system_data_types.7: Add note about length modifiers and conversions to [u]intmax_t, and corresponding example
Commit Message
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 :)
Cheers,
Alex
man7/system_data_types.7 | 52 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 52 insertions(+)
Comments
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
@@ -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
+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,
+an intermediate temporary variable of type
+.I intmax_t
+or
+.I uintmax_t
+should be used.
+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";
+ 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);
+
+ exit(EXIT_SUCCESS);
+}
+.EE
.SH SEE ALSO
.BR feature_test_macros (7),
.BR standards (7)