Message-ID: <448F9EA1.7050908@tlinx.org>
Date: Tue, 13 Jun 2006 22:29:05 -0700
From: Linda Walsh <cygwin AT tlinx DOT org>
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Subject: Cygwin fread on Write-Only File Descriptor returns undefined state
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I think I've run into a problem involving fread.
I open a test file (fopen) with Write-Only access ("w"),
immediately after that, I do an fread on the file handle
of some number of bytes. 

I expect fread to return an error.  But it doesn't. 
It returns 0 as the number of bytes read, and the
error value remains unset when querying with ferror.

In addition to fread not setting the error value, a value
of zero is returned.  Zero is to be returned, *only* on
end-of-file or error.  However, in the test case, neither
value is returned.  Theoretically, this shouldn't happen
(i.e. it's an undefined "set" of return values).

test program follows:
-------------------
#include <stdio.h>
main (int argc, char * argv[]) {

    FILE * output_handle=fopen("/tmp/tmpfile","w");
    if (!output_handle) {
        printf ("can't open output file /tmp/tmpfile\n");
        exit (1);
    }  

    char mybuff[4];
    int retval=fread(mybuff, sizeof(char), 4, output_handle);
    if (!retval) {
        int eof=feof(output_handle);
        int err=ferror(output_handle);
        printf ("(retval,eof,err) = (%d,%d,%d)\n",retval,eof,err);
        if (!eof && ! err) 
            printf ("Undefined error: fread returned zero, but there is 
no eof or error\n");
        exit(2);
    } else
        printf ("Unexpected success in fread: Read %d chars\n",retval);
   
    exit(0);
}
   

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