K&R Milliseconds Calculation - Initialization - Type Conversion

Question

In K&R (second edition) in section 4.9 (pg. 85) when discussing Initialization, an example of calculating the number of milliseconds in a day is given:

long day = 1000L * 60L * 60L * 24L

Why are long literals used here on the RHS of the assignment? In section 2.7 (Type Conversions) on pg 44 the following rules are given:

If either operand is long double, convert the other to `long double

Otherwise if either operand is double , convert the other to double

Otherwise if either operand is float, convert the other to float

Otherwise, convert char and short to int

Then, if either operand is long, convert the other to long.

For the calculation of long day = ..., surely int literals could be used here? Would they not automatically be converted to long based on the rules above?

I wrote the following:

long day1 = 1000L * 60L * 60L * 24L;

long day2 = 1000*60*60*24;

Within the debugger, both day1 and day2 were long. What was the purpose of using long literals?

[Why write 1,000,000,000 as 1000*1000*1000 in C?](https://stackoverflow.com/a/40637622/2410359) may be useful. — chux - Reinstate Monica, Jun 05 '20 at 16:57
Because `1000*60*60*24` _might_ give you 23552 on systems with 16-bit `int`. — , Jun 05 '20 at 16:59
You seem to be assuming that an `int` is 32 bits. That was not the case when K&R was written. Even now, an `int` is allowed to be 16 bits, which is not large enough to hold the number of milliseconds in a day. — user3386109, Jun 05 '20 at 16:59

score 2 · Accepted Answer · answered Jun 05 '20 at 16:59

2

1000*60*60*24 is done using int math which may overflow when int is 16-bit.

1000L * 60L * 60L * 24L does not overflow as long is at least 32-bit.

Code could have been as below to achieve long math with all the Ls.

long day = 1000L * 60 * 60 * 24;

answered Jun 05 '20 at 16:59

chux - Reinstate Monica

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So the RHS needs at least one `long` literal to ensure it does not overflow before being assigned to `day`? – Nate Jun 05 '20 at 18:10
@nate not quite: `1000 * 60 * 60 * 24L` overflows 16-bit math as the product is evaluated left to right. – chux - Reinstate Monica Jun 05 '20 at 18:12
Ah ok! Thanks! Accepted answer. – Nate Jun 05 '20 at 19:08

score 1 · Answer 2 · answered Jun 05 '20 at 16:59

In standard C, an int is only guaranteed to hold 16 bit values, while a long is guaranteed to hold 32 bit values.

K&R predates standard C, but the standard derives from the rules that K&R had in place. So long is required to guarantee the value will fit.

K&R Milliseconds Calculation - Initialization - Type Conversion

2 Answers2