the program crashes as soon i execute

Question

this program is supposed to write to shared memory using win32 API. it is a program given as it is in text book but when i try to execute it it fails. it crashes as i click on execute

the program is supposed to write a string to shared memory

#include<windows.h>  
#include<stdio.h>  

int main(int argc, char *argv[])   
{   
  HANDLE hFile, hMapFile;  
  LPVOID lpMapAddress;
  //mapping of memory
  hFile=CreateFile("temp.txt",GENERIC_WRITE,0,NULL,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL, NULL);    
  hMapFile = CreateFileMapping(hFile,  NULL, PAGE_READWRITE, 0,0, TEXT("SharedObject"));  
  lpMapAddress = MapViewOfFile(hMapFile, FILE_MAP_ALL_ACCESS, 0, 0, 0);   
  //writing into shared memory
  sprintf((char*)lpMapAddress,"shared memory writing");   


  UnmapViewOfFile(lpMapAddress);   
  CloseHandle(hFile);  
  CloseHandle(hMapFile);  
}

Don't you have to map a file of > 0 bytes for memory mapped files? Although the last time I looked at memory mapped files using WINAPI was the late 1990s. — drescherjm, Apr 07 '16 at 18:50
no error message is displayed . It crashes right away @ceros — Shashank Karukonda, Apr 07 '16 at 18:56
***It crashes right away*** Your debugger should help with that. — drescherjm, Apr 07 '16 at 18:57
Good opportunity to [learn how to use a debugger](http://ericlippert.com/2014/03/05/how-to-debug-small-programs/). — Captain Obvlious, Apr 07 '16 at 19:08
i tried to debug but it shows program recieved signal SIGSEGV,segmentation fault — Shashank Karukonda, Apr 07 '16 at 19:08
Well that's what happens when you have shitty code that doesn't check for errors. — Captain Obvlious, Apr 07 '16 at 19:11
Normally you can set your debugger to break on this type of exception and then follow the stack up to your code. From your description (not a Visual Studio message) I expect you are using gdb. I can not help with that. — drescherjm, Apr 07 '16 at 19:29
You didn't check for errors and reaped the usual consequences. — David Heffernan, Apr 07 '16 at 20:06
There are no SIGSEGV segmentation faults on Windows, and there are no error dialogs or messages that contain *"SIGSEGV"* or *"segmentation fault"*. You are either not describing the true behavior, or you are running in an environment that is not Windows (e.g. WINE). — IInspectable, Apr 08 '16 at 08:04

score 4 · Answer 1 · edited May 14 '16 at 13:59

the reason it is crashing is because the file must be created with GENERIC_READ and GENERIC_WRITE access rights since in the CreateFileMapping() function your code specifies PAGE_READWRITE as its third argument(flProtect). This is from MSDN documentation of CreateFileMapping:

The file must be opened with access rights that are compatible with the protection flags that the flProtect parameter specifies

PAGE_READWRITE=>Gives read/write access to a specific region of pages. The file that hFile specifies must be created with the GENERIC_READ and GENERIC_WRITE access rights.

so change

hFile=CreateFile("temp.txt",GENERIC_WRITE,0,NULL,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL, NULL);

to

hFile=CreateFile("temp.txt",GENERIC_WRITE | GENERIC_READ,0,NULL,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL, NULL);

One more thing you can not map a file with a size of zero. Here is from MSDN documentation of CreateFileMapping() function:

If this parameter(dwMaximumSizeLow) and dwMaximumSizeHigh are 0 (zero), the maximum size of the file mapping object is equal to the current size of the file that hFile identifies.

An attempt to map a file with a length of 0 (zero) fails with an error code of ERROR_FILE_INVALID. Applications should test for files with a length of 0 (zero) and reject those files.

If an application specifies a size for the file mapping object that is larger than the size of the actual named file on disk, the file on disk is increased to match the specified size of the file mapping object.

so in your case since the file you are trying to map has initially a size of 0, the CreateFileMapping() function will fail unless you specify the size of a file mapping object in the dwMaximumSizeLow/dwMaximumSizeHigh parameters of CreateFileMapping(). You could do something like this...

HANDLE hFile, hMapFile;  
    LPVOID lpMapAddress;
//mapping of memory
    hFile=CreateFile(L"temp.txt",GENERIC_WRITE | GENERIC_READ,0,NULL,OPEN_ALWAYS,FILE_ATTRIBUTE_NORMAL, NULL);    
    if(hFile!=INVALID_HANDLE_VALUE)
    {
        char* str="shared data to be written";//the data you want to write to the file
        int strLen=::strlen(str);//get the string length of the data you want to write
        hMapFile = CreateFileMapping(hFile,  NULL, PAGE_READWRITE, 0,strLen, TEXT("SharedObject")); //here you also specify the size of the mapping object to be equal to the size of data you want to write
        if (hMapFile != NULL && hMapFile != INVALID_HANDLE_VALUE) 
        {
            lpMapAddress = MapViewOfFile(hMapFile, FILE_MAP_ALL_ACCESS, 0, 0, 0);   
            //writing into shared memory

            if(lpMapAddress!=NULL)
                sprintf((char*)lpMapAddress,"%s","shared file write");   
            else
                printf("error");//error message MapViewOfFile() failed

            UnmapViewOfFile(lpMapAddress);  
            CloseHandle(hMapFile); 
        }
        else
            printf("error");//error message CreateFileMapping() failed

        CloseHandle(hFile);  
    }
    else
        printf("error");//error message CreateFile() failed

i have tried that statement already but the result is still the same !! — Shashank Karukonda, Apr 08 '16 at 03:56
i think there is a problem with the sprintf statement ,when i tired it without that statement the program worked properly — Shashank Karukonda, Apr 08 '16 at 04:06
@shashank karukonda The problem is not the sprintf function it is the fact that you are trying to create a mapping for a file with a size of 0. I have updated my answer to reflect that. — Biruk Abebe, Apr 08 '16 at 05:55

the program crashes as soon i execute

1 Answers1