Valgrind complains reading from a file

Question

When reading in a file, I usually check if read returns a negative iostat to see if I reached the end of the file.

If an intel compiler is used (2022 or 2023 versions of ifort and ifx), Valgrind complains that Conditional jump or move depends on uninitialised value(s). A minimal example would be:

program test_reading
    implicit none
    integer :: err_flag
    character(len = 5) :: input
    do
        read (*, *, iostat = err_flag) input
        if (err_flag < 0) exit ! EOF
    end do
end program

For any arbitrary input (here I tested it with the letter 'e') I get the following warning from Valgrind:

$ valgrind ./a.out
==630192== Memcheck, a memory error detector
==630192== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==630192== Using Valgrind-3.19.0 and LibVEX; rerun with -h for copyright info
==630192== Command: ./a.out
==630192==
e
==630192== Conditional jump or move depends on uninitialised value(s)
==630192==    at 0x41C8FD: for__characterize_LUB_buffer (in /path/to/a.out)
==630192==    by 0x41CB70: for__get_s (in /path/to/a.out)
==630192==    by 0x409495: for_read_seq_lis (in /path/to/a.out)
==630192==    by 0x40422D: MAIN__ (test2.f90:6)
==630192==    by 0x40419C: main (in /path/to/a.out)
==630192==

Why is that and what is the recommended way to read in a file until its end without any warnings from Valgrind?

I don't get this warning when I use gfortran. The optimization level does not have an influence.

Answer 1

This is due to Intel's implementation, and there is nothing you can do about it. I remember similar Valgrind warnings resulting from write (or print) statements with ifort.

Consider using a Valgrind suppression file that ignores for__characterize_LUB_buffer. See, for example, this answer.