将存储在char中的十六进制/二进制值转换为int - 获得奇怪的结果

Question

I am having problems with converting hex/binary values received over socket to an integer.

I am getting the hex value over a socket with the following code:

void get_msg(int sockfd, char *buf)
{
  int n;
  bzero(buf,256);
  n = read(sockfd,buf,255);
  if (n < 0)
    error("ERROR writing socket");
}

I then pass the received binary located in *buf to this function to see them in hex (so that I can check if the calc_msg function below is working properly):

void print_msg(char *buf)
{
  int i;
  char *buffer = malloc(4);
  printf("[ ");
  for(i = 0; i < 4; i++) {
    printf("%02x ", ((const unsigned char *) buf)[i] & 0xff);
  }
  printf("]\n");
}

Now, in an attempt to convert the received message to decimal I call this function:

void calc_msg(char *buf)
{
  int i;
  int dec[3];
  for(i = 0; i < 4; i++) {
    dec[i] = buf[i];
    printf("Transformation %d: %u\n", i, dec[i]);
  }
}

This function only converts the message sometimes. Other times, it give ridiculously high values. Here is an example output:

[ 94 cc 78 28 ]
Transformation 0: 4294967188
Transformation 1: 4294967244
Transformation 2: 120
Transformation 3: 40

As you can see, 94 and cc end up being ridiculous values, whereas 78 and 28 convert just fine. The only relationship I see is that this only happens for higher values. I have not found any useful information using search engines.

Thanks! Surculus

Answer 1

int dec[3]; should be int dec[4]; , for starters.

Furthermore, this probably happens because all values greater than 127 (assuming an 8-bit char, but this is true in general to any value which doesn't fit into a signed char ) are sign extended during the signed upcast ( char -> int ). You'd be better off using unsigned char * for your buffers everywhere.

Answer 2

Hex 94 and cc are 148 and 204 in decimal. You are trying to store it in a signed char. Which cannot store number more than +127. This results in being represented as negative value in dec. When you convert to unsigned int gets converted to large values.