C data type conversion

Question

Can I be sure that data conversion always succeeds? If I have an int and I use mmap(2), is the int converted to off_t and size_t? Why are there all those different sizes? And are there conversion function?

You can definitely not be sure that conversions always succeed. For example, conversions from integer types to floating-point types and from floating-point to integer result in undefined behavior if the values cannot be represented in the new type. — EOF, Oct 08 '15 at 22:43
Can you be sure that conversions always succeed, in general? No. And in your specific example, `off_t` isn't even standard C, it's POSIX. Although `size_t` and `int` are both guaranteed to be at least 16-bits. — Iskar Jarak, Oct 08 '15 at 22:47
Also, by _"Why are there all those different sizes?"_, what do you mean? Do you mean why are different types not all the same size, or why are there different types, or...? — Iskar Jarak, Oct 08 '15 at 22:49
Of interest: https://github.com/git/git/commit/aafa5df0df39036c6500846acd3db5b75d264a3b — VonC, Jun 11 '21 at 20:56

score 2 · Accepted Answer · edited May 23 '17 at 11:58

I am not sure about the usage of the mmap function you are doing since mmap returns a pointer.

Regarding conversion you can basically convert any variable to anything using a cast, however it is your responsibility to verify that this will work for example:
```
int i;
double d;    
d = 42.3;
i = (int)d;
printf("i = %d", i);
```

Output will be 42, without the cast (int) some compilers would probably complain and warn you that you will lose floating precision.

About the why of all these different sizes, first thing size_t is meant to represent a size so you can grossly think of it as an unsigned int, regarding why not use a unsigned int rather than a typedef'ed "size_t", see this post unsigned int vs size_t.

C data type conversion

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