#82 FreeRTOS-Plus-FAT-SL wrong FAT entry

[Heiko]

Settings:
Sector size = cluster size = 512 byte
Fat12

Process:
pFile = f_open("name.bin", "w+"); // Create file. In my case cluster 3 (sector 42)
f_write(data, 1, 512, pFile); // Allocate cluster 4 (sector 43) at the end
f_close(pFile);
pFile = f_open("name.bin", "r+");
f_seek(pFile, 512, F_SEEK_SET);
f_write(data, 1, 512, pFile); // cause a wrong fat entry at the next f_open(); f_write();

What happened:
f_seek() set gl_file.relpos to 512 but didn’t change gl_file.pos

fn_write() increment gl_file.pos.sector, copies the data to the local buffer (gl_sector) and calls _f_emptywritebuffer()

_f_emptywritebuffer() writes the data to cluster 5 (that’s the correct cluster). But gl_file.pos.cluster was not incremented so _f_getclustervalue() returns cluster 4 for the next cluster. Because the next cluster is 4 there will be no allocation for the next cluster.

At the next "f_open(); f_seek(); f_write();" _f_emptywritebuffer() writes to cluster 5 (that’s also the correct cluster) and allocates cluster 11. The FAT entries are now cluster 3 -> cluster 4 -> cluster 11 (-> = points to). But Cluster 5 is be missing in the FAT chain.

[Heiko]

I fixed the issue by editing the function _f_fseek().

Code:
static FatErrorCodeType _f_fseek ( long offset )
{
unsigned long cluster;
unsigned long tmp;
FatErrorCodeType ret = F_NO_ERROR;
long remain;

if ( offset < 0 )
{
    offset = 0;
}

if ( ( (unsigned long) offset <= gl_file.filesize )
    && ( (unsigned long) offset >= gl_file.abspos )
        && ( (unsigned long) offset < gl_file.abspos + F_SECTOR_SIZE ) )
{
    gl_file.relpos = (unsigned short)( (unsigned long)offset - gl_file.abspos );
}
else
{
    if ( gl_file.modified )
    {
        ret = _f_writeglsector( (unsigned long)-1 );
        if ( ret )
        {
            gl_file.mode = F_FILE_CLOSE; /*cant accessed any more*/
            return ret;
        }
    }

    if ( gl_file.startcluster )
    {
        gl_file.abspos = 0u;
        gl_file.relpos = 0u;
        gl_file.prevcluster = 0u;
        gl_file.pos.cluster = gl_file.startcluster;
        remain = (long)gl_file.filesize;

        tmp = gl_volume.bootrecord.sector_per_cluster;
        tmp *= F_SECTOR_SIZE;   /* set to cluster size */

        /*calc cluster*/
        gl_volume.fatsector = (unsigned long)-1;
        while ( (unsigned long)offset >= tmp )
        {
            ret = _f_getclustervalue( gl_file.pos.cluster, &cluster );
            if ( ret )
            {
                gl_file.mode = F_FILE_CLOSE;
                return ret;
            }

            if ( (long) tmp > remain )
            {
                break;
            }

            if ( (long) tmp == remain )
            {
                /* cluster boarder so check if the next cluster exists */
                if ( cluster >= F_CLUSTER_RESERVED )
                {
                    ret = _f_alloccluster( &cluster );
                    if ( ret )
                    {
                        return ret;
                    }

                    ret = _f_setclustervalue( cluster, F_CLUSTER_LAST );
                    if ( ret )
                    {
                        return ret;
                    }

                    ret = _f_setclustervalue( gl_file.pos.cluster, cluster );
                    if ( ret )
                    {
                        return ret;
                    }
                }
            }

            remain -= (long)tmp;
            offset -= (long)tmp;
            gl_file.abspos += tmp;
            if ( cluster >= F_CLUSTER_RESERVED )
            {
                break;
            }

            gl_file.prevcluster = gl_file.pos.cluster;
            gl_file.pos.cluster = cluster;
        }

        _f_clustertopos( gl_file.pos.cluster, &gl_file.pos );
        if ( remain && offset )
        {
            while ( ( offset > (long) F_SECTOR_SIZE )
                    && ( remain > (long) F_SECTOR_SIZE ) )
            {
                gl_file.pos.sector++;
                offset -= (long)F_SECTOR_SIZE;
                remain -= (long)F_SECTOR_SIZE;
                gl_file.abspos += F_SECTOR_SIZE;
            }
        }

        if ( remain < offset )
        {
            gl_file.relpos = (unsigned short)remain;
            ret = _f_extend( (long)gl_file.filesize + offset - remain );
        }
        else
        {
            gl_file.relpos = (unsigned short)offset;
        }
    }
}

return ret;

} / _f_fseek /

[Richard Barry]

Thank you for the information - this is being looked at internally.

Regards.

[Richard Barry]

Fixed in V1.0.1, which is already available in SVN, and will be included in FreeRTOS V8.0.1.