[LLVMbugs] [Bug 14074] New: std::filebuf:sputn fails to write 0xFF as the first byte into the filebuf, irregardless of the openmode

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=14074

             Bug #: 14074
           Summary: std::filebuf:sputn fails to write 0xFF as the first
                    byte into the filebuf, irregardless of the openmode
           Product: libc++
           Version: unspecified
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
        AssignedTo: hhinnant at apple.com
        ReportedBy: andrew at spacemonkey.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


std::filebuf::sputn fails to write 0xFF as the first byte into the buffer. It
however reports the byte as written via the return code.  

The openmode does not affect the behavior.

This breaks libraries which use the filebuf directly (i.e.
boost::iostreams::device::file_sink).

This works correctly using libstdc++ as well as the standard library shipped
with Visual C++.


Steps:
  1) Instantiate a filebuf
  2) Call sputn with a buffer starting with 0xFF

Observed behavior:
  sputn() returns the byte as having been written but no byte is put into the
buffer.

Expected behavior:
  The byte should be written into the buffer.


$ cat badfilebuf.cpp 
#include <fstream>
#include <iostream>

int main() {
    std::filebuf buf;
    buf.open("buf", std::ios::out | std::ios::binary);
    std::cout << buf.sputn("\xFF", 1) << " bytes written" << std::endl;
    buf.close();

    return 0;
}

$ clang++ badfilebuf.cpp 
$ ./a.out
1 bytes written
$ ls -l buf
-rw-r--r--  1 andrew  staff  1 Oct 12 15:08 buf
$ clang++ badfilebuf.cpp -stdlib=libc++
$ ./a.out
1 bytes written
$ ls -l buf
-rw-r--r--  1 andrew  staff  0 Oct 12 15:08 buf

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