What is wrong with this zlib string decompression? - iphone

Question

i'm trying to create a simple stringdecompression algorithm for my app.
/*
Decompresses the source buffer into the destination buffer. sourceLen is
the byte length of the source buffer. Upon entry, destLen is the total size
of the destination buffer, which must be large enough to hold the entire
uncompressed data. (The size of the uncompressed data must have been saved
previously by the compressor and transmitted to the decompressor by some
mechanism outside the scope of this compression library.) Upon exit, destLen
is the actual size of the uncompressed buffer.
uncompress returns Z_OK if success, Z_MEM_ERROR if there was not
enough memory, Z_BUF_ERROR if there was not enough room in the output
buffer, or Z_DATA_ERROR if the input data was corrupted or incomplete.
*/
[Base64 initialize];
NSData * data = [Base64 decode:#"MDAwMDAwNTB42vPMVkhKzVNIBeLsnNTMPB0IpVCWWZyVqpAJkalKTVUoS8xTSMpJLC0HALWrEYi="];
NSString * deBase64 = [[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding];
int lengteOP = [[deBase64 substringWithRange:NSMakeRange(0,8)] intValue];
NSUInteger lengteIP = [deBase64 length];
const unsigned char *input = (const unsigned char *) [[deBase64 substringFromIndex:8] cStringUsingEncoding:NSASCIIStringEncoding];
unsigned char * dest;
uncompress(dest, lengteOP, input, lengteIP);
A get a EXC_BADD_ACCESS error when i try this.
The string is build with code in delphi using ZLib, same as the library in the iPhone sdk
its a base64 encode string with the first 8 characters representing the length of the string followed by the zlib-ed string.

Answer 1

I did not run the code but your issue is likely to be dest. Here is a snippet from the documentation.
Upon entry, destLen is the total size
of the destination buffer, which must
be large enough to hold the entire
uncompressed data.
The destination needs to have the memory allocated before calling the function, otherwise it will attempt to write data to invalid memory causing EXC_BAD_ACCESS.
Try the following:
unsigned char * dest = malloc(sizeof(unsigned char) * lengteOP);
uncompress(dest, lengteOP, input, lengteIP);
//Use dest (create NSString with proper encoding for example)
free(dest);

Answer 2

- finished code
/*
Decompresses the source buffer into the destination buffer. sourceLen is
the byte length of the source buffer. Upon entry, destLen is the total size
of the destination buffer, which must be large enough to hold the entire
uncompressed data. (The size of the uncompressed data must have been saved
previously by the compressor and transmitted to the decompressor by some
mechanism outside the scope of this compression library.) Upon exit, destLen
is the actual size of the uncompressed buffer.
uncompress returns Z_OK if success, Z_MEM_ERROR if there was not
enough memory, Z_BUF_ERROR if there was not enough room in the output
buffer, or Z_DATA_ERROR if the input data was corrupted or incomplete.
ZEXTERN int ZEXPORT uncompress OF((Bytef *dest, uLongf *destLen,
const Bytef *source, uLong sourceLen));
*/
[Base64 initialize];
NSData * data = [Base64 decode:#"MDAwMDAwNTB42vPMVkhKzVNIBeLsnNTMPB0IpVCWWZyVqpAJkalKTVUoS8xTSMpJLC0HALWrEYi="];
NSString * deBase64 = [[NSString alloc] initWithData:data encoding:NSASCIIStringEncoding];
uLongf lengthOriginal = [[deBase64 substringWithRange:NSMakeRange(0,8)] floatValue];
uLongf * lengteOP = malloc(sizeof(uLongf));
lengteOP = &lengthOriginal;
NSUInteger lengteIP = [deBase64 length];
NSString * codedString = [deBase64 substringFromIndex:8];
const unsigned char *input = (const unsigned char *) [codedString cStringUsingEncoding:NSISOLatin1StringEncoding];
unsigned char * dest = malloc((sizeof(unsigned char) * lengthOriginal));
uncompress(dest, lengteOP, input, lengteIP);
NSString * bla = #"Decoded string :";
NSLog([bla stringByAppendingString:[NSString stringWithCString:dest encoding:NSASCIIStringEncoding]]);
free(dest);
free(lengteOP);