kind of thing something like
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
int main()
{
char data[] = {'f','b','r'}; //Random data we want to send
int fd;
int i;
fd = open("/dev/ttyACM0", O_WRONLY); //Opening device file
if (fd == -1)
{
perror("cannot open /dev/ttyACM0");
return -1;
}
for(i = 0 ; i < 3 ; i++)
{
write(fd, &(data[i]), 1);
write(fd, ",", 1);
sleep(1);
}
close(fd);
return 0;
}
on error open returns a special value -1 so you should abort writing to it.
I'm pretty sure in your case there will be a permission denied error, since normally the /dev/tty* belong to group dialout and they have group write permission by default, but since probably your user doesn't belong to that group you don't have write access to /dev/ttyACM0.