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

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Alex,

On 10/30/20 2:46 PM, Alejandro Colomar wrote:
> 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?

I'm not sure about the kernel, but I think we don't need to 
worry in the manual page.

Thanks,

Michael

> 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@gmail.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)
>>


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/