Kris wrote: I must have missed this in the documentation somewhere. What is the default argument passing convention for structs and classes? Presumably they are both by reference? I ask because it appears as though some of my structs are being passed by value, and I can't change the argument type to 'inout' because the struct is statically declared as a const ('inout' causes a compile error).
In computing, D-Bus (for 'Desktop Bus'), is a software bus, is an inter-process communication. Two processes can use a D-Bus connection to exchange messages directly between them, but this is not the. From the kernel mediating, closing race conditions, and allowing D-Bus to be used during boot and shutdown (as. D Programming Language. Learn; Documentation. Language Reference; Library Reference.
Any help would be much appreciated. - Kris Default which is 'in' works like a copy (by value). Now.supposedly D will optimise the code if no copy is really needed.
For classes since they are really a pointer to a class the pointer is passed, so changes made to that class will effect the passed in one. -Anderson. This identifies a syntactic weakness in D then, as Walter alluded to when he wrote 'It (perhaps?) eliminates the need for reference (&) declarations.' Vis-a-vis 'out/inout' in the reference documentation. One should presumably strive to avoid using pointers as arguments (unless there's particular reason for doing so), so one would then move on to using 'inout' instead to force a call-by-reference. By doing so, the user is not forced into thinking about '&' et.
And all is indeed sweetness and light. However, you can't pass a struct as an 'inout' argument if it is declared const, for obvious reasons. Therefore you either have to remove the const declaration from the struct, or live with the pushing gobs of unnecessary data onto the stack. Neither of these are appropriate long-term solutions. This is hardly a showstopper, but it would be good to get it onto the list for post-v1.0 fixes. Sauls' wrote in message news:c65ick$ 6ul$1@ digitaldaemon. As J said, the default (in) is always pass-by-value (although it behaves like reference for classes, because of how they're stored).
You can always use a pointer. Yes yes I know. The function code would really work identically (since D does away with '-' syntax in favor of '.' ) but the caller would have to toss in the darn '&'. It'd work though. Or at least should. Sauls -Invironz.
Time To Get Rid Of errno Posted Aug 21, 2015 0:11 UTC (Fri) by ldo (guest, #40946) Parent article: The fundamental problem is that the errno convention has outlived its usefulness. The Linux kernel calls return an error code directly, but to be POSIX-compatible, glibc has to squirrel these away in errno. Which requires all this complicated wrapper code, as well as a whole extra mechanism to make errno thread-safe. I recently hit the situation where the write(2) call didn’t write all the bytes I gave it to disk, with no error indication in errno. The only says this can happen. If, for example, there is insufficient space on the underlying physical medium, or the RLIMITFSIZE resource limit is encountered (see setrlimit(2)), or the call was interrupted by a signal handler after having written less than count bytes. In other words, you don’t know why it happened.
( to post comments). Ofcourse, if a language wants to skip libc then they can. That just reinforces the fact that the syscall interface can never be changed to return more information.
FWIW I disagree with the OP, it is nice to be able to know that a syscall either succeeded or failed and not some halfway state. If you did a write and the write was short the write still succeeded. POSIX does specify that if the write size is less than PIPEBUF length then it will succeed or fail atomically.
If you have to write your code to handle the case where some data was written but you also have to handle an error code, that just feels more fragile. All the cases where it would be useful to return more information the specific syscall has made allowances for it, for example recvmsg. The fact that there are syscalls that are badly designed is a problem with the interface and not the mechanism. I find the ip/tc tools use of netlink here pretty bad, they return EINVAL and you have to hope there's something useful in the kernel log. Would it have killed them to add an extra field for 'extended error code'? EINTR is a familiar one.
And there are some well-understood processes that want a light touch. But overwhelmingly, the right thing to do in the face of failure is to back off, report it, and wait for somebody to ask to try again after they've re-plugged the cable or entered the right address or something. Have everything already set up for a failure before you go in, so you only need to do more if it succeeds, not if it fails. In the real world of system calls there are so many things to go wrong that the odds that whatever clever response you've coded won't make things worse are heavily against you. My problem is not so much with errno and short read/write calls. That behavior is maybe surprising, but it is well-defined, and it has been well-defined pretty much ever since there was such a thing as UNIX.
