Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Signed-off-by: Alejandro Colomar <colomar.6.4.3@xxxxxxxxx>
---
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(+)
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
+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)
--
2.28.0
