4.6.1.22 Smart-io-format Option

The -msmart-io-format="fmt" option, where fmt is a string containing formatted, printf-style conversion specifications, notifies the compiler that the listed specifications are used by smart IO functions and that the IO library code linked in must be able to process them. See 5.11.2 Smart IO Routines for more information on this feature.

To reduce code size, the compiler customizes library code associated with the print and scan families of smart IO functions, based on the conversion specifications present in the format strings collated across all calls to these functions. This feature is fully automatic and cannot be disabled.

In some situations, the compiler is unable to determine usage information from the formatted IO function call. In such cases, the -msmart-io-format="fmt" option can be used to indicate the required conversion specifications for the linked IO functions, for example, -msmart-io-format=fmt="%d%i%s." Without this option, the compiler makes no assumptions about IO usage and ensures that fully functional IO functions are linked into the final program image.

For example, consider the following calls to smart IO functions.

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 the format strings, nor the arguments. In these instances, the -msmart-io-format option can be used and will potentially allow more optimal formatted IO functions to be generated, thus reducing the program's code size. For example, if 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.

The fmt string may contain any valid printf-style conversion specification, including flags and modifiers (for example "%-13.9ls"), and should reflect exactly those conversion specifications used by the functions whose usage is ambiguous. Failure to include a specification in the fmt argument where it has been used by the formatted IO functions might result in code failure.

If fmt is an empty string or contains no discernible conversion specifications, an error will be issued. Characters other than those in valid conversion specifications are permitted, but will be ignored.

This option may be used multiple times on the command line. The conversion specifications used with each option are accumulated.