Do you know how to reproduce the warnings when compiling that C script ?

I didn't want to die wondering (even if it killed me), so I eventually came up with this C program that reproduces the warnings:

/* try.c */
#include <stdio.h>
#include <stdlib.h>

void foo(double);

int main(void) {

 /* The value assigned to 'd' has no *
  * effect on the warning message.   */
 double d = 0.;
 foo(d);                 
}

void foo(double d) {
 char buf[127];
 sprintf (buf, "%.*g\n", 126, d);
 printf("%s\n", buf);
}
[download]

Build with: gcc -o try.exe try.c -Wformat-overflow
That compilation produces the following noise:

try.c: In function ‘foo’:
try.c:17:17: warning: ‘%.*g’ directive writing between 1 and 133 bytes
+ into a region of size 127 [-Wformat-overflow=]
  sprintf (buf, "%.*g", 126, d);
                 ^~~~
try.c:17:16: note: assuming directive output of 132 bytes
  sprintf (buf, "%.*g", 126, d);
                ^~~~~~
try.c:17:2: note: ‘sprintf’ output between 2 and 134 bytes into a dest
+ination of size 127
  sprintf (buf, "%.*g", 126, d);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[download]

which is essentially the same as the warnings I received when compiling perl.

Apparently, the perl compilation process (with -O2 optimization) determines that the number of digits being requested is 126.
It is then correctly calculated that the number of bytes written will be between 1 and 133 - which allows for the decimal point, a possible leading '-', a possible 'e', and a possible exponent of (maximum) size of 4.
In those warnings, you'll see that the "1 and 133" changes to "2 and 134" - when the terminating NULL byte is included in the count.

I haven't investigated just how the perl source compilation process makes the determination that the digit count has to be 126. It might just be a bug in the -O2 optimization - certainly, no warnings are emitted if the optimization level is reduced to less that -O2.

As I mentioned previously, if the number of digits specified in the "%g" formatting is higher than 91, then the processing switches to a different block of code, so the buffer size of 127 is certainly large enough.
I haven't looked into how or why that change occurs when digits > 91. It's not often that people will request more digits than 91 - so I'm not presently inclined to wade through the whys and wherefores of that processing path. It seems to be working correctly, and IMO that's good enough for now, at least.

Update: Now that I understand how the warning is being created, I think it should be fairly simple to amend the perl source so that this warning is eliminated.

Cheers,
Rob

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