Why does printf specifier format %n not work?

Question

This is my code:

#include <stdio.h>

int main(void) {
    int n;

    fprintf(stdout, "Hello%n World\n", &n);
    fprintf(stdout, "n: %d\n", n);

    return 0;
} 

This is my output:

Hellon: 0

1. Why does the fprintf format specifier "%n" not work?
2. Why is the string to be printed interrupted?

ISO/IEC 9899:201x C11 - 7.21.6.1 - The fprintf function

The conversion specifiers and their meanings are:

(...)

%n The argument shall be a pointer to signed integer into which is written the number of characters written to the output stream so far by this call to fprintf. No argument is converted, but one is consumed. If the conversion specification includes any flags, a field width, or a precision, the behavior is undefined. ...

(...)

This is my compiler version used on Code::Blocks:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=C:/Program\ Files/mingw-w64/x86_64-8.1.0-posix-seh-rt_v6-rev
0/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/8.1.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../../../src/gcc-8.1.0/configure --host=x86_64-w64-mingw32 --bu
ild=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --prefix=/mingw64 --with-sysr
oot=/c/mingw810/x86_64-810-posix-seh-rt_v6-rev0/mingw64 --enable-shared --enable
-static --disable-multilib --enable-languages=c,c++,fortran,lto --enable-libstdc
xx-time=yes --enable-threads=posix --enable-libgomp --enable-libatomic --enable-
lto --enable-graphite --enable-checking=release --enable-fully-dynamic-string --
enable-version-specific-runtime-libs --disable-libstdcxx-pch --disable-libstdcxx
-debug --enable-bootstrap --disable-rpath --disable-win32-registry --disable-nls
 --disable-werror --disable-symvers --with-gnu-as --with-gnu-ld --with-arch=noco
na --with-tune=core2 --with-libiconv --with-system-zlib --with-gmp=/c/mingw810/p
rerequisites/x86_64-w64-mingw32-static --with-mpfr=/c/mingw810/prerequisites/x86
_64-w64-mingw32-static --with-mpc=/c/mingw810/prerequisites/x86_64-w64-mingw32-s
tatic --with-isl=/c/mingw810/prerequisites/x86_64-w64-mingw32-static --with-pkgv
ersion='x86_64-posix-seh-rev0, Built by MinGW-W64 project' --with-bugurl=https:/
/sourceforge.net/projects/mingw-w64 CFLAGS='-O2 -pipe -fno-ident -I/c/mingw810/x
86_64-810-posix-seh-rt_v6-rev0/mingw64/opt/include -I/c/mingw810/prerequisites/x
86_64-zlib-static/include -I/c/mingw810/prerequisites/x86_64-w64-mingw32-static/
include' CXXFLAGS='-O2 -pipe -fno-ident -I/c/mingw810/x86_64-810-posix-seh-rt_v6
-rev0/mingw64/opt/include -I/c/mingw810/prerequisites/x86_64-zlib-static/include
 -I/c/mingw810/prerequisites/x86_64-w64-mingw32-static/include' CPPFLAGS=' -I/c/
mingw810/x86_64-810-posix-seh-rt_v6-rev0/mingw64/opt/include -I/c/mingw810/prere
quisites/x86_64-zlib-static/include -I/c/mingw810/prerequisites/x86_64-w64-mingw
32-static/include' LDFLAGS='-pipe -fno-ident -L/c/mingw810/x86_64-810-posix-seh-
rt_v6-rev0/mingw64/opt/lib -L/c/mingw810/prerequisites/x86_64-zlib-static/lib -L
/c/mingw810/prerequisites/x86_64-w64-mingw32-static/lib '
Thread model: posix
gcc version 8.1.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)

Answer 1

As documented in the Microsoft documentation , the %n is disabled by default in the Microsoft C library used on your MinGW system:

Important

Because the %n format is inherently insecure, it is disabled by default. If %n is encountered in a format string, the invalid parameter handler is invoked, as described in Parameter Validation. To enable %n support, see _set_printf_count_output .

Whether %n is actually unsafe as claimed by Microsoft is highly debatable. The examples shown to support this claim combine this printf function with the use of a variable format string that can by changed by the attacker via a buffer overflow error.

On some Microsoft systems (but maybe not the latest), you could fix your program this way:

#include <stdio.h>

int main(void) {
    int n;

    _set_printf_count_output(1);

    fprintf(stdout, "Hello%n World\n", &n);
    fprintf(stdout, "n: %d\n", n);

    return 0;
} 

For a more portable approach, here is a work around to avoid using %n and still get the same results:

#include <stdio.h>

int main(void) {
    int n;

    n = fprintf(stdout, "Hello");
    fprintf(stdout, " World\n");
    fprintf(stdout, "n: %d\n", n);

    return 0;
} 

Output:

Hello World
n: 5