warnings in test programs

[carns]

With gcc 10.2, on Ubuntu, I get the following warnings in the test programs:

../tests/margo-test-init-ext.c: In function ‘readfile’:
../tests/margo-test-init-ext.c:26:5: warning: ignoring return value of ‘fread’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   26 |     fread(string, 1, fsize, f);
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~

../tests/unit-tests/helper-server.c: In function ‘HS_start’:
../tests/unit-tests/helper-server.c:37:13: warning: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   37 |             write(p[1], addr_size, sizeof(*addr_size));
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../tests/unit-tests/helper-server.c:38:13: warning: ignoring return value of ‘write’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   38 |             write(p[1], addr, *addr_size);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../tests/unit-tests/helper-server.c:53:13: warning: ignoring return value of ‘read’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   53 |             read(p[0], addr_size, sizeof(*addr_size));
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../tests/unit-tests/helper-server.c:54:13: warning: ignoring return value of ‘read’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   54 |             read(p[0], addr, *addr_size);
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

These are trivial, but part of why I opened an issue is that I'm curious about what CFLAGS make them appear. Counterintuitively, there are no warnings emitted on my laptop if I have -Wall set, but they do appear if my CFLAGS are empty at configure time.

[roblatham00]

it's in the warning string: -Wunused-result

[roblatham00]

fun discussion here https://stackoverflow.com/questions/7271939/warning-ignoring-return-value-of-scanf-declared-with-attribute-warn-unused-r -- it's an attribute on the stdio functions but one only sees the warning with -O2 optimizations. Perhaps it is only with that additional level of optimization that the compiler can track used/unused values?