php extension >> bug

Question

To write a PHP extension, I use >> in it but unexpectedly it goes wrong.

code:

printf("%08x ", (W[16]));
printf("%08x ", (W[16]) >> 17);
printf("%08x ", 2425545216 >> 17);

result:

9092e200 40c04849 00004849

note:

W[16]=0x9092e200 = 2425545216, In c ,the code works right. But in php extension ,>> dosen't padding 0 to the left.

php_version: PHP: 7.1.7 thank you for your help.

Do not edit your question to include an answer. Upvote any answers you found helpful, and mark accepted the one answer that answered your question. See https://stackoverflow.com/help/someone-answers. — miken32, Sep 26 '17 at 20:39

score 3 · Accepted Answer · answered Aug 08 '17 at 04:06

In C, the result of bitwise right shift operation on a signed integer is implementation-defined. See answers to this question, for instance.

The value 0x9092e200 can be interpreted as unsigned integer 2425545216 as well as signed integer -1869422080. For example, the following code outputs -1869422080 2425545216:

int x = 0x9092e200;
printf("%d %d",  x, (unsigned)x);

So if you want to fill the vacant bits with zeroes, cast W[16] to unsigned, e.g.:

printf("%08x ",  ((unsigned int)W[16]) >> 17);

php extension >> bug

1 Answers1