为什么回调函数不更改全局结构中的值？

问：

我调用了这个结构，其中包含指向数组的指针和该数组的长度，现在当调用回调函数时，它确实将输出复制到分配的缓冲区中，但 len 没有改变StringchargrabOutputptr

#define  BUF_SIZE 1024

typedef struct String
{
    char * ptr;
    int len;
} String;


HANDLE Child_IN_R = NULL;
HANDLE Child_IN_W = NULL;
HANDLE Child_OUT_R = NULL;
HANDLE Child_OUT_W = NULL;
HANDLE handle = NULL;                
String str;
DWORD grabOutput(LPVOID lpParam )
{
    BOOL success = FALSE;
    DWORD dwRead, total =0;
    char buffer[BUF_SIZE];
    while(1)
    {
        success = ReadFile(Child_OUT_R, buffer, BUF_SIZE, &dwRead, NULL);
        if (!success) break;
        memcpy(str.ptr+total, buffer, dwRead);
        total += dwRead;
        str.ptr = realloc(str.ptr, total+BUF_SIZE); 
    }
    str.len = total; // this does not change the value of str.len 
    return 0;
}

为什么这段代码不改变全局的值？str.lenString str