journald: fix memfd_create_syscall for 32 bit targets

[chrta]

Motivation

I would like to use tracing-journald on a 32 bit target (armv7).

Solution

On 32 bit targets (e.g. armv7) the syscall in memfd_create_syscall()
returns an i32, so this compilation error is printed:

| error[E0308]: mismatched types
|   --> .../tracing-journald-0.2.3/src/memfd.rs:27:9
|    |
| 25 |   fn memfd_create_syscall(flags: c_uint) -> i64 {
|    |                                             --- expected `i64`
|    |                                           because of return type
| 26 |       unsafe {
| 27 | /         syscall(
| 28 | |             SYS_memfd_create,
| 29 | |             "tracing-journald\0".as_ptr() as *const c_char,
| 30 | |             flags,
| 31 | |         )
|    | |_________^ expected `i64`, found `i32`
|    |
| help: you can convert an `i32` to an `i64`
|    |
| 31 |         ).into()
|    |          +++++++
|
| For more information about this error, try `rustc --explain E0308`.
| error: could not compile `tracing-journald` due to previous error
|

[hawkw]

While this works, I don't think it's really the most correct solution. The memfd_create system call returns a C int type, so I think that, instead of calling .into() on the return value, we should just change the definition of the memfd_create_syscall function so that, rather than returning an i64, it returns the c_int type from libc. This will always be the correct type on the current platform, and we can remove the into() call.

[chrta]

Unfortunately c_int type from libc does not work, since it is always i32, also for 64 bit targets. So in this case compilation for 64 bit targets fails.

Edit: But i would also like a better solution. But i did not find any type that is either a i32 or i64 depending on the target platform.

[hawkw]

Unfortunately c_int type from libc does not work, since it is always i32, also for 64 bit targets. So in this case compilation for 64 bit targets fails.

Edit: But i would also like a better solution. But i did not find any type that is either a i32 or i64 depending on the target platform.

Ah, sorry, my bad, I meant to say that it should be c_long, rather than c_int. The syscall function returns c_long, and I believe the corresponding linux libc function is also defined to return a long.

[ZanderBrown]

Yes, syscall(2) returns long however it's also true that memfd_create(2) is int

Thus it should be perfectly safe to force the c_long into c_int, in fact RawFd requires you to. But equally this should definitely be implemented in terms of c_long and c_int rather than plain rust types.

[chrta]

Thank you for the explanations, i am completely new to rust, so this is a little bit more difficult for me. I retried with converting the returned c_long into a c_int, because in the create() function the fd is expected to be convertible into RawFd.

[hawkw]

the cast to c_int seems fine to me, i think

[Ralith]

LGTM, thanks!