Writing to allocated char buffer - c

Question

I have following code:
framingStatus compressToFrame(char* inputBuffer, size_t inputBufferLength, char* frame, size_t* frameLength)
{
dword crc32 = GetMaskedCrc(inputBuffer,inputBufferLength);
size_t length = snappy_max_compressed_length(inputBufferLength);
char* compressed = (char*)malloc(length);
snappy_status status = snappy_compress(inputBuffer,inputBufferLength,compressed,&length);
if( status!=SNAPPY_OK )
return FS_ERROR;
frame[0] = 0x00; // Typ ramki skompresowany
frame[1] = length&0xff;
frame[2] = (length&0xff00)>>8;
frame[3] = (length&0xff00)>>16;
frame[4] = crc32&0xff;
frame[5] = (crc32&0xff00)>>8;
frame[6] = (crc32&0xff0000)>>16;
frame[7] = (crc32&0xff000000)>>24;
frame[8] = '\0'; // Pomoc dla strcat
strcat(frame,compressed);
*frameLength = length+8;
free(compressed);
return FS_OK;
}
Before calling this function I allocate memory for buffer named frame. All is ok, but assign instructions frame[x] = ... don't seem to write anything to buffer called frame and strcat concatenate compressed data to empty buffer without header I need.
Why assign instructions frame[x] = ... etc. don't give any result?
[EDIT:]
Can you suggest what function I have to use if I want to concatenate frame header with compressed data?
[EDIT2:]
Code presented below works just fine.
framingStatus compressToFrame(char* inputBuffer, size_t inputBufferLength, char* frame, size_t* frameLength)
{
dword crc32 = GetMaskedCrc(inputBuffer,inputBufferLength);
size_t length = snappy_max_compressed_length(inputBufferLength);
char* compressed = (char*)malloc(length);
snappy_status status = snappy_compress(inputBuffer,inputBufferLength,compressed,&length);
if( status!=SNAPPY_OK )
return FS_ERROR;
frame[0] = 0x00; // Typ ramki skompresowany
frame[1] = length;
frame[2] = length >> 8;
frame[3] = length >> 16;
frame[4] = crc32;
frame[5] = crc32 >>8;
frame[6] = crc32 >>16;
frame[7] = crc32 >>24;
memcpy(&frame[8],compressed,length);
*frameLength = length+8;
free(compressed);
return FS_OK;
}

Answer 1

You have
frame[0] = 0x00;
which is the same as
frame[0] = '\0';
No matter what you add after the first character, frame becomes a 0 length string.

Answer 2

strcat is for strings, not general binary bytes. Because frame first byte is zero, strcat will copy compressed starting at frame[0] and will stop copying when it sees a zero in compressed.
Try memcpy instead.
memcpy(&frame[8], compressed, length);
Also, since the length of frame is passed as an argument, you might want to be checking the total length you are copying to frame to make sure there's no illegal overwrite in that case.

Answer 3

As others already pointed out, you use binary data and not text strings. Therefore, strcat function is inappropriate here, use memcpy instead.
Furthermore, you should use unsigned char instead of plain char.
Additionally, you don't need to mask the values before shifting
frame[2] = (length&0xff00)>>8;
could be just
frame[2] = length >> 8;
And in this case, it is even buggy
frame[3] = (length&0xff00)>>16;
Same here
frame[5] = crc32 >> 8;
frame[6] = crc32 >> 16;
frame[7] = crc32 >> 24;

Answer 4

frame[0] = 0x00;
will make the first character as end of string character therefore your string frame is empty.
frame[0] = 0x00;
is same as writing,
frame[0] = '\0';