Bug 24886

I wrote a simple program to demonstrate the problem.  Basically, I use mmap to exchange information between files. After this is done, I try to msync the information back to desk.  The new information does not seem to be saved. The return value from msync is 0, and no error registed through errno.

I used simple text files with this program.

< source code begin >
#include <stdio.h>
#include <sys/types.h>
#include <sys/uio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <limits.h>
#include <errno.h>
#include <unistd.h>

bool ResizeFile( int fd, int new_sz  );

int main()
{

	int hOldFile=0, hNewFile=0;
	char *ofbuf=NULL, *nfbuf=NULL;
	long new_sz=155, ofsz=133;
	char *ofile="oldf.txt";
	char *nfile="newf.txt";

	hOldFile = open( ofile, O_RDWR );
	hNewFile = open( nfile, O_RDWR );

	if( hOldFile )
		ofbuf = (char*)mmap(NULL, ofsz, PROT_READ|PROT_WRITE, /*MAP_DENYWRITE*/MAP_PRIVATE, hOldFile, 0);

	if ( !ResizeFile( hOldFile, new_sz ) )
	{
	        close(hOldFile);
        	close(hNewFile);
		return 1;
	}

	nfbuf = (char*) mmap(NULL, new_sz, PROT_READ|PROT_WRITE, MAP_PRIVATE, hNewFile, 0);

	for( int i=0; i<new_sz; i++ )
		ofbuf[i] = nfbuf[i];

	if ( msync( ofbuf, /*MPPH.new_sz*/ 0 , MS_SYNC)  )
		perror( "Error msync ofbuf" );
	munmap( ofbuf, new_sz );
	close(hOldFile);

	if( msync( nfbuf, new_sz, MS_SYNC ) )
		perror( "Error msync nfbuf" );
	munmap( nfbuf, new_sz );
	close(hNewFile);
}


bool SetEndOfFile( int fd )
{
	long int new_size;

	new_size = lseek(fd,0L,SEEK_CUR);
	if( -1 ==  new_size )
		return ( -1 );
	
	if ( write( fd, "\0", 1)   <= 0 )
        	return ( -1 );

 	if( ftruncate(fd, new_size) != -1 )
		   return true;
	return false;
}

bool ResizeFile( int fd, int new_sz  )
{
	if ( new_sz != lseek( fd, new_sz, SEEK_SET ) )
        	return false;
	
	if (!SetEndOfFile(fd) )
        	return false;

	if ( 0 != lseek(fd,0,SEEK_SET))
        	return false;

	return true;
}

How-To-Repeat: I used two text files, 
<oldf.txt, 133bytes>
auto testing: 

Functional spec and Software design spec as guide to features testing.

Generator testing:
command line testing.
<newf.txt, 155bytes>
auto testing: 

Functional spec and Software design spec as guide to features testing.

Generator testing:
command line testing.

Just added a line.