Thread: Access violation when reading [00000000]?

Hi,

It's a code for copying a file. It had been working fine but now something goes wrong and it causes the main process to crash, even without an error message.

I can't figure out what's wrong here, so I am adding the code here.

Code:

#include <windows.h>
#include <stdio.h>
#include <commctrl.h>
#include <stdlib.h>
#include <time.h>

CopyAFile(char * src, char * trg){
char c = 0;

FILE *fp = fopen(src, "rb");
FILE *fp1 = fopen(trg, "wb");

if(!(fp && fp1)){
MessageBox(0, "fopen()", 0, 0);
return 0;
}

fseek(fp, 0, SEEK_SET);

      while(!feof(fp)){
      fscanf(fp, "&#37;c", &c);
      fprintf(fp1, "%c", c);  //It crashes here, the first time fprintf() called
      }

fclose(fp);
fclose(fp1);

return 1;

}

Here is where it crashes, ntdll.dll. From OllyDbg.

Code:

7C901E24   > FF75 10        PUSH DWORD PTR SS:[EBP+10]
7C901E27   . 57             PUSH EDI
7C901E28   . 56             PUSH ESI
7C901E29   . E8 26FDFFFF    CALL ntdll.7C901B54
7C901E2E   .^EB AE          JMP SHORT ntdll.7C901DDE
7C901E30   > 0FB706         MOVZX EAX,WORD PTR DS:[ESI]
7C901E33   . 8B4D 10        MOV ECX,DWORD PTR SS:[EBP+10]
7C901E36   . 0301           ADD EAX,DWORD PTR DS:[ECX]
7C901E38   . 3D 00FE0000    CMP EAX,0FE00
7C901E3D   .^0F87 EEEDFFFF  JA ntdll.7C900C31
7C901E43   . 807D 14 00     CMP BYTE PTR SS:[EBP+14],0
7C901E47   . 0F85 977A0300  JNZ ntdll.7C9398E4
7C901E4D   > 8B4E 0C        MOV ECX,DWORD PTR DS:[ESI+C]
7C901E50   . 8D46 08        LEA EAX,DWORD PTR DS:[ESI+8]
7C901E53   . 8B10           MOV EDX,DWORD PTR DS:[EAX]
7C901E55   . 894D 08        MOV DWORD PTR SS:[EBP+8],ECX
7C901E58   . 8B09           MOV ECX,DWORD PTR DS:[ECX]     ;; DS:[00000000]=???
;;Access violation reading 00000000
7C901E5A   . 3B4A 04        CMP ECX,DWORD PTR DS:[EDX+4]
7C901E5D   . 8955 0C        MOV DWORD PTR SS:[EBP+C],EDX

It would be greatly appreciated if you can point out my mistake.

Thanks...

I corrected that & after I posted the message, I know it shouldn't be there. BTW, what's wrong with feof()?

PS. I don't wan't to use CopyFile() here, so please don't suggest it.

Ok, I corrected it, thanks.

The main problem is not related to this, anyway. Just before the fprintf() called, I can see that the file pointer, the char 'c' and the format string are all correct. I don't really see what is causing a crash like this.

Fixed it. I was using malloc() to initialize a few strings, in some other parts of the program. Like this:

char * string = (char *) malloc(1024);

Apparently this was poisoning the stack. I don't know how, I always had problems with malloc. So I changed them into something like this.

char string[1024];

The problem is fixed now. Thanks for the answers.

For future reference, anytime you get a 0x00000000 access violation error in Windows it is basically saying you have a null pointer somewhere. Good place to start looking is inside object init code and memory allocation code.

I will post if I find anything. Thanks.

Originally Posted by lruc

I've never done anything lower than C++ so what exactly is that second set of code that you posted? I was also kind've curious about .o files. I opened it in notepad once and was really strange. What is this?

The second set of code was assembly language.

.o files are binary-format not-quite-executables, so strange is probably an understatement.

Thank you so much. You have no idea how many questions you just answered for me.