Unexplained "no such file or directory" in compiled code - g++

Question

The following copied and modified code reads the space bar to activate data capture from attached test equipment. Without the intervening code summarized below, it runs fine. I have avoided "using namespace std" by creating a kbrd namespace and using only the standard commands I need. The code compiles without difficulty, but when I run it with the necessary code to capture keyboard data and initialize the test gear, it runs to "Check point Alpha" and waits. When I hit space bar (or any other key!), it prints:
Check Point Beta
No such file or directory.
Very similar code using the left mouse button to collect data runs as expected. I am constructing a small computer, using a "paper" display for portable use outdoors, and would like to have code that does not need a mouse...
I have run out of things to try to fix this. Your help is requested and will be much appreciated.
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <linux/input.h>
#include <string.h>
#include <stdio.h>
#include <iostream>
#include <fstream>
#include <signal.h>
#include <float.h>
#include <sa_api.h>
#pragma comment(lib,"sa_api.lib")
#undef max
#undef min
namespace kbrd{
const char *dev = "/dev/input/event3";
struct input_event ev;
ssize_t n;
int fd;}
using std::cin;
using std::cout;
using std::__cxx11::string;
using std::endl;
using std::ofstream;
using std::ios;
int main(void)
{
[[Code which
Initiates equipment API, prompts screen & reads keyboard to
gather details to be stored with data from attached test
equipment...]]`
while (N_az <Num_Samples) {
kbrd::fd = open(kbrd::dev, O_RDONLY);
if (kbrd::fd == -1) {
fprintf(stderr, "Cannot open %s: %s.\n", kbrd::dev,
strerror(errno));
return EXIT_FAILURE;
}
//and then read keyboard events from the device:
cout << "Check Point Alpha" << endl;
while (1) {
kbrd::n = read(kbrd::fd, &kbrd::ev, sizeof kbrd::ev);
if (kbrd::n == (ssize_t)-1) {
if (errno == EINTR)
continue;
else
break;
} else
if (kbrd::n != sizeof kbrd::ev) {
errno = EIO;
break;
}
cout << "Check Point Beta" << endl;
if (kbrd::ev.type == EV_KEY && kbrd::ev.value == 0 /*&&
kbrd::ev.value <= 2 */&& kbrd::ev.code == 57){
cout << "Check Point Charlie" << endl;
continue;
[[ Code to read attached test gear & store data, one point at a
time...]]
N_az = N_az + 1;
}
else {
fflush(stdout);
fprintf(stderr, "%s.\n", strerror(errno));
return EXIT_FAILURE;
}
}
}
return EXIT_SUCCESS;
}

Answer 1

When I hit space bar (or any other key!), it prints:
Check Point Beta
No such file or directory.
because your code is
cout << "Check Point Beta" << endl;
if (kbrd::ev.type == EV_KEY && kbrd::ev.value == 0 /*&&
kbrd::ev.value <= 2 */&& kbrd::ev.code == 57){
cout << "Check Point Charlie" << endl;
...
}
else {
fflush(stdout);
fprintf(stderr, "%s.\n", strerror(errno));
return EXIT_FAILURE;
}
that means (kbrd::ev.type == EV_KEY && kbrd::ev.value == 0 /*&& kbrd::ev.value <= 2 */&& kbrd::ev.code == 57) is false, No such file or directory is coming from strerror so you have an error somewhere before and probably not in the read, reset errno to 0 before the read. Warning when there is no error errno is not reset to 0, it is only set in case of an error, so it is your responsibility to reset it to 0.
Just temporarily write the values of kbrd::ev.type / kbrd::ev.value / kbrd::ev.code before the test and run your program to know what are the values when you press the space bar allowing you to modify your code in consequence