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Re: can't read sequential files

Dave Korn wrote:
On 01 November 2007 06:43, zirtik wrote:

After adding the line:

if (fp==NULL) { printf("error, NULL pointer!\n"); return(1); }

and then rebuilding the code, everything worked. But it's strange that if I
delete that code segment and never check whether the fp pointer is NULL or
not, I always get a segmentation fault. Can this be some kind of an
optimization problem? I don't know why it happens. Thank you for the

A NULL pointer is never valid in C, and a segfault is what you get if you try to make use of one (by dereferencing it). You get a NULL pointer back from fopen when it fails; a lot of library routines do this to indicate failure, because any other value could be a valid pointer. The other library routines, such as fread and fwrite, will assume that you have done your error checking and won't be passing them a NULL pointer, so they won't bother to check what file pointer you pass them, they'll just go ahead and try and use it. So if you get a NULL pointer back from fopen and you don't check for it, your code carries on and passes that same pointer to fread, which tries to use it as if it pointed to a real FILE object, and crashes.

  The comp.lang.c FAQ has an entire section on NULL pointer, section 5.  It
should be available at
but the website seems to be temporarily down right now; there's also a copy at

I think what the OP is saying is that if he adds the check for null, then his code works normally, including the file read operation, (ie, the pointer is not null), but if he removes the check, then he gets a segfault. Strange behavior like this is typically the result of memory corruption from earlier in the program. If you can post your entire code (preferably a minimal example that exhibits the behavior), then we can probably locate the problem.


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