| Summary: | istream::sync() doesn't work | ||
|---|---|---|---|
| Product: | gcc | Reporter: | MyAdEss |
| Component: | libstdc++ | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED INVALID | ||
| Severity: | normal | ||
| Priority: | P3 | ||
| Version: | 4.5.1 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | ||
| Attachments: | information file generated by g++-4.5 -v -save-temps sync.cpp | ||
I don't see a requirement in the standard for the behaviour you expect. istream::sync() calls streambuf::sync(), the behaviour of which depends on the specific streambuf type. Since the type of streambuf used by std::cin is unspecified, the behaviour of cin.sync() is also unspecified. (In reply to comment #1) > I don't see a requirement in the standard for the behaviour you expect. > > istream::sync() calls streambuf::sync(), the behaviour of which depends on the > specific streambuf type. Since the type of streambuf used by std::cin is > unspecified, the behaviour of cin.sync() is also unspecified. Oh, I guess you are right. Standard doesn't specify behaviour for cin's streambuf type. It is implementation-specific. It worked for me on different platforms and compilers. I was also reading it's definition on cplusplus.com but I should look what standard says. Anyway, it would be nice if it worked as I expected. cin.ignore() statements inside loops could be avoided. Thank you for your fast reply. (In reply to comment #2) > > Oh, I guess you are right. Standard doesn't specify behaviour for cin's > streambuf type. It is implementation-specific. It worked for me on different > platforms and compilers. I was also reading it's definition on cplusplus.com > but I should look what standard says. cplusplus.com has always been a poor reference > Anyway, it would be nice if it worked as I expected. cin.ignore() statements > inside loops could be avoided. It couldn't be avoided in portable code, because other implementation still might not have the semantics given by cplusplus.com |
Created attachment 22500 [details] information file generated by g++-4.5 -v -save-temps sync.cpp istream::sync() doesn't work properly. It returns zero value, so it looks like it synchronizes the buffer, but it does not. When I ask user for input, I should be able to get part of this input and ignore the rest with cin.sync(). Then I should be able to ask for input again without reading those old ignored characters. However when I do that, no characters are discarded and user is not even asked for the second input. I have this problem with g++ versions 4.5.1-7ubuntu2 and 4.4.4-14ubuntu5. ----------- Example ----------- -----source------ char a,b; cout << "Enter word: "; a = cin.get(); cin.sync(); cout << "Enter another word: "; b = cin.get(); cout << a << " " << b; -----expected input------ Enter word: hello Enter another word: world ----expected output------ h w -----real input------ Enter word: hello Enter another word: -----real output------ h e ----------- ----------- sync.cpp: ----------- #include <iostream> using namespace std; int main () { char first, second; cout << "Please, enter a word: "; // this gets the first character of the first word first=cin.get(); // this should discard all unread characters cin.sync(); cout << "Please, enter another word: "; // this should wait for another word, but it gets // the second character of the first word instead second=cin.get(); cout << "The first word began by " << first << endl; cout << "The second word began by " << second << endl; return 0; } ----------- Generated .ii file as attachment ----------- ----------- Output of g++-4.5 -v -save-temps sync.cpp ----------- Using built-in specs. COLLECT_GCC=g++-4.5 COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.5.1/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.5.1-7ubuntu2' --with-bugurl=file:///usr/share/doc/gcc-4.5/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.5 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.5 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-gold --with-plugin-ld=ld.gold --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.5.1 (Ubuntu/Linaro 4.5.1-7ubuntu2) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/4.5.1/cc1plus -E -quiet -v -D_GNU_SOURCE sync.cpp -D_FORTIFY_SOURCE=2 -mtune=generic -march=x86-64 -fpch-preprocess -fstack-protector -o sync.ii ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../../../x86_64-linux-gnu/include" ignoring nonexistent directory "/usr/include/x86_64-linux-gnu" #include "..." search starts here: #include <...> search starts here: /usr/include/c++/4.5 /usr/include/c++/4.5/x86_64-linux-gnu /usr/include/c++/4.5/backward /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/4.5.1/include /usr/lib/gcc/x86_64-linux-gnu/4.5.1/include-fixed /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/4.5.1/cc1plus -fpreprocessed sync.ii -quiet -dumpbase sync.cpp -mtune=generic -march=x86-64 -auxbase sync -version -fstack-protector -o sync.s GNU C++ (Ubuntu/Linaro 4.5.1-7ubuntu2) version 4.5.1 (x86_64-linux-gnu) compiled by GNU C version 4.5.1, GMP version 4.3.2, MPFR version 3.0.0-p3, MPC version 0.8.2 GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 GNU C++ (Ubuntu/Linaro 4.5.1-7ubuntu2) version 4.5.1 (x86_64-linux-gnu) compiled by GNU C version 4.5.1, GMP version 4.3.2, MPFR version 3.0.0-p3, MPC version 0.8.2 GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 50e33501336f77ceb788d88860a1807b COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' as -V -Qy --64 -o sync.o sync.s GNU assembler version 2.20.51 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.20.51-system.20100908 COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.5.1/:/usr/lib/gcc/x86_64-linux-gnu/4.5.1/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.5.1/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.5.1/:/usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-shared-libgcc' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/4.5.1/collect2 --build-id --no-add-needed --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro /usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../../../lib64/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../../../lib64/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.5.1/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.5.1 -L/usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../../../lib64 -L/lib/../lib64 -L/usr/lib/../lib64 -L/usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../.. sync.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/4.5.1/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.5.1/../../../../lib64/crtn.o ----------- uname -a ----------- Linux floop-kubuntu 2.6.35-22-generic #35-Ubuntu SMP Sat Oct 16 20:45:36 UTC 2010 x86_64 GNU/Linux My distribution is Kubuntu 10.10 64-bit Thank you for your help.