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system_data_types.7: Describe use of printf()/scanf() modifiers for integer data types 

Add note about length modifiers and conversions to [u]intmax_t,
and add a corresponding example.

Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Alejandro Colomar 2020-09-21 15:32:57 +02:00 committed by Michael Kerrisk
parent 104fcc3c16
commit f5d2c2efb2
1 changed files with 73 additions and 0 deletions
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man7/system_data_types.7
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@ -629,6 +629,79 @@ 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 an integer type
that doesn't have a length modifier,
an intermediate temporary variable of type
.I intmax_t
or
.I uintmax_t
should be used.
When copying from the temporary variable to the destination variable,
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 of suseconds_t */
if (tmp < -1 || tmp > 1000000) {
fprintf(stderr, "Scaned value might overflow!\en");
exit(EXIT_FAILURE);
}
/* Copy the value to the suseconds_t variable 'us' */
us = tmp;
/* Even though suseconds_t can hold the value -1,
it represents an error code */
if (us < 0) {
fprintf(stderr, "Scanned an error code!\en");
exit(EXIT_FAILURE);
}
/* 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)