Question

Hi I am using base 64 encoding for my string but I am getting crash sometimes. It is not reproducing frequently. I tried using different ways but not get success.

Here is crash :

malloc: * error for object 0x6880830: incorrect checksum for freed object – object was probably modified after being freed.
* set a breakpoint in malloc_error_break to debug

And here is code snippet:

-(NSString *)Base64Encode:(NSData *)data{

      if([data length])
  {
  //Point to start of the data and set buffer sizes
int inLength = [data length];
int outLength = ((((inLength * 4)/3)/4)*4) + (((inLength * 4)/3)%4 ? 4 : 0);
const char *inputBuffer = [data bytes];
char *outputBuffer = malloc(outLength);
outputBuffer[outLength] = 0;
//64 digit code
static char Encode[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
//start the count
int cycle = 0;
int inpos = 0;
int outpos = 0;
char temp;

//Pad the last to bytes, the outbuffer must always be a multiple of 4
outputBuffer[outLength-1] = '=';
outputBuffer[outLength-2] = '=';

/* 
 Text content   M           a           n
 ASCII          77          97          110
 8 Bit pattern  01001101    01100001    01101110

 6 Bit pattern  010011  010110  000101  101110
 Index          19      22      5       46
 Base64-encoded T       W       F       u
 */


while (inpos < inLength){
    switch (cycle) {
        case 0:
            outputBuffer[outpos++] = Encode[(inputBuffer[inpos]&0xFC)>>2];
            cycle = 1;
            break;
        case 1:
            temp = (inputBuffer[inpos++]&0x03)<<4;
            outputBuffer[outpos] = Encode[temp];
            cycle = 2;
            break;
        case 2:
            outputBuffer[outpos++] = Encode[temp|(inputBuffer[inpos]&0xF0)>> 4];
            temp = (inputBuffer[inpos++]&0x0F)<<2;
            outputBuffer[outpos] = Encode[temp];
            cycle = 3;                  
            break;
        case 3:
            outputBuffer[outpos++] = Encode[temp|(inputBuffer[inpos]&0xC0)>>6];
            cycle = 4;
            break;
        case 4:
            outputBuffer[outpos++] = Encode[inputBuffer[inpos++]&0x3f];
            cycle = 0;
            break;                          
        default:
            cycle = 0;
            break;
    }
}
NSString *pictemp = [NSString stringWithUTF8String:outputBuffer];
free(outputBuffer);
return pictemp;

}
else {
    return @"";
}

}

Answer 1

You are writing beyond the end of your output buffer:

char *outputBuffer = malloc(outLength);
outputBuffer[outLength] = 0;

If you allocate outLength + 1 bytes you should be fine.