Question

我想编码包含字段编号和连线类型的标签，以用于协议缓冲区。我现在遇到的问题是，每当我的'tag'值低于'8'时，fprintf会在正确的值旁边写入其他值。即，而不是38，它打印38c0 3.如果标签的值是8或更高，脚本工作正常。下面是省略了无关紧要的行的代码。

int uint32_pack (uint8_t *fieldnumber, uint32_t value, uint8_t *out);

int main(){

uint32_t initvalue = 2;
int return_rv;
uint8_t *tag = (uint8_t *) malloc(sizeof(uint8_t));    
uint8_t *tempout= (uint8_t *) malloc(sizeof(uint32_t));
*tag = 7; //value to be encoded (won't work for values less than 8)

return_rv = uint32_pack (tag, initvalue, tempout);

free(tempout);

    }

/* === pack() === */
/* Pack an unsigned 32-bit integer in base-128 encoding, and return the number
 of bytes needed: this will be 5 or less. */

int uint32_pack (uint8_t *fieldnumber, uint32_t value, uint8_t *out)
{
  unsigned rv = 0;
  FILE *wiretypetag;
  int secondaryvalue;

  wiretypetag = fopen("wiretype.txt","w");


    //encodes wire type and the field number
    if (*fieldnumber <16){
       *fieldnumber <<= 3;
       fprintf(wiretypetag,"%x",fieldnumber[0]);
       }
    if (*fieldnumber < 32 && *fieldnumber > 15){
       *fieldnumber <<= 3;
       secondaryvalue = 0x01;
       fprintf(wiretypetag,"%x %x",fieldnumber[0],secondaryvalue);
           }
    if (*fieldnumber < 48 && *fieldnumber > 31){
        *fieldnumber += 0x10;
        *fieldnumber &= 0x1F;
        *fieldnumber <<= 3;
        secondaryvalue = 0x02;
        fprintf(wiretypetag,"%x %x",fieldnumber[0], secondaryvalue);
        }       
    if (*fieldnumber < 64 && *fieldnumber > 47){
        *fieldnumber &= 0x1F;
        *fieldnumber <<= 3;
        secondaryvalue = 0x03;
        fprintf(wiretypetag,"%x %x",fieldnumber[0], secondaryvalue);
        }       

  /* assert: value<128 */   
    out[rv++] = value;


    if (rv == 1){         
           fprintf(outfile,"%x",out[0]);
           }
    if (rv == 2){
           fprintf(outfile,"%x %x",out[0], out[1]);
           }    
    if (rv == 3){
           fprintf(outfile,"%x %x %x",out[0],out[1],out[2]);
           }
    if (rv == 4){
           fprintf(outfile,"%x %x %x %x",out[0],out[1],out[2],out[3]);
           }
    if (rv == 5){
           fprintf(outfile,"%x %x %x %x %x",out[0],out[1],out[2],out[3],out[4]);
           }

    fclose(wiretypetag);

    return rv;
}

Answer 1

 if (fieldnumber <16){

应该是

if (*fieldnumber <16){

Answer 2

您可以使用else在以下位置简化代码：

//encodes wire type and the field number
if (*fieldnumber <16){
   *fieldnumber <<= 3;
   fprintf(wiretypetag,"%d",fieldnumber[0]);
   }
if (*fieldnumber < 32 && *fieldnumber > 15){
   *fieldnumber <<= 3;

替换为：

//encodes wire type and the field number
if (*fieldnumber < 16) {
   *fieldnumber <<= 3;
   fprintf(wiretypetag, "%d", fieldnumber[0]);
   }
else if (*fieldnumber < 32) {
   *fieldnumber <<= 3;
...

这是一个常见的习语，你应该在适当的时候使用它 - 就像现在一样。它将提高代码的易读性。

~~这可能不是整个问题;它可能不是。~~

问题解释

事实上，使用else if链可以纠正代码。由于第一个if使用*fieldnumber运算符修改了<<=，因此在评估第二个条件时，*fieldnumber不再是7而是56，因此

的条件

if (*fieldnumber < 64 && *fieldnumber > 47){

也会执行，打印出更多信息。

else if链将通过确保只执行一个替代方案来解决这个问题。

因为你有：

unsigned rv = 0;

[...]
/* assert: value<128 */   
out[rv++] = value;

if (rv == 1){         
       fprintf(outfile,"%x",out[0]);
       }

应执行此fprintf()，但永远不应执行rv等于2,3,4,5的以下内容。

为什么fprintf打印额外的值

2 个答案:

问题解释