为什么此Objective-C代码会引发malloc错误? [英] Why does this Objective-C code raise a malloc error?

问题描述

我使用此方法在object-c中编码base64字符串，但该应用有时崩溃了:

I used this method to encode base64 string in object-c, but the app was crashed sometimes:

- (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;

}

错误是:

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

当我调试时，它会在这一行停止:

and when I debug, it stop in this line:

free(outputBuffer); 

您知道是什么原因导致撞车吗?

Do you know what caused crash here ?

推荐答案

也许是问题所在

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

在第一行中，您分配 outLength 个字节，但是在第二行中，您将写到比缓冲区末尾多一个字节的位置.取决于页面边界和 malloc 中其他神秘事件的发生，可能是正确的，也可能不是.那可以解释为什么它不会每次崩溃.

In the first line you allocate outLength bytes, but in the second line you write to a position one byte beyond the end of the buffer. Depending on page boundaries and other mysterious happenings inside malloc, that might be OK or it might not. That would explain why it doesn't crash every time.

尝试以下方法:

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

这可能会解决您的问题.

That might fix your problem.

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