Using MapViewOfFile, pointer eventually walks out

Question

All,

I'm using MapViewOfFile to hold part of a file in memory. There is a stream that points to this file and writes to it, and then is rewound. I use the pointer to the beginning of the mapped file, and read until I get to the null char I write as the final character.

int fd;
yyout = tmpfile();
fd = fileno(yyout);
#ifdef WIN32
    HANDLE fm;
    HANDLE h = (HANDLE) _get_osfhandle (fd);

    fm = CreateFileMapping(
             h,
             NULL,
             PAGE_READWRITE|SEC_RESERVE,
             0,
             4096,
             NULL);
    if (fm == NULL) { 
            fprintf (stderr, "%s: Couldn't access memory space! %s\n", argv[0],  strerror (GetLastError()));
            exit(GetLastError());
    }
    bp = (char*)MapViewOfFile(
              fm,
              FILE_MAP_ALL_ACCESS,
              0,
              0,
              0);
    if (bp == NULL) { 
            fprintf (stderr, "%s: Couldn't fill memory space! %s\n", argv[0],  strerror (GetLastError()));
            exit(GetLastError());
    }

Data is sent to the yyout stream, until flushData() is called. This writes a null to the stream, flushes, and then rewinds the stream. Then I start from the beginning of the mapped memory, and read chars until I get to the null.

void flushData(void) {
    /* write out data in the stream and reset */ 
    fprintf(yyout, "%c%c%c", 13, 10, '\0');
    fflush(yyout);
    rewind(yyout);
    if (faqLine == 1) {
        faqLine = 0; /* don't print faq's to the data file */
    }
    else {
        char * ps = bp;
        while (*ps != '\0') {
                fprintf(outstream, "%c%c", *ps, blank);
                ps++;
            }
        fflush(outfile);
    }
    fflush(yyout);
    rewind(yyout);
}

After flushing, more data is written to the stream, which should be set to the start of the memory area. As near as I can determine with gdb, the stream is not getting rewound, and eventually fills up the allocated space.

Since the stream points to the underlying file, this does not cause a problem initially. But, when I attempt to walk the memory, I never find the null. This leads to a SIGSEV. If you want more details of why I need this, see here.

Why am I not reusing the memory space as expected?

Answer 1

I think this line from the MSDN documentation for CreateFileMapping might be the clue.

A mapped file and a file that is accessed by using the input and output (I/O) functions (ReadFile and WriteFile) are not necessarily coherent.

You're not apparently using Read/WriteFile, but the documentation should be understood in terms of mapped views versus explicit I/O calls. In any case, the C RTL is surely implemented using the Win32 API.

In short, this approach is problematic.

I don't know why changing the view/file size helps; perhaps it just shifts the undefined behaviour in a direction that happens to be beneficial.

Answer 2

Well, after working on this for a while, I have a working solution. I don't know why this succeeds, so if someone comes up with something better, I'll be happy to accept their answer instead.

fm = CreateFileMapping(
         h,
         NULL,
         PAGE_READWRITE|SEC_RESERVE,
         0,
         16384,
         NULL);

As you can see, the only change is to the size declared from 4096 to 16384. Why this works when the total chars input at a time is no more than 1200, I don't know. If someone could provide details on this, I would appreciate it.

Answer 3

When you're done with the map, simply un-map it.

UnmapViewOfFile(bp);