I think I have a method in FAQ here describing how to deal with huge files using fll from Cetin Basoz.

See Removing double quotes from tab delimited file FAQ #8016

>>>I tried FREAD instead of FGETS and it looks like it works fine, I tried to read 8192 bytes with FREAD then write them with FWRITE without any modifications and I got same file size.
>>
>>With FREAD some occurences may be split between two sequential reads in which case they will not be replaced.
>
>Not tested, but I wonder if this might be a faster approach?
>

>LPARAMETERS cInFile, cOutFile, cFind, cReplace
>
>LOCAL  cVar, aPos[1], nCtr, nOccurs, nHndIn, nHndOut, nEnd
>
>m.nHnd = 0
>m.nHndOut = 0
>m.cVar = FILETOSTR(m.cInFile)
>m.nOccurs = OCCURS(m.cFind,m.cVar)
>
>FOR m.nCtr = 1 TO m.nOccurs
>   aPos[m.nCtr] = IIF(m.nCtr > 1,;
>                      AT(m.cFind,m.cVar,m.nCtr) - aPos[m.Ctr - 1], ;
>                      AT(m.cFind,m.cVar,m.nCtr) - LEN(m.cFind))
>ENDFOR
>
>DO WHILE m.nHndOut < 1
>   m.nHndOut = FCREATE(m.cOutFile)
>ENDFOR
>
>DO WHILE m.nHndIn < 1
>   m.nHndIn = FOPEN(m.cInFile)
>ENDDO
>
>m.nEnd = FSEEK(m.nHndIn,0,2)
>= FSEEK(m.nHndIn,0,o)
>
>= FWRITE(m.nHndOut,FREAD(m.nHndIn,LEN(m.cFind)),LEN(m.cFind))
>
>FOR m.nCtr = 1 TO m.nOccurs
>   = FWRITE(m.nHndOut,STRTRAN(FREAD(m.nHndIn,aPos[m.nCtr]),m.cFind,m.cReplace)aPos[m.nCtr])
>ENDFOR
>
>= FWRITE(m.nHndOut,FREAD(m.nHndIn,m.nEnd-FSEEK(m.nHndIn,0,1)),m.nEnd-FSEEK(m.nHndIn,0,1))
>
>= FCLOSE(m.nHndIn)
>= FCLOSE(m.nHndOut)
>

>Of course, it would only work if the strings being replaced were the same length.
>
>Regards,