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cgcclanguage-lawyerscanf

`sscanf` for `%%` does not work as expected

Consider the following program:

#include <stdio.h>

// [Example 5 from C17; Page 238](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2310.pdf)

int main(void) {
  int n = 0;
  int i = 0;

  n = sscanf("foo %bar 42", "foo%%bar%d", &i);

  printf("n = %d\ni = %d\n", n, i);

  return 0;
}

According to the C17 Standard, it should print

n = 1
i = 42

However, on my platform it prints

n = 0
i = 0

gcc -v on my platform gives:

Using built-in specs.
COLLECT_GCC=D:\mingw64\bin\gcc.exe
COLLECT_LTO_WRAPPER=d:/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/12.1.0/lto-wrapper.exe
OFFLOAD_TARGET_NAMES=nvptx-none
Target: x86_64-w64-mingw32
Configured with: ../configure --prefix=/R/winlibs64-11.3.0ucrt/inst_gcc-12.1.0/share/gcc --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --enable-offload-targets=nvptx-none --with-pkgversion='MinGW-W64 x86_64-ucrt-posix-seh, built by Brecht Sanders' --with-tune=generic --enable-checking=release --enable-threads=posix --disable-sjlj-exceptions --disable-libunwind-exceptions --disable-serial-configure --disable-bootstrap --enable-host-shared --enable-plugin --disable-default-ssp --disable-rpath --disable-libstdcxx-debug --disable-version-specific-runtime-libs --with-stabs --disable-symvers --enable-languages=c,c++,fortran,lto,objc,obj-c++,jit --disable-gold --disable-nls --disable-stage1-checking --disable-win32-registry --disable-multilib --enable-ld --enable-libquadmath --enable-libada --enable-libssp --enable-libstdcxx --enable-lto --enable-fully-dynamic-string --enable-libgomp --enable-graphite --enable-mingw-wildcard --enable-libstdcxx-time --disable-libstdcxx-pch --with-mpc=/e/Prog/wi
nlibs64-11.3.0ucrt/custombuilt --with-mpfr=/e/Prog/winlibs64-11.3.0ucrt/custombuilt --with-gmp=/e/Prog/winlibs64-11.3.0ucrt/cu
stombuilt --with-isl=/e/Prog/winlibs64-11.3.0ucrt/custombuilt --enable-libstdcxx-backtrace --enable-install-libiberty --enable
-__cxa_atexit --without-included-gettext --with-diagnostics-color=auto --enable-clocale=generic --with-libiconv --with-system-
zlib --with-build-sysroot=/R/winlibs64-11.3.0ucrt/gcc-12.1.0/build_mingw/mingw-w64 CFLAGS='-I/e/Prog/winlibs64-11.3.0ucrt/cust
ombuilt/include/libdl-win32 -D__USE_MINGW_ACCESS' CXXFLAGS=-D__USE_MINGW_ACCESS LDFLAGS='-Wl,--disable-nxcompat -Wl,--disable-high-entropy-va -Wl,--disable-dynamicbase'
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.1.0 (MinGW-W64 x86_64-ucrt-posix-seh, built by Brecht Sanders)

ADDED: I tried another platform, it does print

n = 1
i = 42

On this platform, gcc -v shows:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 11.4.0-1ubuntu1~22.04' --with-bugurl
=file:///usr/share/doc/gcc-11/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c
++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-11 --program-prefix=x86_64-linu
ithout-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
ithout-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)
ithout-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
ithout-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)

So it seems that this is a bug of the compiler on the first platform?

Solution

  • The behavior of your first platform does not conform to the C standard. C 2018 7.21.6.2 7 and 8 say:

    A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps:

    Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [, c, or n specifier.

    %% is a conversion specification (because 7.21.6.2 3 says the format string is composed of directives which are one or more white-space characters, an ordinary multibyte character, or a conversion specification, and that each conversion specification is introduced by the character %) and does not include [, c, or n, so input white-space characters before it should be skipped.

    Equivalent language appears in versions of the standard going back to 1990.

    You should test compiling with -std=c17 just in case there is some issue with default compilations using the C library in a non-conforming mode. (I do not think that is likely, but it is theoretically possible.)