為什么這個Objective-C代碼會引發malloc錯誤？

Question

我用這個方法在object-c中編碼base64字符串，但是應用程序有時會崩潰：

- (NSString *) base64Encode
{
    //Point to start of the data and set buffer sizes
    int inLength = [self length];
    int outLength = ((((inLength * 4)/3)/4)*4) + (((inLength * 4)/3)%4 ? 4 : 0);
    const char *inputBuffer = [self 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] = '=';

    /* http://en.wikipedia.org/wiki/Base64
     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;

}

錯誤是：

malloc: *** error for object 0x164084: incorrect checksum for freed object - object was probably modified after being freed.

當我調試時，它停在這一行：

free(outputBuffer); 

你知道是什么原因造成了撞車嗎？

Answer 1

也許這就是問題所在：

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

在第一行中，您分配outLength字節，但在第二行中，您將寫入超出緩沖區末尾一個字節的位置。 根據頁面邊界和malloc內部的其他神秘事件，這可能是好的，也可能不是。 這可以解釋為什么它不會每次都崩潰。

試試這個：

char *outputBuffer = malloc(outLength + 1);
outputBuffer[outLength] = 0;

這可能會解決您的問題。

Answer 2

您可以使用以下命令修改malloc'd outputBuffer的結尾：

outputBuffer[outLength] = 0;

如果outLength為3，則可以設置outputBuffer[0] ， outputBuffer[1]和outputBuffer[2]但不能設置outputBuffer[3] 。

將malloc更改為：

char *outputBuffer = malloc(outLength+1);

或者將初始化更改為：

outputBuffer[outLength-1] = 0;