Reading a file and then writing something back

[Jeff Epler]

Python's file object is based on ISO C's file I/O primitives
(fopen, fread, etc) and inherits both the requirements of the standard
and any quirks of your OS's C implementation.

According to this document
http://www.lysator.liu.se/c/rat/d9.html#4-9-5-3
a direction change is only permitted after a "flushing" operation
(fsetpos, fseek, rewind, fflush).  file.flush calls C's fflush.

I believe that this C program is equivalent to your Python program:

#include <stdio.h>

int main(void) {
    char line[21];
    FILE *f = fopen("testfile", "w");
    fputs("kevin\n", f); 
    fputs("dan\n", f);
    fputs("pat\n", f);
    fclose(f);

    f = fopen("testfile", "r+");
    fgets(line, 20, f); printf("%s", line);
    fgets(line, 20, f); printf("%s", line);

    fflush(f);

    if(fputs("chris\n", f) == EOF) { perror("fputs"); }
    fclose(f);

    return 0;
}

On my Linux machine, it prints
    kevin
    pat
and testfile's third and final line is "chris".

On a windows machine nearby (compiled with mingw, but using msvcrt.dll)
it prints
    kevin
    dan
    fputs: No error
and testfile's third and final line is "pat".

If I add fseek(f, 0, SEEK_CUR) after fflush(f), I don't get a failure
but I do get the curious contents
    kevin
    dan
    pat
    chris

If I use just fseek(f, 0, SEEK_CUR) I get no error and correct
contents in testfile.

I don't have a copy of the actual C standard, but even Microsoft's
documentation says
    When the "r+", "w+", or "a+" access type is specified, both reading
    and writing are allowed (the file is said to be open for “update”).
    However, when you switch between reading and writing, there must be
    an intervening fflush, fsetpos, fseek, or rewind operation. The
    current position can be specified for the fsetpos or fseek
    operation, if desired.
        http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccore98/HTML/_crt_fopen.2c_._wfopen.asp
so it smells like a bug to me.  Do you happen to be using Windows?  I
guess you didn't actually say.

Jeff
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