linux-kernel - Re: [PATCH 2/2] futex.2: Use appropriate types

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Date:   Fri, 30 Oct 2020 14:46:39 +0100
From:   Alejandro Colomar <colomar.6.4.3@...il.com>
To:     mtk.manpages@...il.com
Cc:     linux-man@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] futex.2: Use appropriate types

BTW, apparently the kernel doesn't use 'const' for 'utime'
('timeout' in the manual page),
but effectively, it doesn't modify it, AFAICS.

Should the kernel use 'const'?
Is there a reason for the kernel not using 'const'?
Should we do anything about it in the manual page?

Thanks,

Alex

On 2020-10-30 13:39, Alejandro Colomar wrote:
> The Linux kernel uses the following:
> 
> kernel/futex.c:3778:
> SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
> 		struct __kernel_timespec __user *, utime, u32 __user *, uaddr2,
> 		u32, val3)
> 
> Since there is no glibc wrapper, use the same types the kernel uses.
> 
> Signed-off-by: Alejandro Colomar <colomar.6.4.3@...il.com>
> ---
>   man2/futex.2 | 27 ++++++++++++++-------------
>   1 file changed, 14 insertions(+), 13 deletions(-)
> 
> diff --git a/man2/futex.2 b/man2/futex.2
> index 837adbd25..73de71623 100644
> --- a/man2/futex.2
> +++ b/man2/futex.2
> @@ -26,12 +26,13 @@ futex \- fast user-space locking
>   .nf
>   .PP
>   .B #include <linux/futex.h>
> +.B #include <stdint.h>
>   .B #include <sys/time.h>
>   .PP
> -.BI "int futex(int *" uaddr ", int " futex_op ", int " val ,
> +.BI "long futex(uint32_t *" uaddr ", int " futex_op ", uint32_t " val ,
>   .BI "          const struct timespec *" timeout , \
>   " \fR  /* or: \fBuint32_t \fIval2\fP */"
> -.BI "          int *" uaddr2 ", int " val3 );
> +.BI "          uint32_t *" uaddr2 ", uint32_t " val3 );
>   .fi
>   .PP
>   .IR Note :
> @@ -581,8 +582,8 @@ any of the two supplied futex words:
>   .IP
>   .in +4n
>   .EX
> -int oldval = *(int *) uaddr2;
> -*(int *) uaddr2 = oldval \fIop\fP \fIoparg\fP;
> +uint32_t oldval = *(uint32_t *) uaddr2;
> +*(uint32_t *) uaddr2 = oldval \fIop\fP \fIoparg\fP;
>   futex(uaddr, FUTEX_WAKE, val, 0, 0, 0);
>   if (oldval \fIcmp\fP \fIcmparg\fP)
>       futex(uaddr2, FUTEX_WAKE, val2, 0, 0, 0);
> @@ -1765,11 +1766,11 @@ Child  (18535) 4
>   #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \e
>                           } while (0)
>   
> -static int *futex1, *futex2, *iaddr;
> +static uint32_t *futex1, *futex2, *iaddr;
>   
>   static int
> -futex(int *uaddr, int futex_op, int val,
> -      const struct timespec *timeout, int *uaddr2, int val3)
> +futex(uint32_t *uaddr, int futex_op, uint32_t val,
> +      const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3)
>   {
>       return syscall(SYS_futex, uaddr, futex_op, val,
>                      timeout, uaddr2, val3);
> @@ -1779,9 +1780,9 @@ futex(int *uaddr, int futex_op, int val,
>      become 1, and then set the value to 0. */
>   
>   static void
> -fwait(int *futexp)
> +fwait(uint32_t *futexp)
>   {
> -    int s;
> +    long s;
>   
>       /* atomic_compare_exchange_strong(ptr, oldval, newval)
>          atomically performs the equivalent of:
> @@ -1794,7 +1795,7 @@ fwait(int *futexp)
>       while (1) {
>   
>           /* Is the futex available? */
> -        const int one = 1;
> +        const uint32_t one = 1;
>           if (atomic_compare_exchange_strong(futexp, &one, 0))
>               break;      /* Yes */
>   
> @@ -1811,13 +1812,13 @@ fwait(int *futexp)
>      so that if the peer is blocked in fpost(), it can proceed. */
>   
>   static void
> -fpost(int *futexp)
> +fpost(uint32_t *futexp)
>   {
> -    int s;
> +    long s;
>   
>       /* atomic_compare_exchange_strong() was described in comments above */
>   
> -    const int zero = 0;
> +    const uint32_t zero = 0;
>       if (atomic_compare_exchange_strong(futexp, &zero, 1)) {
>           s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);
>           if (s  == \-1)
>