Re: gl_{,SN}PRINTF_DIRECTIVE_N wrongly fail on Ubuntu 18.04

[Bruno Haible <bruno@clisp.org>]

Hi Adrian,

> rpl_fprintf is wrongly being used on Ubuntu 18.04 due to:
> 
> $ cat test.c
> /* gl_PRINTF_DIRECTIVE_N */
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> static char fmtstring[10];
> static char buf[100];
> int main ()
> {
>   int count = -1;
>   /* Copy the format string.  Some systems (glibc with _FORTIFY_SOURCE=2)
>      support %n in format strings in read-only memory but not in writable
>      memory.  */
>   strcpy (fmtstring, "%d %n");
>   if (sprintf (buf, fmtstring, 123, &count, 33, 44, 55) < 0
>       || strcmp (buf, "123 ") != 0
>       || count != 4)
>     return 1;
>   return 0;
> }
> $ gcc -O2 test.c -o test && ./test
> *** %n in writable segment detected ***
> Aborted
> $ 

gnulib works as designed. gnulib is designed to override system function so as
to make them POSIX compliant. POSIX [1] specifies that support for %n in the
*printf functions is mandatory. As you have shown with the test program, glibc
does not provide %n support when the CFLAGS contain _FORTIFY_SOURCE=2; gnulib
rectifies that.

Bruno

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/printf.html