c - Printing inverse of a character -


this question has answer here:

i know why printf statement displays character value 4 byte value (fffffffe) in following code. since character of 1 byte, thought should getting result of fe 1 byte value.

int main(void) {     unsigned char a4=1;     char b4=1;     printf("inverted a4 = %x\t b4= %x\n",~a4, ~b4);     return(0); }

when any arithmetic or bit operation on char quantity, silently converted int first. called integer promotion. affects short (and signed char, unsigned char, , unsigned short) well. (technically, if sizeof(unsigned short) == sizeof(unsigned int) unsigned short promote unsigned int rather int, you're not trip on system characteristic anymore, though it's still allowed c standard.)

this also happens char , short when passed through anonymous arguments printf (or other variadic function).

so, code

unsigned char a4=1; char b4=1; printf("inverted a4 = %x\t b4= %x\n",~a4, ~b4);

is equivalent to

... printf("inverted a4 = %x\t b4= %x\n", (int) ~(int)a4, (int) ~(int)b4);

(the cast after ~ doesn't have written anyway emphasize conversion happen both because of arithmetic , because of argument passing.)

there no way turn behavior off. effect wanted, write

static_assert(char_bit == 8); printf("inverted a4 = %02x\tb4 = %02x\n",        (~(unsigned int)a4) & 0xffu,        (~(unsigned int)b4) & 0xffu);

the difference between , code suggested mikecat , nick bit operations done on unsigned quantities. bit operations have undefined behavior when applied signed quantities, , can never remember rules are, avoid doing of them signed quantities. (the change %x %02x aesthetics.)

systems on static_assert fails vanishingly rare nowadays, again, still allowed c standard. include document program's expectations. (the 0xffu masks , %02x formatters wrong on system different value char_bit.)


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