Using ssize_t vs int | 易学教程

问题

Code

I've got a function which I can write in one of the four possible ways:

int do_or_die(int retval);
int do_or_die(ssize_t retval);
ssize_t do_or_die(int t retval);   
ssize_t do_or_die(ssize_t retval);

And then it will be called with both of these ways for library functions:

written = do_or_die(write(...)); // POSIX write returns ssize_t
printed = do_or_die(printf(...)); // printf returns int

Questions

Which prototype should I use?
What types should I give to written and printed?

I want to have the most robust and standard code, while still having just one do_or_die function.

I am using C99 in this case, but if answer is different for C11, then I'd like to know that too, for future.

回答1:

There's no guarantee in the C or POSIX standards that sizeof(int) >= sizeof(ssize_t), nor the other way around. Typically ssize_t is larger than int, but the safe and portable option in C99 is to use intmax_t instead for the argument and the return value.

The only guarantees you have wrt. the relationship between int and ssize_t are:

int can store values of at least the range [-2^15 ... 2^15-1] per ISO C
ssize_t can store values of at least the range [-1 ... 2^15-1] per POSIX (see _POSIX_SSIZE_MAX).

(Interestingly, there isn't even a guarantee that ssize_t can store the negative counterparts of its positive range. It's not a signed size_t, but a "size type" with an error value.)

回答2:

Use types in a way:

you don't mix signed and unsigned types together and
you don't truncate values from larger types while storing them in smaller types (overflow/underflow)

ssize_t might be an alias for int, yet it is not standard C and might be environment specific.

If your program will run in specific environment, check whether sizeof(ssize_t) <= sizeof(int) and use int. Otherwise, use some other type T where sizeof(T) is greater or equal than both sizeof(int) and sizeof(ssize_t).

来源：https://stackoverflow.com/questions/19224655/using-ssize-t-vs-int

标签

type-conversion

standards