free()抱怨无效指针

Question

我试图通过连接一些值来逐个构建一个字符串,以获得类似的东西:

|0x0F64:0x0063:0x1A|0x7CC4:0x0073:0x1A|0x0A51:0xA29A:0x9C|0xD49D:0x0058:0x10|

我想使用动态变量,因为NB_ELEMENT可能会得到改进.

#include <stdio.h>
#include <stdlib.h>

#define NB_ELEMENT 4
typedef struct
{
   unsigned short  u16Val1;
   unsigned short  u16Val2;
   unsigned char   u8Val3;
}stElement;

stElement element[NB_ELEMENT] = {{ 0 }};

int main()
{
    element[0].u16Val1 = 3940;
    element[0].u16Val2 = 99;
    element[0].u8Val3 = 26; 

    element[1].u16Val1 = 31940;
    element[1].u16Val2 = 115;
    element[2].u8Val3 = 26;           

    element[2].u16Val1 = 2641;
    element[2].u16Val2 = 41620;
    element[2].u8Val3 = 156; 

    element[3].u16Val1 = 52429;
    element[3].u16Val2 = 88;
    element[3].u8Val3 = 16;

    char *str = malloc(NB_ELEMENT * sizeof element);

    snprintf(str, sizeof element, "|0x%04x:0x%04x:0x%x\n",element[0].u16Val1,
                                                      element[0].u16Val2,
                                                      element[0].u8Val3);

    str += sprintf(str, "|0x%04x:0x%04x:0x%x\n",element[1].u16Val1,
                                            element[1].u16Val2,
                                            element[1].u8Val3);

    fprintf(stdout, "%s\n", str);
    free(str);
    return 0;
}

上面的代码返回:

*** Error in `./TEST': free(): invalid pointer: 0x0000000000cac023 ***

但是,我设法打印|0x0f64:0x0063:0x1a但是avec添加str + = sprintf ..,发生错误.

Answer 1

问题出在这里:

str += sprintf(str, "|0x%04x:0x%04x:0x%x\n",element[1].u16Val1,
                                        element[1].u16Val2,
                                        element[1].u8Val3);

这会修改指针str.因此,当你把它交给free以后,它不是malloc给你的指针,这使得free工作变得相当困难.

解决方案:不要修改指针.如果要通过添加写入指针的字符数来跟踪缓冲区中的位置,请使用不同的指针:

char *str = malloc(NB_ELEMENT * sizeof element);
// ...
char * p = str;
// ...
p += sprintf(p, "|0x%04x:0x%04x:0x%x\n",element[1].u16Val1,
                                        element[1].u16Val2,
                                        element[1].u8Val3);
// ...
free(str);