I'm asking the user to type in two variables. First an unsigned integer a
and second an unsigned character b
. The reading in of a
works fine, but after reading in b
, a
is 0.
I found out that the pointer to a
is one greater than the pointer to b
. I realized that when b
is greater than 255 a
is not 0 anymore. So it seem to me that scanf
reads in more than one byte for b
and overwrites a
.
#include <stdio.h>
int main ()
{
unsigned int a;
unsigned char b;
printf("a: ");
scanf("%u", &a); /* 255 */
printf("b: ");
scanf("%hhu", &b); /* 17 */
printf("a: %u\n", a); /* a: 0 */
printf("b: %u\n", b); /* b: 17 */
printf("pointer a: %u\n", &a); /* pointer a: 6422316 */
printf("pointer b: %u\n", &b); /* pointer b: 6422315 */
return 0;
}
Since I'm new to programming in C, I'm not sure what information is necessary to understand my problem. I'm using a 64 bit processor an this is the used compiler:
gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=c:/mingw/bin/../libexec/gcc/mingw32/8.2.0/lto-wrapper.exe
Target: mingw32
Configured with: ../src/gcc-8.2.0/configure --build=x86_64-pc-linux-gnu --host=mingw32 --target=mingw32 --prefix=/mingw --disable-win32-registry --with-arch=i586 --with-tune=generic --enable-languages=c,c++,objc,obj-c++,fortran,ada --with-pkgversion='MinGW.org GCC-8.2.0-3' --with-gmp=/mingw --with-mpfr=/mingw --with-mpc=/mingw --enable-static --enable-shared --enable-threads --with-dwarf2 --disable-sjlj-exceptions --enable-version-specific-runtime-libs --with-libiconv-prefix=/mingw --with-libintl-prefix=/mingw --enable-libstdcxx-debug --with-isl=/mingw --enable-libgomp --disable-libvtv --enable-nls --disable-build-format-warnings
Thread model: win32
gcc version 8.2.0 (MinGW.org GCC-8.2.0-3)
The problem is in this line:
scanf("%hhu", &b); /* 17 */
Microsoft's C runtime library does not implement %hhu
(it was not part of the C standard prior to C99). This causes the input to be interpreted as a short
(due to how MSVCRT interprets hh
as h
in the format specifier), and writes past the limits of b
and overwrites parts of a
.
To fix this problem, change the type of a
to unsigned short
and use %hu
as the format specifier OR pass the -D__USE_MINGW_ANSI_STDIO
flag to your compiler.
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