2 Chars to Short in C | 易学教程

问题

I've got 2 chars.

Char 128 and Char 2.

How do I turn these chars into the Short 640 in C?

I've tried

unsigned short getShort(unsigned char* array, int offset)
{
    short returnVal;
    char* a = slice(array, offset, offset+2);
    memcpy(&returnVal, a, 2);
    free(a);
    return returnVal;
}

But that didn't work, it just displays it as 128. What's the preferred method?

回答1:

I found that the accepted answer was nearly correct, except i'd run into a bug where sometimes the top byte of the result would be 0xff...

I realized this was because of C sign extension. if the second char is >= 0x80, then converting 0x80 to a short becomes 0xff80. Performing an 'or' of 0xff80 with anything results in the top byte remaining 0xff.

The following solution avoids the issue by zeroing out the top byte of b during its implicit conversion to a short.

short c = (((short)a) << 8) | (0x00ff & b);

回答2:

Probably the easiest way to turn two chars, a and b, into a short c, is as follows:

short c = (((short)a) << 8) | b;

To fit this into what you have, the easiest way is probably something like this:

unsigned short getShort(unsigned char* array, int offset)
{
    return (short)(((short)array[offset]) << 8) | array[offset + 1];
}

回答3:

I see that there is already an answer, but I'm a bit puzzled about what was going on with your original attempt. The following code shows your way and a technique using a union. Both seem to work just fine. I suppose you might have been running into an endianness problem. Anyway, perhaps this demonstration will be useful even if your problem is already solved.

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

int main()
{
    short returnVal;
    char a[2];
    union {
        char ch[2];
        short n;
    } char2short;

    a[0] = 128;
    a[1] = 2;

    memcpy(&returnVal, a, 2);

    printf("short = %d\n", returnVal);

    char2short.ch[0] = 128;
    char2short.ch[1] = 2;

    printf("short (union) = %d\n", char2short.n);

    return 0;
}

Outputs:

short = 640
short (union) = 640

回答4:

I see that you are not actually trying to shift bits but assemble the equivelant of hex values together, like you would color values in CSS.

Give this code a shot:

    char b1=128,b2=2;
    char data[16];

    sprintf((char *)data,"%x%x",(BYTE)b2,(BYTE)b1);
    short result=strtol(data,(char **)NULL, 16);

来源：https://stackoverflow.com/questions/17768625/2-chars-to-short-in-c

标签

int

type-conversion

short