[comp.lang.c] fseek doesn't work properly

jonas@his.UUCP (Jonas Mellin) (10/20/89)

We have a couple of HP9000/8XX machines with HPUX 3.1. 

The problem:
	fseek moves the filepointer to beginning of the file
	regardless of the parameters 

Solution:
	if setbuf is used to either:
	1)	set no buffer
		or
	2)	set user declared buffer
	on the file;
	fseek will work properly in forward searches, but it will
	not work in relative searches nor in searches
	from the end of the file.

Question:
	Does anyone else have this problem?

Here is a testprogram which works if the comments are removed from the
line containing the call to 'setbuf'. The program creates the file
'test' and then is then meant to update the file.

#include <stdio.h>

#define N

struct dummy_struct {
  int a;
  char b[N];
} a;

void main()
{
  FILE *fp;
  int i;
  char buf[BUFSIZ];

  fp=fopen("test","w");

  for (a.a='A'; a.a<'z'; a.a++) {
    for (i=0; i<N; i++)
      a.b[i]=a.a;
    fwrite(&a,sizeof(a),1,fp);
  }

  fclose(fp);

  fp=fopen("test","r+");
  /* Remove comments on next line to get the program working */
  /*setbuf(fp,(char *)buf); */
  rewind(fp);
  fread(&a,sizeof(a),1,fp);
  i=0;
  while (!feof(fp)) {
    printf("%d %c\n",i,a.b[0]);
    a.a+=2;
    fseek(fp,(long)i*sizeof(a),0);
    /* fseek(fp,(long)-sizeof(a),1); */
    fwrite(&a,sizeof(a),1,fp);
    fread(&a,sizeof(a),1,fp);
    i++;
  }

  fclose(fp);
}

Disclaimer: Even a blind man may know how to C.
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cpcahil@virtech.UUCP (Conor P. Cahill) (10/20/89)

In article <496@his.UUCP>, jonas@his.UUCP (Jonas Mellin) writes:
> The problem:
> 	fseek moves the filepointer to beginning of the file
> 	regardless of the parameters 
> Question:
> 	Does anyone else have this problem?


>     fseek(fp,(long)i*sizeof(a),0);
>     /* fseek(fp,(long)-sizeof(a),1); */
>     fwrite(&a,sizeof(a),1,fp);
>     fread(&a,sizeof(a),1,fp);


The problem probably is that you are using both the fread and fwrite
immediatly after each other.

My trusty manual specifies (on fopen(3s)):

	When a file is opened for update, both input and output may be
	done on the resulting stream.  However, output may not be directly 
	followed by input without an intervening fseek, or rewind.....

RTM.


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| Conor P. Cahill     uunet!virtech!cpcahil      	703-430-9247	!
| Virtual Technologies Inc.,    P. O. Box 876,   Sterling, VA 22170     |
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| Conor P. Cahill     uunet!virtech!cpcahil      	703-430-9247	!
| Virtual Technologies Inc.,    P. O. Box 876,   Sterling, VA 22170     |
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