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#include <stdio.h>
/* ... */
nblocks = 1 + ((nbytes - 1) >> 9);  /* hardBUFSIZ to= modify,512 uses "magic number"= 2^9 */

The programmer's assumption underlying this code is that "everyone knows that BUFSIZ equals 512," and right-shifting nine bits is the same (for positive numbers) as dividing by 512. However, if BUFSIZ changes to 1024 on some systems, modifications are difficult and error-prone.

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#include <stdio.h>
/* ... */
nblocks = 1 + (nbytes - 1) / BUFSIZ;

Most modern C compilers will optimize this code appropriately.

Risk Assessment

Improper use of macros may result in unexpected arithmetic resultsHardwiring constants renders code potentially nonportable; in fact it will produce unexpected under any circumstances in which the constant changes.

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