What is the 'I' (capital i) flag in C printf?

user_13813463 0
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While compiling the following code:

#include <stdio.h>
int main() {
  printf("99% Invisible");
  return 0;
}

in gcc 7.5.0 I get the following warnings:

test.c: In function ‘main’:
test.c:4:16: warning: ' ' flag used with ‘%n’ gnu_printf format [-Wformat=]
   printf("99% Invisible");
                ^
test.c:4:16: warning: 'I' flag used with ‘%n’ gnu_printf format [-Wformat=]
test.c:4:16: warning: format ‘%n’ expects a matching ‘int *’ argument [-Wformat=]
   printf("99% Invisible");
             ~~~^

What is going on here? I don't see mention of a " " flag or an "I" flag anywhere in documentation. The code outputs 99visible, essentially ignoring the space and I in the format string and following the %n format.

edit: People seem to be misunderstanding the question. I know how to printf a literal %, and what %n do. I am just curious what is happening here.

(also, for those who know the context: I know the system in question didn't use C, I am just curious as to what printf is doing here).

printf c
3 Answers

The I flag is a GNU extension to printf. From the man page:

glibc 2.2 adds one further flag character.

I

For decimal integer conversion (i, d, u) the output uses the locale's alternative output digits, if any. For example, since glibc 2.2.3 this will give Arabic-Indic digits in the Persian ("fa_IR") locale.

So when the compiler checks the format string, it sees % In as a format specifier, i.e. the space and I flags applied to the n conversion specifier. Since neither flag is applicable to the n conversion specifier, the compiler emits a warning for each.

user_16871190
user_127344670

It appears that with your compiler when a % character is encountered in a printf format string, it scans forward to find a valid format specifier, then interprets everything in-between as a modifier. If those are not valid modifiers, an error is flagged.

As others have pointed out, replace "99% Invisible" with "99%% Invisible" to fix the problem.

user_33422480
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