[PATCH v2] system_data_types.7: Add note about length modifiers and conversions to [u]intmax_t, and corresponding example

[Michael Kerrisk (man-pages)]

Hello Alex,

On 9/21/20 10:19 AM, Alejandro Colomar wrote:
> Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
> Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
> ---
> 
> Hi Michael,
> 
> I added the part about range checking, and used a type with defined
> limits to show a complete example.

Thanks! Still a few coments.

> Thanks,
> 
> Alex
> 
> 
>  man7/system_data_types.7 | 62 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 62 insertions(+)
> 
> diff --git a/man7/system_data_types.7 b/man7/system_data_types.7
> index dd1d01aab..ba1338179 100644
> --- a/man7/system_data_types.7
> +++ b/man7/system_data_types.7
> @@ -629,6 +629,68 @@ 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 integer types described in this page don't have
> +a corresponding length modifier for the
> +.BR printf (3)
> +and the
> +.BR scanf (3)
> +families of functions.
> +To print a value of an integer 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 type/

> +an intermediate temporary variable of type
> +.I intmax_t
> +or
> +.I uintmax_t
> +should be used.
> +When copying from the temporary variable to the actual variable,

s/actual/destination/

> +the value could overflow.
> +If POSIX provides lower and upper limits to the type,
> +the user should check that the value is within those limits,
> +before actually copying the value.
> +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 an integer type that doesn't have a length modifier.
> +The appropriate conversions from and to
> +.IR intmax_t ,
> +and the appropriate range checkings,
> +are used as explained in the notes section above:
> +.PP
> +.EX
> +#include <stdint.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <sys/types.h>
> +
> +int
> +main (void)
> +{
> +    static const char *const str = "500000 us in half a second";
> +    suseconds_t us;
> +    intmax_t    tmp;
> +
> +    /* Scan the number from the string into the temporary variable */
> +    sscanf(str, "%jd", &tmp);
> +
> +    /* Check that the value is within the valid range */
> +    if (tmp < -1 || tmp > 1000000)

I think the first part of the check here should be 'tmp < 0'.
(Yes, the defined range for the type must allow -1, but speaking
of -1 microseconds is nonsensical, right?

> +        exit(EXIT_FAILURE);
> +
> +    /* Copy the value to the suseconds_t variable 'us' */
> +    us = tmp;
> +
> +    /* Print the value */
> +    printf("There are %jd us in half a second.\en", (intmax_t) us);
> +
> +    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/