On 2024-12-06 19:16, Keith Thompson via Cygwin wrote:
The use of "1$", "2$" et al in printf format specifiers is a
POSIX-specific feature.
On Cygwin (newlib) this is handled correctly in most cases, but one
example I tried misbehaves.
The output is correct on other implementations, including glibc and
musl on Ubuntu.
This C program:
#include <stdio.h>
int main(void) {
long long a = 123456789876543210;
double b=1.0/3;
printf("a:%2$8.8lld b:%1$10.2g\n", b, a);
}
should produce this output:
a:123456789876543210 b: 0.33
Under Cygwin (fully updated), with "gcc c.c -o c && ./c", the output is:
a:140732550844138 b: 7.1e-315
Confirmed with gcc 12.4 and minor tweaks to constant data types: printf is
ignoring arg positions:
$ gcc --version && tail posnprintf.c && gcc -o posnprintf{,.c} && ./posnprintf
gcc (GCC) 12.4.0
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
#define I18LL 123456789876543210LL
#define THIRD 1.0/3.0
int main(void) {
long long a = I18LL;
double b = THIRD;
printf("expected\ta:%18lld\tb:%10.6f\n", I18LL, THIRD);
printf("parameters\ta:%8.8lld\tb:%10.2g\n", a, b);
printf("positional\ta:%2$8.8lld\tb:%1$10.2g\n", b, a);
printf("ignoreposn\tb:%8.8lld\ta:%10.2g\n", b, a);
}
posnprintf.c: In function ‘main’:
posnprintf.c:12:33: warning: format ‘%lld’ expects argument of type ‘long long
int’, but argument 2 has type ‘double’ [-Wformat=]
12 | printf("ignoreposn\tb:%8.8lld\ta:%10.2g\n", b, a);
| ~~~~~~^ ~
| | |
| long long int double
| %8.8f
posnprintf.c:12:43: warning: format ‘%g’ expects argument of type ‘double’, but
argument 3 has type ‘long long int’ [-Wformat=]
12 | printf("ignoreposn\tb:%8.8lld\ta:%10.2g\n", b, a);
| ~~~~~^ ~
| | |
| double long long int
| %10.2lld
expected a:123456789876543210 b: 0.333333
parameters a:123456789876543210 b: 0.33
positional a:4599676421641834218 b: 2.1e-300
ignoreposn b:4599676419421066581 a: 2.1e-300
--
Take care. Thanks, Brian Inglis Calgary, Alberta, Canada
La perfection est atteinte Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer but when there is no more to cut
-- Antoine de Saint-Exupéry
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