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

Differences Between 16-Bit Device C and ANSI C
The weak attribute may be applied to functions as well as variables:
extern int __attribute__((__weak__)) compress_data(void *buf);
int process(void *buf) {
if (compress_data) {
if (compress_data(buf) == -1) /* error */
}
/* process buf */
}
In the above code, the function compress_data will be used only if it is linked in from
some other module. Deciding whether or not to use the feature becomes a link-time
decision, not a compile time decision.
The affect of the weak attribute on a definition is more complicated and requires
multiple files to describe:
/* weak1.c */
int __attribute__((__weak__)) i;
void foo() {
i = 1;
}
/* weak2.c */
int i;
extern void foo(void);
void bar() {
i = 2;
}
main() {
foo();
bar();
}
Here the definition in weak2.c of i causes the symbol to become a strong definition.
No link error is emitted and both i’s refer to the same storage location. Storage is
allocated for weak1.c’s version of i, but this space is not accessible.
There is no check to ensure that both versions of i have the same type; changing i in
weak2.c to be of type float will still allow a link, but the behavior of function foo will
be unexpected. foo will write a value into the least significant portion of our 32-bit float
value. Conversely, changing the type of the weak definition of i in weak1.c to type
float may cause disastrous results. We will be writing a 32-bit floating point value into
a 16-bit integer allocation, overwriting any variable stored immediately after our i.
In the cases where only weak definitions exist, the linker will choose the storage of the
first such definition. The remaining definitions become in-accessible.
The behavior is identical, regardless of the type of the symbol; functions and variables
behave in the same manner.
© 2008 Microchip Technology Inc. DS51284H-page 19