Question

fread() issues with multiple line files

Asked by: madscientist

Hi,

I'm trying to create a function for reading a plain text file using dynamic memory assignment, and it works fine ... but only if the file contains a single line. As soon as I go to 2 or more lines my gremlins error message pops up. What am I doing wrong?

Thanks.

#include <stdlib.h>
#include <stdio.h>
 
char *file_contents_in;
 
int read_from_file(FILE *fp) {
 
	long filelen;
  void *file_data;
 
  fseek(fp, 0L, SEEK_END);
  filelen = ftell(fp);
  rewind(fp);
  printf("Chars in file: %d.\n\n", filelen);
  file_data = calloc(filelen + 1, sizeof(char));
 
  if (file_data == NULL) { puts("not enough memory..."); /* Not enough memory. Report error & exit */ }
 
  if (fread(file_data, filelen, 1, fp)) {
    file_contents_in = (char *) file_data;
    return 1;
  } else {
    return 0;
  }
 
}
 
int main() {
 
  FILE *fp;
 
	fp = fopen("hello.txt", "r");
  
  if (read_from_file(fp)) {
    puts(file_contents_in);
  } else {
    puts("woops, the gremlins be out to play");
  }
 
}

                                  
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Asked On
2009-02-07 at 12:30:49ID24122725
Tags

C programming

,

fread

,

muliple line

Topic

C Programming Language

Participating Experts
1
Points
50
Comments
8

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Answers

 

by: ozoPosted on 2009-02-07 at 12:45:01ID: 23580131

what error messages?

 

by: madscientistPosted on 2009-02-07 at 12:54:25ID: 23580181

Apologies, when I use a multiple lined text file the error message from line 37 is displayed. I still get the output from line 14 and I don't get an error message from line 17 either.

 

by: ozoPosted on 2009-02-07 at 13:13:13ID: 23580290

it is working for me with multiple lines in hello.txt
can you show what fread returns, and the output of perror?

 

by: ozoPosted on 2009-02-07 at 13:25:34ID: 23580347

can you also show the result of feof(fp) and ferror(fp), and the return value from fread if you change the arguments from read(file_data, filelen, 1, fp)) to read(file_data, 1, filelen, fp))

 

by: madscientistPosted on 2009-02-07 at 13:39:04ID: 31544092

Reversing the arguments of fread() from fread(file_data, filelen, 1, fp) to fread(file_data, 1, filelen, fp) solved the problem.

 

by: madscientistPosted on 2009-02-07 at 13:42:29ID: 23580419

Thanks for the answer. Slightly confused about why it was an issue but at my level of knowledge I'm fine with it just working...

 

by: ozoPosted on 2009-02-07 at 14:31:41ID: 23580567

I can recreate the error by making filelen too large, which would prevent fread from returning even one full block, block, although it can return less than that many one byte blocks.
I would guess that this may be caused by ftell and fread counting lengths differently, which might happen on systems that handle files differently depending on whether they were opened in "r" or "rb" mode

 

by: madscientistPosted on 2009-02-07 at 14:34:21ID: 23580575

Oh, ok. Thanks for the extra info!

20120131-EE-VQP-002

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