Solved

Null pointer assignment

Posted on 1998-09-24
1
374 Views
Last Modified: 2012-05-04

/***** Program Merge many files to one file *****/

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
#include<io.h>
#include<dos.h>
#include<string.h>
#include <fcntl.h>
#include <sys\stat.h>

#define BUFSIZE 54

int main(int argc,char *argv[])
{
  extern int errno;
  void *point;
  FILE *fpw,*fpr;
  int  count;
  int  handle;
  int  errchk;
  long  filesize;

  for(count=1;count<argc-1;++count)
  {
    free(point);
    handle = open(argv[count],O_RDONLY|O_TEXT);
    filesize = filelength(handle)-3;
    printf("\n%d----%d\n",handle,filelength(handle));
    if((point = malloc(filelength(handle))) == NULL)
    {
      printf("Not enough memory to allocate buffer\n");
      exit(1);  /* terminate program if out of memory */
    }


    /****** READ FROM SOURCE FILE ******/
    if((errchk = read(handle,point,filelength(handle))) == -1)
    {
      printf("Read Failed.\n ----%d",errno);
      exit(1);
    }
    else
    {
      close(handle);
    }


 /****** WRITE TO TARGET FILE ******/

    if((fpw = fopen(argv[argc-1],"a+t")) == NULL)
    {
      fprintf(stderr, "Cannot open input file.\n");
      return(1);
    }
    else
    {
      fwrite(point,filesize,1,fpw);
      fclose(fpw);
      free(point);
    }
  }
  return(0);
}
 Above is my merging program source code for merge detail in many files to one file.It work completely but after close target file appear message "Null pointer assignment" on screen.Can you correct this error message or any error may have on source code ?.

0
Comment
Question by:dissathep
1 Comment
 
LVL 2

Accepted Solution

by:
JYoungman earned 50 total points
ID: 1253025
You've made several mistakes.  The "null pointer assignment" is just the most obvious symtom of the bugs.  

You do free(point) twice with the same value.  Doing this a second time is a bug.  The first time round your main loop, you do free(point) without initialising point to anything other than its default random value.   This will be what causes that message.

There are other problems with this code, too.

You didn't check the return value of filelength() for a failure status (it can fail, can't it?)

>> printf("\n%d----%d\n",handle,filelength(handle));

If filelength() returns a long, the second format should be %ld.
If it does return a long and a negative value means failure,
you should not pass this value directly to malloc(), because the argument of malloc() is an unsigned quantity.

You do not need to include <conio.h>, nor <io.h>, and probably not <dos.h>.

>>  printf("\n%d----%d\n",handle,filelength(handle));

Use perror() instead.

Lastly, there is no reason to malloc() a buffer big enough to read the whole input file into, you could just read it in piecewise, appending data to the output file as you go. There is no need to reopen the output file once each time round the loop.  You could just open it once at the start and close it at the end.

0

Featured Post

Free Trending Threat Insights Every Day

Enhance your security with threat intelligence from the web. Get trending threat insights on hackers, exploits, and suspicious IP addresses delivered to your inbox with our free Cyber Daily.

Join & Write a Comment

Summary: This tutorial covers some basics of pointer, pointer arithmetic and function pointer. What is a pointer: A pointer is a variable which holds an address. This address might be address of another variable/address of devices/address of fu…
Windows programmers of the C/C++ variety, how many of you realise that since Window 9x Microsoft has been lying to you about what constitutes Unicode (http://en.wikipedia.org/wiki/Unicode)? They will have you believe that Unicode requires you to use…
The goal of this video is to provide viewers with basic examples to understand and use pointers in the C programming language.
Video by: Grant
The goal of this video is to provide viewers with basic examples to understand and use for-loops in the C programming language.

758 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

21 Experts available now in Live!

Get 1:1 Help Now