in_voltageX_raw fails with Resource temporarily unavailable

After reboot, command line: cat /sys/bus/iio/devices/iio:device0/in_voltage0_raw works fine and returns correct 0-4095 ADC values. But as soon as I try to read those values from a compiled C program I get the “Resource temporarily unavailable” error response. After giving up on open() and fopen() C commands I’ve reverted to:
Enter code here...
{value= system(“cat /sys/bus/iio/devices/iio:device0/in_voltage0_raw”);}

And after that the manual read in linux command line, cat /sys/bus/iio/devices/iio:device0/in_voltage0_raw, fails the same way: “Resource temporarily unavailable”

my “uname -a” returns: Linux arm 4.14.108-ti-r104 #1 SMP PREEMPT Tue Apr 9 18:45:06 UTC 2019 armv7l armv7l armv7l GNU/Linux