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Re: fread() bug?


On Jan 5, 2008, at 8:29 AM, Norm Green wrote:

I've found what I consider to be a stream IO bug. Here's what's happening:
[snip]
Is this is a bug? Are there any work arounds?

I wouldn't think this is a bug. Stream IO is buffered. Just because you flushed the output doesn't mean you flushed the input. I would expect it to read from its buffer. Try fpurge(3). It seems to be a BSD function, though.

I wrote a simple test case to demonstrate this problem, which passes on
Solaris. 


Something like this?

steve$ cat write.c
#include <stdio.h>
#include <unistd.h>
#include <stdint.h>
#include <stdlib.h>

#define STR(x) XSTR(x)
#define XSTR(x) #x
#define ASSERT( x ) \
do { if( !(x) ) { perror( STR(__LINE__) ": " #x ); exit( 1 ); } } while( 0 )

int main( int argc, const char **argv )
{
	FILE *w = fopen( "foo", "w" );
	ASSERT( w );
	int32_t num = 0xAABBCCDD;
	ASSERT( fwrite(&num, 4, 1, w) == 1 );
	ASSERT( fflush(w) == 0 );
	FILE *r = fopen( "foo", "r" );
	ASSERT( r );
	ASSERT( fread(&num, 4, 1, r) == 1 );
	ASSERT( num == 0xAABBCCDD );
	ASSERT( fseek(w, 0, SEEK_SET) == 0 );
	num = 0x11223344;
	ASSERT( fwrite(&num, 1, 1, w) == 1 );
	ASSERT( fflush(w) == 0 );
	if( argc > 1 )
		ASSERT( fpurge(r) == 0 );
	ASSERT( fseek(r, 0, SEEK_SET) == 0 );
	ASSERT( fread(&num, 1, 1, r) == 1 );
	printf( "%#x\n", num );
	ASSERT( num == 0x11223344 );
	ASSERT( fclose(r) == 0 );
	ASSERT( fclose(w) == 0 );
	return 0;
}
steve$ gcc -Wall write.c
steve$ ./a.out
0xaa223344
31: num == 0x11223344: Undefined error: 0
steve$ ./a.out foo
0x11223344

fpurge(3) seems to advance the file pointer so you have to seek after you purge.

--
Steve Checkoway

    "Anyone who says that the solution is to educate the users
    hasn't ever met an actual user." -- Bruce Schneier




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