malloc问题导致变量被覆盖

Question

我的程序中有3个全局变量。由于某种原因，当我在pad上运行string2bin时，binaryMessage的内容被覆盖，我找不到原因。我增加了我的malloc's的大小，但这并没有帮助。我究竟做错了什么？

请注意，randomPad()会创建一串随机的“十六进制”值，该值应转换为表示十六进制字符串的二进制字符串。

char * temp; 
char * binaryMessage; 
char * pad; 

void process_message(char *s) 

int main (int argc, const char * argv[]) { 

     temp = (char *)malloc(sizeof(char *) *2048); 

     binaryMessage = (char *)malloc(sizeof(char *) *2048); 

     pad = (char *)malloc(sizeof(char *) * 2048); 

     process_message("test"); 

} 


char * char2bin (unsigned char c){ 
     static char bin[CHAR_BIT + 1] = {0}; 
     int i; 

     for (i = CHAR_BIT - 1; i >= 0; i--) { 
      bin[i] = (c % 2) + '0'; 
      c /= 2; 
     } 

     return bin; 
} 

char* string2bin(char* str){ 
     int i; 
     int len = strlen(str); 

     sprintf(temp,""); 
     for(i=0; i< len;i++){ 
      sprintf(temp, "%s%s",temp,char2bin(str[i])); 
     } 
     return temp; 
} 

char* randomPad(){ 
     int i; 
     const char *hex_digits = "ABCDEF"; 
     char * p = (char*)malloc(sizeof(char*)*242); 

     // clear old pad data 
     //sprintf(pad,""); 

     // create random string of 242 hex chars to use as pad 
     for(i = 0; i< 242; i++){ 
      sprintf(p,"%s%c",p,hex_digits[ (rand() % 16) ]); 
     } 
     return p; 
} 
void process_message(char *s){ 
     binaryMessage = string2bin(s); 
     printf("m %s\nbm %s\n",s, binaryMessage); 

     //get random one time pad 
     pad = string2bin(randomPad()); 

     printf("m %s\nbm %s\n",s, binaryMessage); 
} 

我的预期输出是：

m test 
bm 01110100011001010111001101110100 
m test 
bm 01110100011001010111001101110100 

我的实际输出为

m test 
bm 01110100011001010111001101110100 
m test 
bm 0011011100110001001110010100000100110010001110000011100001000101001100110100010000111000001101010100001100110010001100110011011100110111001100010011100000110100010001100011000101000100001101010011100000110101010000100011011000111000001101110011100100110010001110000011001100110010001100010011001101000100001100110011010101000110001100000011000100111000010001000011000101000011010000100011001000110101001110010011000100110100001110000011010000110010010000110011010001000001001101110100000101000001001101010011100000111001001101010011000100110000001101000011010101000001001101010100001001000100001110010011011000110111001110000011000100110101001100110100010101000100001101010011010100110011001110010011010001000010010000010100001100110011001100010011001000110000010001010100001100110001001101100011100001000100001100010100011000110100001100010100010100111001010001000011011000110000001110010011011100110001010001100011001101000001010001010100011000110000010000110011010000110101010000010011100001000010001100000011100100110010010001010011001000110110001101010011001001000011010001100100010001000110001110000011011101000100010001010100011000110010001100010011000001000101001110000011011100110111010001010011010000110010001101110011011101000101001101110011000100111000001110010100010101000010001100100011010100110010001101100011011001000011001101000100010100110110010000010011001000110011001100100011010000110110001101100011011101000110001100110011100100110001001110000011001101000110001100010011100000110101010001100011010000110000001100000011011101000010001110010100010101000001010000100011011100111000010000110100011000110010010000110100001100110111010001100011000101000100001101010011000100110011001100010100000101000101001100110011000000110001001110010011000100110111010000110100001000111000010001100100011000110010001100110100001101000100001110000011000001000101001110000100001001000100010001100100011000110001001100010011000001000110 

Answer 1

除了alk指出的错误之外，还有四个问题。

1-此：

char * p = (char*)malloc(sizeof(char*)*242); 

应该是这样的：

char * p = malloc(sizeof(char)*243); // Do NOT cast malloc 

2-此：

sprintf(p,"%s%c", p, hex_digits[ (rand() % 16) ]); 

应该是这样的：

sprintf(p+i,"%c",hex_digits[ (rand() % 16) ]); 

3 - 你错过这样的：

*(p+242) = '\0'; 
return p; 

4-最后，这行代码：

binaryMessage = string2bin(s); 

替换binaryMessage指针与temp指针值。

我可以提供的最好的建议？摆脱所有的全局变量。而且你需要给随机数发生器加上一些不可预知的东西（例如当前时间），否则你每次都会得到相同的“随机”数字序列。

Answer 2

这些线是关键：

sprintf(temp, "%s%s", temp, char2bin(str[i])); 

... 

sprintf(p, "%s%c", p, hex_digits[ (rand() % 16) ]); 

从sprintf()'s man page：

个

一些程序过于草率依赖代码，如下面的

 sprintf(buf, "%s some further text", buf); 

将文本追加到buf中。然而，该标准明确指出，如果源和目标缓冲区调用的sprintf（），的snprintf（），vsprintf中（），并 vsnprintf（）重叠时，结果是不确定的。根据所使用的gcc（1）的版本和采用的编译器选项，上述调用将会产生预期结果，如而不是。