Q34920: Mixed Expressions Can Cause Unsigned Division

Article: Q34920
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 12-OCT-1988

The ANSI C standard specifies that when a signed and unsigned integer
are involved in an arithmetic expression, the signed integer is to be
converted to the type of the unsigned before expression evaluation. If
the signed integer is negative, the sign will be lost and the value
will change, even though the bit pattern will remain the same.

This situation is especially a problem with functions that return a
value of type size_t, which is an alias for unsigned int in our
implementation. Note that this is not a problem or a code generation
error, this is the expected behavior.

For example, if the statement

i = (4 - strlen("1234567890")) / 2;

is executed, the value of i will be 32765 rather than -3 as one might
expect because the expression (4 - strlen("1234567890")) is an
unsigned integer expression with the value of 65530 rather than a
signed expression with the value of -6. If you look at the code
generated, you'll notice that an instruction that performs unsigned
division is generated rather than a one that performs signed division.

To avoid this behavior, use a typecast on the return value
of strlen() so that it is treated as an int. For the example
above, the statement

i = (4 - (int) strlen("1234567890")) / 2;

would have generated the desired value of -3. Note that even this code
would have failed if the length of the string was greater than 32767;
however, the incorrect code above fails if the length of the string is
greater than four.

Runtime functions that return values of type size_t, unsigned, or
unsigned long include the following:

strtoul, _getlinestyle, fread, fwrite, _clear87, _control87,
_status87, _fmsize, _memavl, _memmax, _msize, _nmsize,
stackavail, strcspn, strlen, strspn, _bios_disk,
_bios_equiplist, _bios_keybrd, _bios_memsize, _bios_printer,
_bios_serialcom, _bios_timeofday, _dos_allocmem, _dos_close,
_dos_creat, _dos_creatnew, _dos_findfirst, _dos_findnext,
_dos_freemem, _dos_getdate, _dos_getdiskfree,
_dos_getfileattr, _dos_getftime, _dos_gettime, _dos_open,
_dos_read, _dos_setblock, _dos_setdate, _dos_setfileattr,
_dos_setftime, _dos_settime, _dos_write, FP_OFF, FP_SEG,
long_lrotl, long_lrotr, _rotl, and _rotr

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