Cast char to short | 易学教程

问题

char *buffer; 
short num; 
memcpy(&num, buffer, sizeof(short));

*buffer - pointer to the buffer, where number is situated in HEX view. I want to put this number in the variable num without calling memcpy. How can I do it?

number = (short) buffer; //DOESN'T work!

回答1:

For two byte short:

number = (short)(
           ((unsigned char)buffer[0]) << 8 | 
           ((unsigned char)buffer[1])
          );

For different short:

for (int i = 0; i < sizeof(short); i++)
        number = (number << 8) + ((unsigned char) buffer[i]);

or you'll have some macros for each size.

Also, see tristopia's comment about this making assumptions about endianness.

回答2:

All answers so far suggested using *(short *)buf, but that's not any good - it breaks the strict aliasing rule (the alignment of short is greater than that of char, so you can't do this without invoking undefined behavior).

The short answer is: you'd be better off using memcpy(), but if you really don't want that, then you can use unions and "type punning" (note that this may result in a trap representation of the buffer of bytes which may or may not be what you want):

union type_pun {
    char buf[sizeof(short)];
    short s;
};

union type_pun tp;
tp.buf[0] = 0xff;
tp.buf[1] = 0xaa; // whatever, if `short' is two bytes long
printf("%hd\n", tp.s);

回答3:

Based on your memcpy of sizeof(short) bytes, I'm guessing you want to get the first sizeof(short) bytes from where buffer is pointing at.

number = * (short *) buffer;

will do that for you, as other have pointed out.

You cannot take the pointer's address and put it in a short, so you need to dereference it to get the value in the memory instead.

回答4:

Assuming that with "situated in HEX view" you mean the number is stored in the string like "89AB", you can use the strtol function.

char* end;
num = (short)strtol(buffer, &end, 16);

This function converts the string to a long integer. There is no corresponding function that converts to a short directly, so you'll have to do a (short) anyway, but that wasn't the problem, was it?

来源：https://stackoverflow.com/questions/16851340/cast-char-to-short

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