一道百度的改错题

（15分）下面一段代码，想在调用f2(1)时打印err1，调用f2(2)时打印err4，但是代码中有一些问题，请做尽可能少的修改使之正确。
    
1    static int f1(const char *errstr, unsigned int flag) {
2        int copy, index, len;
3        const static char **__err = {“err1”, “err2”, “err3”, “err4”};
4    
5        if(flag & 0x10000)
6            copy = 1;
7        index = (flag & 0x300000) >> 20;
8     
9        if(copy) {
10            len = flag & 0xF;
11            errstr = malloc(len);
12            if(errstr = NULL)
13                return -1;
14            strncpy(errstr, __err[index], sizeof(errstr));
15        } else
16            errstr =  __err + index;    
17    }
18
19    void f2(int c) {
20        char *err;
21    
22        swtch(c) {
23        case 1:
24            if(f1(err, 0x110004) != -1)
25                printf(err);
26        case 2:
27            if(f1(err, 0x30000D) != -1)
28                printf(err);
29        }
30 }  

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

#define DEBUG


static int f1( char **errstr, unsigned int flag) //change char *errstr -->char **errstr

{
        int copy, index, len;
        //int i=0;

        
       
        // static char **__err = {"err1", "err2","err3","err4"}; //此行换为下行

        static char __err[4][10] = {"err1", "err2","err3","err4"};
       
        if(flag & 0x10000) copy = 1;
        else copy=0; //add 如果不加的话 copy 没有初始化

       
       
        index = (flag & 0x300000) >> 20;
       
        #ifdef DEBUG
            printf("copy %u index:%u\n",copy,index);
        #endif

        if(copy)
        {
                len = flag & 0xf;
                *errstr =(char*) malloc(len+1);
                if( (*errstr) == NULL ) //change = ---> ==

                return -1;
                strncpy(*errstr, __err[index-1], len); //change -1 sizeof(errstr)=4 -->len

       
        }
        else
                {
                        //*errstr = __err+index; appear :

                        //cannot convert char (*)[10] to char * in assignment

                           
                        *errstr = __err [index];
                }
               
               
                return 0;
}

void f2(int c)
{
        char *err;
       
        switch(c)
        {
        case 1:
       
                if(f1(&err, 0x110004) != -1)
                printf("%s\n",err);
                break; //add

       
        case 2:
       
                if(f1(&err, 0x30000D) != -1) //change f2 ->f1

                printf("%s\n",err);
               
                break; //add

        }
        free(err); //add

}

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

    f2(1);
    f2(2);
    return 0;
}
  看来 这类改错题 还是不能 小瞧的。