4.10.1.1 Smart IO For AVR Devices

When using MPLAB XC8 C Compiler, multiple IO library variants, representing increasingly complex subsets of IO functionality, are available and are linked into your program based on the -msmart-io option and how you use the smart IO functions in your project's source code.

When the smart IO feature is disabled (-msmart-io=0), a full implementation of the IO functions will be linked into your program. All features of the IO library functions will be available, and these may consume a significant amount of the available program and data memory on the target device.

When the smart IO feature is enabled (-msmart-io=1 or -msmart-io), the compiler will link in the least complex variant of the IO library that implements all of the IO functionality required by the program, based on the conversion specifications detected in the program's IO function format strings. This can substantially reduce the memory requirements of your program, especially if you can eliminate in your program the use of floating-point features in calls to smart IO functions. This is the default setting.

The compiler analyzes the usage of each IO function independently, so while the code for a particular program might require that the printf function be full featured, only a basic implementation of the snprintf function might be required, for example.

If the format string in a call to an IO function is not a string literal, the compiler will not be able to detect the exact usage of the IO function, and a full-featured variant of the IO library will be linked into the program image, even with smart IO enabled. In this instance, the -msmart-io-format="fmt" option can be used to specify those conversion specifications that have been used in non-literal format strings, allowing a more optimal library to be linked. You must ensure that your program only uses the indicated conversion specifications; otherwise, IO functions may not work as expected.

vscanf("%d:%li", va_list1);
vprintf("%-s%d", va_list2);
vprintf(fmt1, va_list3); // ambiguous usage
vscanf(fmt2, va_list4);  // ambiguous usage

When processing the last two calls, the compiler cannot deduce any usage information from either of the format strings. If it is known that the format strings pointed to by fmt1 and fmt2 collectively use only the %d, %i and %s conversion specifiers, the -msmart-io-format=fmt="%d%i%s" option should be issued.

These options should be used consistently across all program modules to ensure an optimal selection of the library routines included in the program image.