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writing from a pointer to a file

Posted on 2014-04-14
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Last Modified: 2014-04-16
Hi

 I am trying to write a certain number of bytes, pointed by a pointer, p, to a file called myfile.

int numbytes=sizeof(float)*56;

FILE *myfile;
myfile=fopen("out.txt","w");  //returns a <Bad Ptr>
memcpy(myfile,memoryPtr,numbytes);

I noticed that myfile has a value 0x57251448 but it also says <Bad Ptr>. However, i read some confusing remarks on the web saying that might not necessarily mean something bad i,e <Bad Ptr> might just mean NULL pointer.

However, when i proceed with that in mind and I do memcpy, Visual Studio crashes and I am sent to some dbgheap.c file where I get an error at

/* check if the heap was not allocated by _aligned routines */
        if ( nBlockUse == _NORMAL_BLOCK)
        {
            if ( CheckBytes((unsigned char*)((uintptr_t)pUserData & ~(sizeof(uintptr_t) -1)) -nAlignGapSize,_bAlignLandFill, nAlignGapSize))
            {
                // We don't know (yet) where (file, linenum) pUserData was allocated
                _RPT1(_CRT_ERROR, "The Block at 0x%p was allocated by aligned routines, use _aligned_free()", pUserData);
                errno = EINVAL;
                return;
            }


What's the best way to write a chunk of memory , pointed to by a pointer,  to a file? And what is wrong with the current way I am doing things?
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Question by:LuckyLucks
  • 2
3 Comments
 
LVL 86

Assisted Solution

by:jkr
jkr earned 334 total points
ID: 39999388
There are two issues with your approach:

- You need to open your file in binary tranylation mode ("wb" instead of "w")
- You can't 'memcpy()' to a file

That should be more like

int numbytes=sizeof(float)*56;

FILE *myfile;
myfile=fopen("out.bin","wb");  // if that returns a <Bad Ptr>, be sure you have the permissions to create a file in the current folder
fwrite(memoryPtr,numbytes,1,myfile); // write the memory chunk
fclose(myfile);

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LVL 32

Assisted Solution

by:phoffric
phoffric earned 166 total points
ID: 39999508
I have never encountered a case where fopen did not work when returning a valid pointer. Try this.. When making library api calls, check to see what it returns. Notice the check on fopen() from this example taken from http://www.cplusplus.com/reference/cstdio/fopen/?kw=fopen
/* fopen example */
#include <stdio.h>
int main ()
{
  FILE * pFile;
  pFile = fopen ("myfile.txt","w");
  if (pFile!=NULL)                     // CHECK RETURN VALUE OF fopen()
  {
    fputs ("fopen example",pFile);
    fclose (pFile);
  }
  return 0;
}

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LVL 86

Accepted Solution

by:
jkr earned 334 total points
ID: 39999751
>>I have never encountered a case where fopen did not work when returning a valid
>>pointer.

That's why I added

// if that returns a <Bad Ptr>, be sure you have the permissions to create a file in the current folder

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This - along with File System Virtualization on Windows - can indeed cause that problem.
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