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Message-ID: <20140831001324.GL12888@brightrain.aerifal.cx>
Date: Sat, 30 Aug 2014 20:13:24 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH 2/8] additions to src/time
On Sat, Aug 30, 2014 at 08:46:35PM +0200, Jens Gustedt wrote:
> This adds two functions, thrd_sleep and timespec_get. Both have easy
> functional equivalences in POSIX, but these have subtle differences in
> the handling of errors.
>
> thrd_sleep forces concrete numerical values as error return
>
> timespec_get has a call interface that is incompatible with POSIX because
> it has a bogus coding for its clock constants, and also this clock
> constants must be returned in case of success. For the moment we only
> implement one single clock, TIME_UTC, and map this to
> CLOCK_REALTIME. This is the clock that we later need to measure time for
> the timedlock and timedwait.
>
> Also, other than for clock_gettime, C doesn't specify touching errno for
> timespec_get. Because CLOCK_REALTIME can be obtained very efficiently
> through VDSO and messing with errno is clearly the wrong step to go, we
> try to avoid this.
> ---
> src/time/thrd_sleep.c | 26 ++++++++++++++++++++++++++
> src/time/timespec_get.c | 31 +++++++++++++++++++++++++++++++
> 2 files changed, 57 insertions(+)
> create mode 100644 src/time/thrd_sleep.c
> create mode 100644 src/time/timespec_get.c
>
> diff --git a/src/time/thrd_sleep.c b/src/time/thrd_sleep.c
> new file mode 100644
> index 0000000..3dbfe47
> --- /dev/null
> +++ b/src/time/thrd_sleep.c
> @@ -0,0 +1,26 @@
> +#include <time.h>
> +#include <errno.h>
> +#include "syscall.h"
> +#include "libc.h"
> +#include "threads.h"
> +
> +/* Roughly __syscall already returns the right thing: 0 if all went
> + well or a negative error indication, otherwise. */
> +int thrd_sleep(const struct timespec *req, struct timespec *rem)
> +{
> + int ret = __syscall(SYS_nanosleep, req, rem);
> + switch (ret) {
> + case 0:
> + return 0;
> + /* error described by POSIX: */
> + /* EINTR The nanosleep() function was interrupted by a signal. */
> + /* The C11 wording is: */
> + /* -1 if it has been interrupted by a signal */
Purely coding style, but for musl we generally do multi-line comments
in this form:
/* line 1
* line 2
* line 3 */
Using // would be a second-choice alternative. I don't like the
right-alignment of */ with spaces because it will look like a mess to
someone reading it with a non-fixed-width font.
> diff --git a/src/time/timespec_get.c b/src/time/timespec_get.c
> new file mode 100644
> index 0000000..20080a0
> --- /dev/null
> +++ b/src/time/timespec_get.c
> @@ -0,0 +1,31 @@
> +#include <time.h>
> +#include "syscall.h"
> +#include "atomic.h"
> +
> +static int syscall_clock_gettime(clockid_t clk, struct timespec *ts)
> +{
> + return __syscall(SYS_clock_gettime, clk, ts);
> +}
> +
> +void *__vdsosym(const char *, const char *);
> +
> +/* There is no other implemented value than TIME_UTC, all other values
> + are considered erroneous. */
> +int timespec_get(struct timespec * ts, int base)
> +{
> + if (base != TIME_UTC) return 0;
> + int ret;
> +#ifdef VDSO_CGT_SYM
> + static int (*cgt)(clockid_t, struct timespec *);
> + if (!cgt) {
> + void *f = __vdsosym(VDSO_CGT_VER, VDSO_CGT_SYM);
> + if (!f) f = (void *)syscall_clock_gettime;
> + a_cas_p(&cgt, 0, f);
> + }
> + /* The vdso variants never fail, and thus never set errno. */
> + ret = cgt(CLOCK_REALTIME, ts);
> +#else
> + ret = syscall_clock_gettime(CLOCK_REALTIME, ts);
> +#endif
> + return ret < 0 ? 0 : base;
I'd rather not duplicate this code but just call __clock_gettime.
Unlike __clock_gettime, your duplicate code does not handle
pre-clock_gettime kernels and will return an error on them.
Rich
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