Date: Thu, 21 Dec 2000 15:30:02 -0800 (PST) From: Ben Smithurst <ben@freebsd.org> To: freebsd-doc@freebsd.org Subject: Re: docs/23717: printf(3) is unclear about asprintf(3) return value Message-ID: <200012212330.eBLNU2442437@freefall.freebsd.org>
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The following reply was made to PR docs/23717; it has been noted by GNATS.
From: Ben Smithurst <ben@FreeBSD.org>
To: Poul-Henning Kamp <phk@critter.freebsd.dk>
Cc: FreeBSD-gnats-submit@freebsd.org
Subject: Re: docs/23717: printf(3) is unclear about asprintf(3) return value
Date: Thu, 21 Dec 2000 22:46:53 +0000
Poul-Henning Kamp wrote:
> printf(3) says:
>
> [...]
> int
> asprintf(char **ret, const char *format, ...)
> [...]
> Asprintf() and vasprintf() return a pointer to a buffer [...]
>
> It seems to me that asprintf() returns an int...
How about this.
...
-return a pointer to a buffer sufficiently large to hold the
-string in the
-.Fa ret
-argument;
+set
+.Fa *ret
+to be a pointer to a buffer sufficiently large to hold the formatted string.
The full diff with a few other changes is here...
Index: printf.3
===================================================================
RCS file: /usr/cvs/src/lib/libc/stdio/printf.3,v
retrieving revision 1.19
diff -u -r1.19 printf.3
--- printf.3 2000/11/10 17:44:59 1.19
+++ printf.3 2000/12/21 22:44:37
@@ -82,7 +82,7 @@
family of functions produces output according to a
.Fa format
as described below.
-.Fn Printf
+.Fn printf
and
.Fn vprintf
write output to
@@ -114,19 +114,12 @@
.Xr stdarg 3 )
are converted for output.
.Pp
-These functions return
-the number of characters printed
-(not including the trailing
-.Ql \e0
-used to end output to strings).
-.Pp
-.Fn Asprintf
+.Fn asprintf
and
.Fn vasprintf
-return a pointer to a buffer sufficiently large to hold the
-string in the
-.Fa ret
-argument;
+set
+.Fa *ret
+to be a pointer to a buffer sufficiently large to hold the formatted string.
This pointer should be passed to
.Xr free 3
to release the allocated storage when it is no longer needed.
@@ -135,10 +128,10 @@
and
.Fn vasprintf
will return -1 and set
-.Fa ret
+.Fa *ret
to be a NULL pointer.
.Pp
-.Fn Snprintf
+.Fn snprintf
and
.Fn vsnprintf
will write at most
@@ -153,7 +146,7 @@
argument, the string was too short
and some of the printed characters were discarded.
.Pp
-.Fn Sprintf
+.Fn sprintf
and
.Fn vsprintf
effectively assume an infinite
@@ -557,6 +550,27 @@
a field; if the result of a conversion is wider than the field width, the
field is expanded to contain the conversion result.
.Pp
+.Sh RETURN VALUE
+These functions return
+the number of characters formatted
+(not including the trailing
+.Ql \e0
+used to end output to strings).
+If a system error is encountered
+(for example,
+if memory allocations fails for
+.Fn asprintf
+or if a write failure occurs on an output stream),
+-1 will be returned.
+Note that
+.Fn snprintf
+and
+.Fn vsnprintf
+return the number of characters which should have been written to the
+specified buffer,
+which may be more than the number of characters actually written if the
+specified buffer was too small.
+.Pp
.Sh EXAMPLES
To print a date and time in the form `Sunday, July 3, 10:02',
where
@@ -585,14 +599,22 @@
#include <stdarg.h>
char *newfmt(const char *fmt, ...)
{
- char *p;
- va_list ap;
- if ((p = malloc(128)) == NULL)
- return (NULL);
- va_start(ap, fmt);
- (void) vsnprintf(p, 128, fmt, ap);
- va_end(ap);
- return (p);
+ char *p;
+ va_list ap;
+ if ((p = malloc(128)) == NULL)
+ return (NULL);
+ va_start(ap, fmt);
+ if (vsnprintf(p, 128, fmt, ap) >= 128) {
+ /*
+ * The buffer wasn't big enough; handle this case as
+ * appropriate. Here, missing characters is assumed to be a
+ * fatal error so the string is free'd and NULL is returned.
+ */
+ free(p);
+ p = NULL;
+ }
+ va_end(ap);
+ return (p);
}
.Ed
.Sh SEE ALSO
--
Ben Smithurst / ben@FreeBSD.org / PGP: 0x99392F7D
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