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#include <stdio.h>
#include <string.h>
typedef unsigned packed_t;
/*
3 2 1 0
byte byte byte byte
1000 1100 1110 1111
xbyte(..., 2);
>> 8
0000 0000 1000 1100
& 0xFF
0000 0000 0000 1100
0 <= x < 7 -> id
-8 < x < 0 -> minus
*/
int xbyte(packed_t word, int bytenume);
int xbyte(packed_t word, int bytenum) {
int max_bytes = 3;
int max_bits = max_bytes << 3;
int shift_bits = (3 - bytenum) << 3;
return (int) word << shift_bits >> max_bits;
}
/*
Copy integer into buffer if space is available
WARNING: The following code is buggy
size_t is an unsigned. So one of the operands gets casted.
Either
int -> unsigned
or
unsigned -> int
apparently the result of unsigned - int or int - unsigned -> unsigned
*/
void buggy_copy_int(int val, void *buf, int maxbytes) {
/*
Check if maxbytes will cause problems when converted to unsigned.
*/
if (maxbytes < 0) {
return;
}
if (maxbytes >= sizeof(val)) {
memcpy(buf, (void *) &val, sizeof(val));
}
}
int main() {
int maxbytes = 4;
char buf[maxbytes];
int i;
for (i=0; i<maxbytes; ++i) {
buf[i] = 'a';
}
buggy_copy_int(32, buf, maxbytes);
for (i=0; i<maxbytes; ++i) {
printf("%x\n", buf[i]);
}
return 0;
printf("%x\n", xbyte(0x0000FF00, 1));
return 0;
}
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