topic Re: fgets coredumps on nullbyte in textfile in Operating System - HP-UX

fgets coredumps on nullbyte in textfile

Andreas D. Skjervold — Tue, 24 May 2005 06:07:59 GMT

Hi

I have some text files that only contains null bytes hex 00.( By error from an application)

When fgets reads these files it core dumps.
buffer is allokated char of 32000

char buffer[32000]
bufferlen = 32000

The file is opened like this
if ((fp = fopen(szparsefile,"rb")) == NULL)
if(fgets(buffer,bufferlen,fp)==NULL) {

HP-UX 11iv1

Any ideas?

rgds
Andreas

Re: fgets coredumps on nullbyte in textfile

Steven E. Protter — Tue, 24 May 2005 06:38:05 GMT

If its c/c++ check the code for null pointers, or improperly initialized variables.

SEP

Re: fgets coredumps on nullbyte in textfile

Simon Hargrave — Tue, 24 May 2005 06:43:21 GMT

I've just compiled that code and tested it (though the filename should be quoted but I guess you know this). It worked fine, so I don't know your problem. Perhaps if you posted the whole code? This is what I tested: -

#include

char buffer[32000];
int bufferlen;
FILE* fp;

main()
{
if ((fp=fopen("testfile","rb")) == NULL){
if (fgets(buffer,bufferlen,fp)==NULL){
}
}
}

However shouldn't you really be using fread? fgets will give you a null-terminated buffer, and since your file is full of nulls, how will you know where your read buffer ends?

Re: fgets coredumps on nullbyte in textfile

Andreas D. Skjervold — Tue, 24 May 2005 07:31:37 GMT

Attached is the textfile with nullbytes

Re: fgets coredumps on nullbyte in textfile

Stephen Keane — Tue, 24 May 2005 07:38:15 GMT

You are opening a file in binary mode ("rb" in open) then using fgets to read the contents. But fgets is designed for text-mode processing, looking (as it does) for end of line characters to break on (or the size arg, whichever happens first).

Re: fgets coredumps on nullbyte in textfile

TwoProc — Tue, 24 May 2005 07:40:28 GMT

I agree with Simon - you probably should be using fread. But, you should be able to use fgets - you'll get one returned line per null in the file - which would be the same of using fgetc instead. But, if I needed to read a unknown length of nulls I think I'd put in a while loop to do fgetc until I didn't get any more null characters. If you're skipping a block of known length of nulls - just fseek right around them if you know the byte offset in the file where they stop occuring.

Re: fgets coredumps on nullbyte in textfile

Dietmar Konermann — Tue, 24 May 2005 07:57:32 GMT

Hmm, maybe I'm too stupid to read that code sniplet, but shouldn't we check for fp!=NULL?

if ((fp = fopen(szparsefile,"rb")) != NULL)

Otherwise we don't read anything at all, do we?

Re: fgets coredumps on nullbyte in textfile

Simon Hargrave — Tue, 24 May 2005 08:12:22 GMT

Good point on reading the file! But it still works even when I recompiled my test with != (also put a printf in to prove it gets there).

But Andreas, when I asked for you to send the file, I meant the source code, not the file full of nulls ;) That's easy for us to reproduce!

Re: fgets coredumps on nullbyte in textfile

Dietmar Konermann — Tue, 24 May 2005 08:22:26 GMT

My point was that Andreas never calls fgets(), so the buffer never gets initialized or null-terminated.

char buffer[32000]="";

This could change a lot, I think.

Re: fgets coredumps on nullbyte in textfile

Andreas D. Skjervold — Tue, 24 May 2005 08:31:15 GMT

Hi again.

We found the solution... Steven you had it at once. Uninitialized variables..

Thanks for your help. This forum is great.

rgds
Andreas