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. There is no error handling if a writing error occurs.

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 (disabled by default).
• %F: floating point (disabled by default, treated as %f).
• %g: floating point (disabled by default, treated as %f).
• %G: floating point (disabled 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

Usage

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

Modify your application configuration file to override the parameter target.printf_lib with the value minimal-printf as shown below:

{
    "target_overrides": {
        "*": {
            "target.printf_lib": "minimal-printf"
        }
    }
}

Configuration

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

{
    "name": "platform",
    "config": {
        "minimal-printf-enable-64-bit": {
            "help": "Enable printing 64 bit integers when using minimal-printf profile",
            "value": true
        },
        "minimal-printf-enable-floating-point": {
            "help": "Enable floating point printing when using minimal-printf profile",
            "value": false
        },
        "minimal-printf-set-floating-point-max-decimals": {
            "help": "Maximum number of decimals to be printed",
            "value": 6
        }
   }
}

By default, 64 bit integers support is 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": {
        "*": {
            "target.printf_lib": "minimal-printf",
            "platform.minimal-printf-enable-floating-point": false,
            "platform.minimal-printf-set-floating-point-max-decimals": 6,
            "platform.minimal-printf-enable-64-bit": false
        }
    }

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

	Floating point	64 bit integers	Flash	RAM
mbed-printf			32,972	11,608
mbed-printf		X	33,116	11,608
mbed-printf	X	X	35,856	11,608
std printf	X	X	55,766	12,104

Blinky application size on K64F/ARMC6

	Floating point	64 bit integers	Flash	RAM
mbed-printf			33,585	xxxxx
mbed-printf		X	33,679	xxxxx
mbed-printf	X	X	36,525	xxxxx
std printf	X	X	39,128	xxxxx

Blinky application size on K64F/IAR

	Floating point	64 bit integers	Flash	RAM
mbed-printf			31,439	8,493
mbed-printf		X	31,579	8,493
mbed-printf	X	X	33,387	8,493
std printf	X	X	36,643	8,553

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

	Floating point	64 bit integers	Flash	RAM
mbed-printf			19,660	5,368
mbed-printf		X	19,804	5,368
mbed-printf	X	X	22,548	5,368
std printf	X	X	35,292	5,864

Blinky application size on K64F/ARMC6

	Floating point	64 bit integers	Flash	RAM
mbed-printf			18,764	xxxxx
mbed-printf		X	18,764	xxxxx
mbed-printf	X	X	18,764	xxxxx
std printf	X	X	25,403	xxxxx

Blinky application size on K64F/IAR

	Floating point	64 bit integers	Flash	RAM
mbed-printf			19,623	1,737
mbed-printf		X	19,763	1,737
mbed-printf	X	X	21,571	1,737
std printf	X	X	18,059	1,281