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My program doesn't recognize a variable updated within an interrupt routine

When using the optimizer, in a loop like the following one:

uint8_t flag;
...
ISR(SOME_vect) {
  flag = 1;
}
...

    while (flag == 0) {
        ...
    }

the compiler will typically access flag only once, and optimize further accesses completely away, since its code path analysis shows that nothing inside the loop could change the value of flag anyway. To tell the compiler that this variable could be changed outside the scope of its code path analysis (e. g. from within an interrupt routine), the variable needs to be declared like:

volatile uint8_t flag;

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Optimization flags	Size of .text	Time for test #1	Time for test #2
-O3	6898	903 µs	19.7 ms
-O2	6666	972 µs	20.1 ms
-Os	6618	955 µs	20.1 ms
-Os -mcall-prologues	6474	972 µs	20.1 ms

Library call	Reentrant Issue	Workaround/Alternative
rand(), random()	Uses global variables to keep state information.	Use special reentrant versions: rand_r(), random_r().
strtod(), strtol(), strtoul()	Uses the global variable `errno` to return success/failure.	Ignore `errno`, or protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can tolerate it. Or use sccanf() or sccanf_P() if possible.
malloc(), realloc(), calloc(), free()	Uses the stack pointer and global variables to allocate and free memory.	Protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can tolerate it. If using an OS, use the OS provided memory allocator since the OS is likely modifying the stack pointer anyway.
fdevopen(), fclose()	Uses calloc() and free().	Protect calls with cli()/sei() or ATOMIC_BLOCK() if the application can tolerate it. Or use fdev_setup_stream() or FDEV_SETUP_STREAM(). Note: fclose() will only call free() if the stream has been opened with fdevopen().
eeprom_(), boot_()	Accesses I/O registers.	Protect calls with cli()/sei(), ATOMIC_BLOCK(), or use OS locking.
pgm_*_far()	Accesses I/O register RAMPZ.	Starting with GCC 4.3, RAMPZ is automatically saved for ISRs, so nothing further is needed if only using interrupts. Some OSes may automatically preserve RAMPZ during context switching. Check the OS documentation before assuming it does. Otherwise, protect calls with cli()/sei(), ATOMIC_BLOCK(), or use explicit OS locking.
printf(), printf_P(), vprintf(), vprintf_P(), puts(), puts_P()	Alters flags and character count in global FILE `stdout`.	Use only in one thread. Or if returned character count is unimportant, do not use the *_P versions. Note: Formatting to a string output, e.g. sprintf(), sprintf_P(), snprintf(), snprintf_P(), vsprintf(), vsprintf_P(), vsnprintf(), vsnprintf_P(), is thread safe. The formatted string could then be followed by an fwrite() which simply calls the lower layer to send the string.
fprintf(), fprintf_P(), vfprintf(), vfprintf_P(), fputs(), fputs_P()	Alters flags and character count in the FILE argument. Problems can occur if a global FILE is used from multiple threads.	Assign each thread its own FILE for output. Or if returned character count is unimportant, do not use the *_P versions.
assert()	Contains an embedded fprintf(). See above for fprintf().	See above for fprintf().
clearerr()	Alters flags in the FILE argument.	Assign each thread its own FILE for output.
getchar(), gets()	Alters flags, character count, and unget buffer in global FILE `stdin`.	Use only in one thread. ***
fgetc(), ungetc(), fgets(), scanf(), scanf_P(), fscanf(), fscanf_P(), vscanf(), vfscanf(), vfscanf_P(), fread()	Alters flags, character count, and unget buffer in the FILE argument.	Assign each thread its own FILE for input. *** Note: Scanning from a string, e.g. sscanf() and sscanf_P(), are thread safe.

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