path: root/hurd/porting/guidelines.mdwn

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This is a compilation of common porting problems and their solutions.


Additionally to this page, also see the section *General Porting Issues* of
<http://www.debian.org/ports/hurd/hurd-devel-debian><!-- TODO: merge these two
pages.  -->, as well as further Debian-specific [[running/debian/porting]]
information.

There is a separate page about [[System_API_Limitations]].

You may ask on the [[mailing lists/bug-hurd]] mailing list for details or
questions about fixing bugs.


## <a name="Undefined_bits_confname_h_tt_mac"> Undefined `bits/confname.h` macros (`PIPE_BUF`, ...) </a>

If macro `XXX` is undefined, but macro `_SC_XXX` or `_PC_XXX` is defined in `bits/confname.h`, you probably need to use `sysconf`, `pathconf` or `fpathconf` to obtain it dynamicaly.

The following macros have been found in this offending situation (add more if you find them): `PIPE_BUF`

An example with `sysconf`: (when you find a `sysconf` offending macro, put a better example)

    #ifndef XXX
    #define XXX sysconf(_SC_XXX)
    #endif
    /* offending code using XXX follows */

An example with `fpathconf`:

    #ifdef PIPE_BUF
       read(fd, buff, PIPE_BUF - 1);
    #else
       read(fd, buff, fpathconf(fd, _PC_PIPE_BUF) - 1);
    #endif
    /* note we can't #define PIPE_BUF, because it depends
       on the "fd" variable */

## <a name="Bad_File_Descriptor"> Bad File Descriptor </a>

If you get Bad File Descriptor error when trying to read from a file (or accessing it at all), check the `open()` invocation. The second argument is the access method. If it is a hard coded number instead of a symbol defined in the standard header files, the code is screwed and should be fixed to either use `O_RDONLY`, `O_WRONLY` or `O_RDWR`. This bug was observed in the `fortunes` and `mtools` packages for example.

## <a name="PATH_MAX_tt_MAX_PATH_tt_MAXPATHL"> `PATH_MAX` / `MAX_PATH` / `MAXPATHLEN` </a>

Every unconditionalized use of `PATH_MAX`, `MAX_PATH` or `MAXPATHLEN` is a POSIX incompatibility. If there is no upper limit on the length of a path (as its the case for GNU), this symbol is not defined in any header file. Instead, you need to either use a different implementation that does not rely on the length of a string or use `sysconf()` to query the length at runtime. If `sysconf()` returns -1, you have to use `realloc()` to allocate the needed memory dynamically. Usually it is thus simpler to just use dynamic allocation. Sometimes the amount is actually known. Else, a geometrically growing loop can be used: for instance, see [Alioth patch](http://alioth.debian.org/tracker/download.php/30628/410472/303735/1575/cpulimit-path-max-fix.patch) or [Pulseaudio patch](http://bugs.debian.org/cgi-bin/bugreport.cgi?msg=5;filename=patch-pulse;att=1;bug=522100). Note that in some cases there are GNU extensions that just work fine: when the `__GLIBC__` macro is defined, `getcwd()` calls can be just replaced by `get_current_dir_name()` calls.

## <a name="ARG_MAX"> `ARG_MAX` </a>

Same rationale as `PATH_MAX`. There is no limit on the number of arguments.

## <a name="IOV_MAX"> `IOV_MAX` </a>

Same rationale as `PATH_MAX`. There is no limit on the number of iovec items.

## <a name="MAXHOSTNAMELEN_tt_"> `MAXHOSTNAMELEN` </a>

Same as `PATH_MAX`. When you find a `gethostname()` function, which acts on a static buffer, you can replace it with Neal's [xgethostname function](http://ftp.walfield.org/pub/people/neal/xgethostname/) which returns the hostname as a dynamic buffer. For example:

Buggy code:

    char localhost[MAXHOSTNAMELEN];
    ...
    gethostname(localhost, sizeof(localhost));

Fixed code:

    #include "xgethostname.h"
    ...
    char *localhost;
    ...
    localhost = xgethostname();
    if (! localhost)
      {
        perror ("xgethostname");
        return ERROR;
      }
    ...
    /* use LOCALHOST.  */
    free (localhost);

## <a name="NOFILE_tt_"> `NOFILE` </a>

Replace with `getrlimit(RLIMIT_NOFILE,...)`

## <a name="GNU_specific_define_tt_"> </a> GNU specific `#define`

If you need to include specific code for GNU/Hurd using `#if` ... `#endif`, then you can use the `__GNU__` symbol to do so. But think (at least) thrice! before doing so. In most situations, this is completely unnecessary and will create more problems than it may solve. Better ask on the mailing list how to do it right if you can't think of a better solution.

## <a name="sys_errlist_tt_vs_strerror_tt_"> `sys_errlist[]` vs. `strerror()` </a>

If a program has only support for `sys_errlist[]` you will have to do some work to make it compile on GNU, which has dropped support for it and does only provide `strerror()`. Steinar Hamre writes about `strerror()`:

`strerror()` should be used because:

* It is the modern, POSIX way.
* It is localized.
* It handles invalid signals/numbers out of range. (better errorhandling and not a buffer-overflow-candidate/security risk)

`strerror()` should always be used if it is available. Unfortunaly there are still some old non-POSIX systems that do not have `strerror()`, only `sys_errlist[]`.