I have a completely different issue with 'errno'. The bulk of the time that I had to make raw system calls has been in extremely low-level code. When the code executes, I can't make much of a guarantee about the execution environment.
Quite frequently, there is no such thing as an 'errno' variable (e.g. Because I just called clone, and didn't set up thread local storage yet).
This means, I would need libinux to have zero dependencies on any libc code. No accesses to 'errno', no accesses to thread local storage, no cancellation points, no locking, no calls to atfork handlers, nothing! But things get even more complicated than that. By default, the dynamic linker lazily resolves library functions. This means, whenever I make a call into libinux, there is a chance that the dynamic loader gets called and makes all sorts of calls that are incompatible with my particular requirements. In other words, all of libinux would either need to be inline functions, or there needs to be a way to fully resolve its symbols on demand. It is quite possible that my needs are a little unusual, as I have been writing very low-level and Linux specific code.
But that's probably something that people will end up wanting to use libinux for. Other than that, yes, I am fully in favor of libinux giving easy and direct access to all Linux system calls.
That feature is long overdue and would be very welcome. I also feel that having wrapper functions that make system calls easier to use is wonderful. I sometimes need the exact raw system calls; and when I do, I am OK with researching the idiosyncrasies of the kernel API and making sure I get things right. But most of the time, I don't need this much control and I actually appreciate having helpers that allow the compiler to make sure I don't do anything stupid. The syscall ABI isn't really all that better.
If you think using syscall(NRxxx) is the solution to all of your problems, then you haven't looked beyond the x86 cpu on your desktop;). Some functions, like ptrace, have overlap between valid values and errors. In some cases it returns arbitrary data, so you cannot know whether 0xffffffff is because the data was 0xffffffff or -EPERM (on a 32bit system). You simply have to make assumptions that it's always valid based on other syscalls. Glibc doesn't treat.all.
negative values as being errors - it caps it at different values. On x86, it treats -1,-4095 (or should it be -4095,-1?) as an errno value. That way you aren't limiting yourself to 31bits, but (2^32 - 4096) possible valid values. Further, the convention for returning errno values isn't consistent across architectures. Some (most) will normalize into one register, but a few split it - at least ia64 & mips do. That way there is no confusion whether there was an error. Further further, some syscalls have to deal with raw C calling conventions.
Namely, some ABIs (like arm, mips, and ppc) require uint64t to be split on even/odd pairs. So instead of doing: syscall(SYSreadahead, fd, (uint32t)(offset), (uint32t)(offset 32), count) you have to insert a 0 after the fd by hand: syscall(SYSreadahead, fd, 0, (uint32t)(offset), (uint32t)(offset 32), count) further further further, just because you call a specific syscall by name, it does not mean it's going to be the same across architectures. Alpha is a pretty big example of this - there are many syscalls that don't exist like NRgetpid. Instead they named it NRgetxpid. They made a lot of decisions so as to be compatible with OSF (after all, surely OSF is more important than this toy 'linux' project, and will obivously outlive it). Or g'luck trying to do something as simple as mmap - there's NRmmap, NRmmap2, and arches are not consistent as to how the offsets are used (maybe they're shifted?). The syscall(2) man page has a lot of good discussion in it.
You should use the symbols provided by the C library. Stat if you have a path string, fstat if you have a fd, and fstatat if you have a dir fd. As for the syscalls you quoted, there are other stat variants (stat64 and fstatat64 at least). There's really no guarantee your code will compile or run properly when you call the syscalls directly.
C libraries provide stable APIs/ABIs, including emulating newer functionality when the kernel is old (e.g. The.at syscalls could be emulated in userspace when the kernel was too old by utilizing /proc/self/fd/, but you'd have to call glibc's fstatat and not the kernel's syscall(NRnewfstatat)).