mbed-os/platform/source/minimal-printf

Minimal printf and snprintf

Library supports both printf and snprintf in 1252 bytes of flash.

Prints directly to stdio/UART without using malloc. All flags and precision modifiers are ignored.

Supports:

• %d: signed integer [h, hh, (none), l, ll, z, j, t].
• %i: signed integer [h, hh, (none), l, ll, z, j, t].
• %u: unsigned integer [h, hh, (none), l, ll, z, j, t].
• %x: unsigned integer [h, hh, (none), l, ll, z, j, t], printed as hexadecimal number (e.g., ff).
• %X: unsigned integer [h, hh, (none), l, ll, z, j, t], printed as hexadecimal number (e.g., FF).
• %f: floating point (enabled by default).
• %F: floating point (enabled by default, treated as %f).
• %g: floating point (enabled by default, treated as %f).
• %G: floating point (enabled by default, treated as %f).
• %c: character.
• %s: string.
• %p: pointer (e.g. 0x00123456).

Unrecognized format specifiers are treated as ordinary characters.

Floating point limitations:

• All floating points are treated as %f.
• No support for inf, infinity or nan

Configuration

Minimal printf is configured by the following parameters defined in platform/mbed_lib.json:

{
    "name": "platform",
    "config": {
       "minimal-printf-console-output": {
            "help": "Console output when using minimal-printf profile. Options: UART, SWO",
            "value": "UART"
        },
        "minimal-printf-enable-64-bit": {
            "help": "Enable printing 64 bit integers when using minimal-printf profile",
            "value": true
        },
        "minimal-printf-enable-file-stream": {
            "help": "Enable printing to a FILE stream when using minimal-printf profile",
            "value": true
        },
        "minimal-printf-enable-floating-point": {
            "help": "Enable floating point printing when using minimal-printf profile",
            "value": true
        },
        "minimal-printf-set-floating-point-max-decimals": {
            "help": "Maximum number of decimals to be printed",
            "value": 6
        }
   }
}

By default, 64 bit integers, floating point and FILE stream printing are enabled.

If your target does not require some options then you can override the default configuration in your application mbed_app.json and achieve further memory optimisation (see next section for size comparison numbers).

In mbed_app.json:

    "target_overrides": {
        "*": {
            "platform.minimal-printf-enable-file-stream": false,
            "platform.minimal-printf-enable-floating-point": false,
            "platform.minimal-printf-set-floating-point-max-decimals": 6,
            "platform.minimal-printf-enable-64-bit": false
        }
    }

Usage

To replace the standard implementation of the printf functions with the ones in this library:

Compile with mbed-cli using the custom minimal-printf profile. For example, to compile in release mode:

$ mbed compile -t <toolchain> -m <target> --profile release --profile mbed-os/tools/profiles/extensions/minimal-printf.json

Size comparison

Blinky application

https://github.com/ARMmbed/mbed-os-example-blinky application compiled with the different toolchains.

Blinky application size on K64F/GCC_ARM

	File stream	Floating point	64 bit integers	Flash	RAM
mbed-printf				30,944	12,096
mbed-printf			X	31,084	12,096
mbed-printf		X	X	33,824	12,096
mbed-printf	X	X	X	34,304	12,096
std printf	X	X	X	54,158	12,112

Blinky application size on K64F/ARMC6

	File stream	Floating point	64 bit integers	Flash	RAM
mbed-printf				31,543	xxxxx
mbed-printf			X	31,691	xxxxx
mbed-printf		X	X	34,515	xxxxx
mbed-printf	X	X	X	34,647	xxxxx
std printf	X	X	X	37,458	xxxxx

Blinky application size on K64F/IAR

	File stream	Floating point	64 bit integers	Flash	RAM
mbed-printf				28,713	8,546
mbed-printf			X	28,853	8,546
mbed-printf		X	X	30,661	8,546
mbed-printf	X	X	X	32,047	8,594
std printf	X	X	X	35,055	8,462

Blinky bare metal application

https://github.com/ARMmbed/mbed-os-example-blinky-baremetal application compiled with the different toolchains.

Blinky application size on K64F/GCC_ARM

	File stream	Floating point	64 bit integers	Flash	RAM
mbed-printf				17,168	5,880
mbed-printf			X	17,308	5,880
mbed-printf		X	X	20,044	5,880
mbed-printf	X	X	X	22,560	5,880
std printf	X	X	X	35,244	5,864

Blinky application size on K64F/ARMC6

	File stream	Floating point	64 bit integers	Flash	RAM
mbed-printf				19,496	xxxxx
mbed-printf			X	19,644	xxxxx
mbed-printf		X	X	22,468	xxxxx
mbed-printf	X	X	X	23,060	xxxxx
std printf	X	X	X	25,393	xxxxx

Blinky application size on K64F/IAR

	File stream	Floating point	64 bit integers	Flash	RAM
mbed-printf				13,007	1,413
mbed-printf			X	13,147	1,413
mbed-printf		X	X	14,955	1,413
mbed-printf	X	X	X	21,757	1,929
std printf	X	X	X	17,991	1,281