[RFC v1] libio/: Add [v]aprintf()

[Adhemerval Zanella Netto]

> 	Projects have come up with APIs for this over time.  GNU and the
> 	BSDs have it as asprintf(3), but there seems to be consensus
> 	that this API isn't very well designed; evidence of this is that
> 	the behavior is slightly different in the various
> 	implementations, and has changes through history.

Could you add more details of what is wrong with the current asprintf glibc
implementation, besides the initial designed limitation ('int' return i
nstead of ssize_t)?

We don't have any bugs against asprintf on glibc bugzilla.

On 17/03/26 15:59, Alejandro Colomar wrote:
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
> ---
> 
> Hi Paul, Joseph,
> 
> Since nobody replied, I guess people are not strongly opposed to it, but
> they probably want to see a patch before spending time with this.
> 
> Thus, I've written a patch (or a draft of a patch).
> 
> This is the first time I add a function to glibc, and I find it quite
> difficult, so I'll need some help.  I've added the most obvious pieces:
> the prototypes and the implementation.  However, this doesn't work at
> all, so I guess I'm missing stuff.  Would you mind helping me figure out
> what I'm missing (or what's wrong)?
> 
> I tried building this simple test program, but I get linker errors:

You need to export the symbol by adding a version tag and its name on
stdio-common/Versions.

I don't have a strong opinion about this, albeit since gnulib is already
providing and that committee has shown interest I would way for C standard
inclusion. 

> 
> 	#include <stdio.h>
> 	#include <stdlib.h>
> 
> 	int
> 	main(void)
> 	{
> 		char *p;
> 
> 		p = aprintf("foo %d", 42);
> 		if (p == NULL)
> 			exit(1);
> 		puts(p);
> 	}
> 
> 
> Have a lovely night!
> Alex
> 
> 
>  libio/stdio.h    |  8 ++++++++
>  libio/vaprintf.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 61 insertions(+)
>  create mode 100644 libio/vaprintf.c
> 
> diff --git a/libio/stdio.h b/libio/stdio.h
> index 3bf6a1f6..3db363f1 100644
> --- a/libio/stdio.h
> +++ b/libio/stdio.h
> @@ -410,6 +410,14 @@ extern int __asprintf (char **__restrict __ptr,
>  extern int asprintf (char **__restrict __ptr,
>  		     const char *__restrict __fmt, ...)
>       __THROWNL __attribute__ ((__format__ (__printf__, 2, 3))) __wur;
> +
> +/* Write formatted output to a string dynamically allocated with `malloc'.  */
> +extern char *vaprintf (const char *__restrict __f, __gnuc_va_list __arg)
> +     __THROWNL __attribute__ ((__format__ (__printf__, 1, 0)))
> +     __attribute_malloc__;
> +extern char *aprintf (const char *__restrict __fmt, ...)
> +     __THROWNL __attribute__ ((__format__ (__printf__, 1, 2)))
> +     __attribute_malloc__;
>  #endif
>  
>  #ifdef __USE_XOPEN2K8
> diff --git a/libio/vaprintf.c b/libio/vaprintf.c
> new file mode 100644
> index 00000000..0ba147a0
> --- /dev/null
> +++ b/libio/vaprintf.c
> @@ -0,0 +1,53 @@
> +/* Copyright (C) 2026 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.
> +
> +   As a special exception, if you link the code in this file with
> +   files compiled with a GNU compiler to produce an executable,
> +   that does not cause the resulting executable to be covered by
> +   the GNU Lesser General Public License.  This exception does not
> +   however invalidate any other reasons why the executable file
> +   might be covered by the GNU Lesser General Public License.
> +   This exception applies to code released by its copyright holders
> +   in files containing the exception.  */
> +
> +#include "libioP.h"
> +#include <stdarg.h>
> +#include <stddef.h>
> +#include <stdio.h>
> +
> +char **
> +__vaprintf (const char *fmt, va_list args)
> +{
> +  char *p;
> +
> +  return (__vasprintf_internal (&p, fmt, args, 0) < 0 ? NULL : p;
> +}
> +ldbl_weak_alias (__vaprintf, vaprintf)
> +
> +char **
> +__aprintf (const char *fmt, ...)
> +{
> +  char *p;
> +  va_list ap;
> +
> +  va_start(ap, fmt);
> +  p = __vaprintf(fmt, ap);
> +  va_end(ap);
> +
> +  return p;
> +}
> +ldbl_weak_alias (__aprintf, aprintf)
> 
> Range-diff against v0:
> -:  -------- > 1:  e5defcbd libio/: Add [v]aprintf()