Today, only supporting `strerror()` is far better than only supporting `sys_errlist[]`. The best (from a portability viewpoint), however is supporting both. For configure.in, you will need:

    AC_CHECK_FUNCS(strerror)

To `config.h.in`, you need to add:

    #undef HAVE_STRERROR

Then something like:

            #ifndef HAVE_STRERROR
            static char *
            private_strerror (errnum)
                 int errnum;
            {
              extern char *sys_errlist[];
              extern int sys_nerr;

              if (errnum > 0 && errnum <= sys_nerr)
                return sys_errlist[errnum];

              return "Unknown system error";
            }
            #define strerror private_strerror
            #endif /* HAVE_STRERROR */

You can for example look in the latest coreutils (the above is a simplified version of what I found there.) Patches should of course be sent to upstream maintainers, this is very useful even for systems with a working `sys_errlist[]`.

Of course, if you don't care about broken systems (like MS-DOG) not supporting `strerror()` you can just replace `sys_errlist[]` directly (upstream might not accept your patch, but debian should have no problem)

## <a name="C++_error_t"> C++, `error_t` and `E*` </a>

On the Hurd, `error_t` is an enumeration of the `E*` constants. However, C++
does not like `E*` integer macros being directly assigned to that enumeration. In short, replace

        error_t err = EINTR;

by

        error_t err = error_t(EINTR);

## <a name="Missing_termio_h_tt_"> Missing `termio.h` </a>

Change it to use `termios.h` (check for it properly with autoconf `HAVE_TERMIOS_H` or the `__GLIBC__` macro)

Also, change calls to `ioctl(fd, TCGETS, ...)` and `ioctl(fd, TCSETS, ...)` with `tcgetattr(fd, ...)` and `tcsetattr(fd, ...)`.

## <a name="AC_HEADER_TERMIO_tt_"> `AC_HEADER_TERMIO` </a>

The autoconf check for `AC_HEADER_TERMIO` tryes to check for termios, but it's only really checking for termio in `termios.h`. It is better to use `AC_CHECK_HEADERS(termio.h termios.h)`

## <a name="_IOT"> missing `_IOT` </a>

This comes from ioctls.  Fixing this is easy if the structure members can be expressed by using the _IOT() macro, else it's simply impossible... See `bits/termios.h` for an instance:

`#define _IOT_termios /* Hurd ioctl type field.  */ \
  _IOT (_IOTS (tcflag_t), 4, _IOTS (cc_t), NCCS, _IOTS (speed_t), 2)`

The rationale behind is that on the Hurd ioctl numbers actually encode how the
data should be transferred via RPC: here `struct termios` holds 4 members of
type `tcflag_ts`, then `NCCS` members of type `cc_tsi` and finaly 2 members of
type `speed_ts`, so the RPC mecanism will know how to transfer them.

As you can see, this limits the number of contiguous kinds of members to 3, and
in addition to that (see the bitfield described in `ioctls.h`), the third kind
of member is limited to 3 members.  This is a design limitation, there is no way
to overcome it at the moment.

Note: if a field member is a pointer, then the ioctl can't be expressed
this way, and that makes sense, since the server you're talking to
doesn't have direct access to your memory. Ways other than ioctls must
then be found.

## <a name="SA_SIGINFO"> `SA_SIGINFO` </a>

Not implemented, packages may be fixed for working around this: use void sighandler(int num) prototype and sa_handler field.

## <a name="SOL_IP"> `SOL_IP` </a>

Not implemented yet.

## <a name="HZ"> `HZ` </a>

Linuxish and doesn't even make sense since the value may vary according to the running kernel.  Should use `sysconf(_SC_CLK_TCK)` or `CLK_TCK` instead.

## <a name="SIOCDEVPRIVATE"> `SIOCDEVPRIVATE` </a>

Oh, we should probably provide it.

## <a name="MAP_NORESERVE"> `MAP_NORESERVE` </a>

Not POSIX, but we could implement it.

## <a name="O_DIRECT"> `O_DIRECT` </a>

Long story to implement.

## <a name="PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP"> `PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP` </a>

We could easily provide it;

## <a name="PTHREAD_STACK_MIN"> `PTHREAD_STACK_MIN` </a>

We should actually provide it.

## <a name="types"> `linux/types.h` or `asm/types.h` </a>

These are not POSIX, `sys/types.h` and `stdint.h` should be used instead.

## <a name="iopl"> `iopl` </a>

Not supported. Try to replace with `ioperm(0, 65536, 1)` (conditionned with `__GNU__` as that will not work in Linux).

## <a name="iopl"> `semget`, `sem_open` </a>

Not implemented, will always fail.  Use `sem_init()` instead if possible.

## <a name="net/..."> `net/if_arp.h`, `net/ethernet.h`, etc. </a>

Not implemented, not POSIX.  Try to disable the feature in the package.

## <a name="parport"> <linux/parport.h> <linux/ppdev.h> </a>

There is no programming interface for the parallel port on GNU/Hurd yet.

## <a name="errno"> `errno` values </a>

When dealing with `errno`, you should always use the predefined error codes defined with the `E*` constants, instead of manually comparing/assigning/etc with their values.

For example (C/C++):

    /* check whether it does not exist */
    if (errno == 2)
      ...

or Python:

    # check whether it does not exist
    try:
      ...
    except OSError, err:
      err.errno == 2:
        ...

This is wrong, as [the actual values of the `E*` are unspecified (per POSIX)](http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_03.html#tag_02_03). You must always use the predefined constants for the possible errors.

For example (C/C++):

    /* check whether it does not exist */
    if (errno == ENOENT)
      ...

With Python, you can use the [`errno` module](http://docs.python.org/library/errno.html) for the various constants:

    # check whether it does not exist
    try:
      ...
    except OSError, err:
      import errno
      err.errno == errno.ENOENT:
        ...