Re: [PATCH] system_data_types.7: Add note about length modifiers and conversions to [u]intmax_t, and corresponding example

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

Hi Alex,

On 9/20/20 11:40 PM, Alejandro Colomar wrote:
> Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
> Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
> ---
> Hi Michael,
> 
> On 9/20/20 10:20 PM, Michael Kerrisk (man-pages) wrote:
>> PS It occurs to me that this manual page is a suitable place 
>> to explain the general technique of casting integral system
>> data types to [u]intmax_t for the purpose of printf(). Would
>> you like to add this, Alex?
> 
> Sure.  Good idea!
> 
> Hope you like the patch :)

Good in principle, but some tweaks required.

>  man7/system_data_types.7 | 52 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/man7/system_data_types.7 b/man7/system_data_types.7
> index 5128e1f01..1fcc09dae 100644
> --- a/man7/system_data_types.7
> +++ b/man7/system_data_types.7
> @@ -626,6 +626,58 @@ See also:
>  .SH NOTES
>  The structures described in this manual page shall contain,
>  at least, the members shown in their definition, in no particular order.
> +.PP
> +Most of the types described in this page don't have a corresponding

s/types/integer types/

> +length modifier for the
> +.BR printf (3)
> +and the
> +.BR scanf (3)
> +families of functions.
> +To print a value of a type that doesn't have a length modifier,
> +it should be converted to
> +.I intmax_t
> +or
> +.I uintmax_t
> +by an explicit cast.
> +To scan into a variable of a type that doesn't have a length modifier,

s/a type/an integer/

> +an intermediate temporary variable of type
> +.I intmax_t
> +or
> +.I uintmax_t
> +should be used.

Hmmm -- I wonder if we need to say something about range checking.
I mean, what if time_t is narrower than intmax_t in the example below?
(It's not, on my x86-64 system.) The problem of course is how to
construct such a range check in the absence of any appropriate
POSIX constants (e.g., there is no TIME_T_MAX).

> +The example below shows how these conversions should be done.
> +.SH EXAMPLES
> +The program shown below scans from a string and prints a value stored in
> +a variable of a type that doesn't have a length modifier.
> +The appropriate conversions from and to
> +.I intmax_t
> +are used as explained in the notes section above:
> +.PP
> +.EX
> +#include <stdint.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <time.h>
> +
> +int
> +main (void)
> +{
> +    static const char *const str = "There are 60 s in an hour";

Either "60" needs to be 3600 or "hour" needs to be "minute".

> +    time_t   secs;
> +    intmax_t tmp;
> +
> +    /* Scan the number from the string into the temporary variable */
> +    sscanf(str, "There are %jd", &tmp);
> +
> +    /* Copy the value to the time_t variable secs */
> +    secs = tmp;
> +
> +    /* Print the value */
> +    printf("There are %jd seconds in an hour!\en", (intmax_t) secs);

See my previous comment. A change may be required in the line above.

> +
> +    exit(EXIT_SUCCESS);
> +}
> +.EE
>  .SH SEE ALSO
>  .BR feature_test_macros (7),
>  .BR standards (7)

Thanks,

Michael


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