According to ISO C standard (ISO/IEC 9899), printf function contains "%n" format to count the number of output characters.
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.
Example code:
#include <stdio.h>
int main (void) {
int c1, c2;
printf ("hello%n, world%n\n", &c1, &c2);
printf ("% d\t% d\n", c1, c2);
return 0;
}
The C runtime library on both Windows (MSVCRT/UCRT) and Linux (GLIBC) supports "%n" format in printf function. (However, "%n" format is disabled by default when using cl.exe to compile C program in Windows. A nonstandard function _set_printf_count_output by Microsoft is required to enable "%n" format. That is because cl.exe is a C++ compiler, function printf is not in common use in C++ [In C++ std::cout is used instead]. If cl.exe followed C standard, "%n" format should be enabled by default and disabled by using _set_printf_count_output function. In addition, Microsoft creates a nonstandard function printf_s, in which "%n" format is completely disabled)
Unlike MSVCRT/UCRT in Windows and GLIBC in Linux, The Bionic C runtime library in Android does not support "%n" format at all, which parses "%n" format only into two ordinary characters "%" and "n". Why "%n" format is not parsed in ISO standard by Bionic C runtime library?