如何在C＃中复制Perl的解包功能？

Question

I am trying to recreate a Perl script in C# but have a problem creating a checksum value that a target system needs.

In Perl this checksum is calculated using the unpack function:

while (<PACKAGE>) {
    $checksum += unpack("%32C*", $_);
}
$checksum %= 32767;
close(PACKAGE);

where PACKAGE is the .tar file input stream

I need to replicate this in C# but can't find a means of replicating that unpack function.

All help appreciated!

(I know there are much better checksum calculations available but can't change target system so can't change calculation)

Answer 1

There seems to be a library in Mono called DataConvert that was written to provide facilities similar to Perl's pack/unpack. Does this do what you need?

Answer 2

Perkl's unpack is described here and here . From that you should be able to write an equivalent in C#.

Answer 3

To supplement Mitch Wheat's comment, here's a Java implementation (which does a single block only). I'm sure you'll find a way to convert it into C#, and to do multiple blocks.

int sum = 0;
for (byte b : buffer) {
    sum += (int) b & 255;
}
return sum % 32767;

Hope this helps!

Answer 4

In my testing here, unpack with %32C appears to be the additive sum of the bytes, limited to 32 bits.

print unpack("%32C*", 'A');
65
print unpack("%32C*", 'AA');
130

It shouldn't be hard to reproduce that.

Answer 5

Based on the comments of Chris Jester-Young and piCookie, I developed the following function. I hope you find it useful.

int fileCheckSum(const char *fileName)
{
   FILE *fp;
   long fileSize;
   char *fileBuffer;
   size_t result;
   int sum = 0;
   long index;

   fp = fopen(fileName, "rb");
   if (fp == NULL)
   {
      fputs ("File error",stderr); 
      exit (1);
   }

   fseek(fp, 0L, SEEK_END);
   fileSize = ftell(fp);
   fseek(fp, 0L, SEEK_SET);

   fileBuffer = (char*) malloc (sizeof(char) * fileSize);   
   if (fileBuffer == NULL)
   {
      fputs ("Memory error",stderr);
      exit (2);
   }

   result = fread(fileBuffer, 1, fileSize, fp);   
   if (result != fileSize)
   {
      fputs ("Reading error", stderr);
      if (fileBuffer != NULL)
         free(fileBuffer);

      exit (3);
   }

   for (index = 0; index < fileSize; index++)
   {
      sum += fileBuffer[index] & 255;
   }

   fclose(fp);
   if (fileBuffer != NULL)
      free(fileBuffer);

   return sum % 32767;  
}