On 07/12/2024 05:26, Keith Thompson via Cygwin wrote:
Brian Inglis wrote:
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$10.2g\n", b, a);
}
should produce this output:
a:123456789876543210 b: 0.33
Under Cygwin (fully updated), with "gcc c.c -o c && ./c", the output is:
a:140732550844138 b: 7.1e-315
Confirmed with gcc 12.4 and minor tweaks to constant data types: printf is
ignoring arg positions:
[SNIP]
It's not always ignoring arg positions. I think there's an interaction
between the "1$" / "2$" position specification and relatively complex
format specifiers. The following case works correctly:
$ cat c2.c
#include <stdio.h>
int main(void) {
int a = 42;
double b = 1.0/3.0;
printf("a:%2$d b:%1$g\n", b, a);
printf("a:%1$d b:%2$g\n", a, b);
printf("a:%d b:%g\n", a, b);
}
$ gcc c2.c -o c2 && ./c2
a:42 b:0.333333
a:42 b:0.333333
a:42 b:0.333333
$
(And the version of gcc shouldn't matter. printf is implemented in
newlib. The code in question should be in newlib/libc/stdio/vfprintf.c.)
Not sure if this is the same/similar/different to the problem I reported
at [1]
[1] https://sourceware.org/pipermail/newlib/2023/020374.html
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