minimal-printf: fix display decimals between -1 to 0 incorrect issue.

[Linguanghsou]

Summary of changes

Fixes "minimal-printf doesn't display decimals between -1 to 0 correctly" listed in #13783

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[x] Tests / results supplied as part of this PR

double d = -0.00139;
printf("d = %f\r\n",d);

--->
d = -0.001391

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Reviewers

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[ciarmcom]

@Linguanghsou, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Can you add details to the commit message? How is this fixing the issue, I am interested in how the casting here (changing argument type and later casting it) works

[Linguanghsou]

Can you add details to the commit message? How is this fixing the issue, I am interested in how the casting here (changing argument type and later casting it) works

@ciarmcom

static void mbed_minimal_formatted_string_signed(char *buffer, size_t length, int *result, MBED_SIGNED_STORAGE value, FILE *stream)
{
    MBED_UNSIGNED_STORAGE new_value = 0;

    /* if value is negative print sign and treat as positive number */
    if (value < 0) {
        /* write sign */
        mbed_minimal_putchar(buffer, length, result, '-', stream);

        /* get absolute value using two's complement */
        new_value = ~((MBED_UNSIGNED_STORAGE) value) + 1;
    } else {
        new_value = value;
    }

In original mbed_minimal_formatted_string_signed() above, the type of agument value is a signed integer. Given a double value -0.00139 being printed, its integer part is 0 (-0 is equal to 0), so the condition "if (value < 0)" is false and it was handled as a positive decimal like 0.00139.

In modification listed below, the value is a double type. Given a double value -0.00139, the condition "if (value < (double)0)" is true and it is handled as a negative decimal as expected.

static void mbed_minimal_formatted_string_signed(char *buffer, size_t length, int *result, double value, FILE *stream)
{
    MBED_UNSIGNED_STORAGE new_value = 0;
    MBED_SIGNED_STORAGE integer = value;

    /* if value is negative print sign and treat as positive number */
    if (value < (double)0) {
        /* write sign */
        mbed_minimal_putchar(buffer, length, result, '-', stream);

        /* get absolute value using two's complement */
        new_value = ~((MBED_UNSIGNED_STORAGE) integer) + 1;
    } else {
        new_value = integer;
    }

    /* use unsigned long int function */
    mbed_minimal_formatted_string_unsigned(buffer, length, result, new_value, stream);
}

[0xc0170]

Thanks for the explanation, can you amend the commit message as requested?

Pull request has been modified.

[Linguanghsou]

Thanks for the explanation, can you amend the commit message as requested?

Done, please check the commit message.

[0xc0170]

@Linguanghsou I can see two commits and one merge commit, can you rebase to squash all three into one ?

[0xc0170]

There was a different suggestion to fix this: #13783 (comment) . isn't this better than what is proposed here?

[Linguanghsou]

@Linguanghsou I can see two commits and one merge commit, can you rebase to squash all three into one ?

@0xc0170 Please noted that I created #13865 to replace current PR to have only one commit message.
#13865