README.md

Demo program for a bug that appears under Termux on Android.

The %n format for printf does not work.

Files:

• percent-n-test.c is the test program.
• correct-output.txt is the correct output
• incorrect-output.txt is the output under Termux on my Nexus 9, Android 7.1.1

UPDATE : The problem is that the C runtime library used by Android, bionic, deliberately disables the %n format specifier for *print(), as explained here. (There are some known security problems with %n, but this means the C implementation is non-conforming.)

correct-output.txt

Hello, world
a=0 b=5 c=13
PASSED

incorrect-output.txt

nHellon, world
na=100 b=200 c=300
FAILED, variables unchanged

percent-n-test.c

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    int a = 100, b = 200, c = 300;
    printf("%nHello%n, world\n%n", &a, &b, &c);
    printf("a=%d b=%d c=%d\n", a, b, c);
    if (a == 0 && b == 5 && c == 13) {
        puts("PASSED");
        exit(EXIT_SUCCESS);
    }
    else if (a == 100 && b == 200 && c == 300) {
        puts("FAILED, variables unchanged\n");
        exit(EXIT_FAILURE);
    }
    else {
        puts("FAILED, variables changed but incorrect\n");
        exit(EXIT_FAILURE);
    }
}