gcc attribute format - MarekBykowski/readme GitHub Wiki
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format (archetype, string-index, first-to-check)
where string-index is the number of the argument to check against. Then the arguments starting from first-to-check will be checked against. If mismatch GCC will complain. For example for myFormatText2 (const char *, ...) __attribute__((format(printf, 1, 2))); const char * is the 1st argument of myFormatText2 function to check against, and a number of aruguments to type check are 2. See below:
#include <stdio.h>
myFormatText1 (const char *, ...);
myFormatText2 (const char *, ...) __attribute__((format(printf, 1, 2)));
int main(void) {
int a, b;
float c;
a = 5;
b = 6;
c = 9.099999;
myFormatText1("Here are some integers: %d , %d\n", a, b); // No type checking. Types match.
myFormatText1("Here are some integers: %d , %d\n", a, c); // No type checking. Type mismatch, but no warning.
myFormatText2("Here are some integers: %d , %d\n", a, b); // Type checking. Types match.
myFormatText2("Here are some integers: %d , %d\n", a, c); // Type checking. Warning: 181-D: argument is incompatible...
}