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Add examples to show why this is a terrible API. OK jmc@ henning@

OPENBSD_5_5
millert 11 years ago
parent
commit
fb3c12e47e
1 changed files with 60 additions and 1 deletions
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      src/lib/libc/string/stpcpy.3

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src/lib/libc/string/stpcpy.3 View File

@ -1,4 +1,4 @@
.\" $OpenBSD: stpcpy.3,v 1.4 2013/09/25 21:49:31 millert Exp $
.\" $OpenBSD: stpcpy.3,v 1.5 2013/09/25 21:50:18 millert Exp $
.\"
.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" All rights reserved.
@ -103,7 +103,66 @@ if the length of
.Fa src
is greater than or equal to
.Fa len .
.Sh EXAMPLES
The most common use of
.Fn stpcpy
is to build up a string from multiple elements.
The following example builds up a pathname from
directory and file components using
.Fn stpcpy :
.Bd -literal -offset indent
char *dir, *file, pname[PATH_MAX];
\&...
if (strlen(dir) + strlen("/") + strlen(file) >= sizeof(pname))
goto toolong;
stpcpy(stpcpy(stpcpy(pname, dir), "/"), file);
.Ed
.Pp
However, the size check required to avoid a buffer overflow is error
prone since the check can become out of sync with the code that
performs the copy.
.Pp
One might expect that
.Fn stpncpy
could be safely used instead, but it suffers from the same defects as
.Fn strncpy .
The example below using
.Fn stpncpy
is even more prone to error and will not detect when truncation occurs:
.Bd -literal -offset indent
char *dir, *file, pname[PATH_MAX];
char *p1, *p2;
\&...
p1 = stpncpy(pname, dir, sizeof(pname) - 1);
p2 = stpncpy(p1, "/", sizeof(pname) - 1 - (p1 - pname));
stpncpy(p2, file, sizeof(pname) - 1 - (p2 - pname));
pname[sizeof(pname) - 1] = '\e0';
.Ed
.Pp
A safer (and simpler) approach is to use
.Fn snprintf :
.Bd -literal -offset indent
char *dir, *file, pname[PATH_MAX];
int len;
\&...
len = snprintf(pname, sizeof(pname), "%s/%s", dir, file);
if (len >= sizeof(pname))
goto toolong;
.Ed
.Pp
In most cases, it is better to use
.Fn snprintf ,
.Fn strlcpy ,
or
.Fn strlcat .
.Sh SEE ALSO
.Xr snprintf 3 ,
.Xr strcpy 3 ,
.Xr strlcpy 3 ,
.Xr strncpy 3


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