MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

16-Bit C Compiler User’s Guide
Automatic variables are owned by the enclosing function and do not need the boot
attribute. They may be assigned initial values, as shown:
void __attribute__((boot)) chuck_cookies()
{
int hurl;
int them = 55;
char *where = "far";
splat(where);
/* ... */
}
Note that the initial value of where is based on a string literal which is allocated in the
PSV constant section .boot_const. The compiler will set PSVPAG to the correct
value upon entrance to the function. If necessary, the compiler will also restore PSV-
PAG after the call to splat().
const
Many functions do not examine any values except their arguments, and have no effects
except the return value. Such a function can be subject to common subexpression
elimination and loop optimization just as an arithmetic operator would be. These
functions should be declared with the attribute const. For example:
int square (int) __attribute__ ((const int));
says that the hypothetical function square is safe to call fewer times than the program
says.
Note that a function that has pointer arguments and examines the data pointed to must
not be declared const. Likewise, a function that calls a non-const function usually
must not be const. It does not make sense for a const function to have a void return
type.
deprecated
See Section 2.3.1 “Specifying Attributes of Variables” for information on the
deprecated attribute.
far
The far attribute tells the compiler that the function should not be called using a more
efficient form of the call instruction.
format (archetype, string-index, first-to-check)
The format attribute specifies that a function takes printf, scanf or strftime
style arguments which should be type-checked against a format string. For example,
consider the declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
This causes the compiler to check the arguments in calls to my_printf for
consistency with the printf style format string argument my_format.
The parameter archetype determines how the format string is interpreted, and should
be one of printf, scanf or strftime. The parameter string-index specifies
which argument is the format string argument (arguments are numbered from the left,
starting from 1), while first-to-check is the number of the first argument to check
against the format string. For functions where the arguments are not available to be
checked (such as vprintf), specify the third parameter as zero. In this case, the
compiler only checks the format string for consistency.
DS51284H-page 22 © 2008 Microchip Technology Inc.