"End of file" when reading from standard input with an online Fortran compiler

Question

I am running the following code via an online Fortran compiler .

PROGRAM ONE 

IMPLICIT NONE 

REAL:: v

READ (*,*) v


IF ( sqrt(v) > 1. ) THEN
WRITE(*,*) 'At first: sqrt(v) > 1.'

ELSE IF ( sqrt(v) < 1. ) THEN 

WRITE(*,*) 'At first: sqrt(v) < 1. '
ELSE
WRITE(*,*) 'At first: sqrt(v) == 1.'
END IF 

END PROGRAM ONE 

I get the following message.

$gfortran -std=gnu *.f95 -o main
$main
At line 7 of file main.f95 (unit = 5, file = 'stdin')
Fortran runtime error: End of file

Error termination. Backtrace:
#0  0x7fb0b576beda
#1  0x7fb0b576ca85
#2  0x7fb0b576d24d
#3  0x7fb0b58e3513
#4  0x7fb0b58dc459
#5  0x7fb0b58ddbb9
#6  0x400824
#7  0x4009bd
#8  0x7fb0b4c334d9
#9  0x400719
#10  0xffffffffffffffff

Answer 1

When you have a read statement like

READ (*,*) v

you may typically expect the program to pause, waiting for data from standard input . However, it's possible that a normal file has been redirected to standard input, in which case the program tries to read from that file. If that file is empty, then an end-of-file condition (see your runtime error message) will occur, rather than the program waiting.

In your case, with your online compiler, there is a tab next to the source code where you can enter the data which will be used as standard input. Unless you put the input here, you will see this failure.