minimal-printf doesn't display decimals between -1 to 0 correctly

[Linguanghsou]

Description of defect

The output of:

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

with minimal-printf is:
d = 0.001391
but it should be:
d = -0.001391

with setting "target.printf_lib": "std" the output is as expected.

Target(s) affected by this defect ?

NUCLEO_F413ZH

Toolchain(s) (name and version) displaying this defect ?

gcc-arm-none-eabi-9-2019-q4-major

What version of Mbed-os are you using (tag or sha) ?

mbed-os-6.3.0

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed-cli

How is this defect reproduced ?

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

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers.
Internal Jira reference: https://jira.arm.com/browse/IOTOSM-2528

[helpfulchicken]

This issue is also affecting me. I'm having to duplicate print calls for negative and positive cases to deal with this.

Target(s) affected by this defect ?
NRF52_DK

Toolchain(s) (name and version) displaying this defect ?
gcc-arm-none-eabi-9-2019-q4-major

What version of Mbed-os are you using (tag or sha) ?
mbed-os-6.4.0 (#ad40b1b2672d)

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)
mbed-cli 1.10.4

Additional details
I looked into the source code, and I found where this is happening.
In the function "mbed_minimal_formatted_string_double", the integer part of the floating point value is take using this line:

mbed-os/platform/source/minimal-printf/mbed_printf_implementation.c

Line 265 in cdc2d45

MBED_SIGNED_STORAGE integer = value;

This works fine as long as negative values are less than or equal to -1.0.
Otherwise, for values between 0.0 and -1.0, the integer component is "-0", which loses the sign when being saved in a signed integer format.
Again, for the sake of clarity, writing a floating point value to a signed integer results in loss of sign information if the value in question is between 0.0 and 1.0.

Suggested solution
After this line:

mbed-os/platform/source/minimal-printf/mbed_printf_implementation.c

Line 265 in cdc2d45

MBED_SIGNED_STORAGE integer = value;

Insert this code:

/* write sign if integer part is zero and value is negative */
if ((value < 0.0) && (integer == 0))
{
    /* write sign */
    mbed_minimal_putchar(buffer, length, result, '-', stream);
}

That will add the sign character in this scenario, without interfering with the existing method of printing.

[0xc0170]

Thanks @helpfulchicken . I commented in the #13784 (fix for this), if we should consider your fix.