Hi Petr, On Sun, Dec 7, 2008 at 6:37 PM, Petr Baudis <pasky@xxxxxxx> wrote: > The result of this operation is undefined as specified in C99, see > > http://sourceware.org/bugzilla/show_bug.cgi?id=7075 > > Signed-off-by: Petr Baudis <pasky@xxxxxxx> > > diff --git a/man3/printf.3 b/man3/printf.3 > index b2ed86a..f8377a7 100644 > --- a/man3/printf.3 > +++ b/man3/printf.3 > @@ -750,6 +750,9 @@ or > The array must contain a terminating null wide character, unless a > precision is given and it is so small that the number of bytes written > exceeds it before the end of the array is reached. > + > +If the source array and the target buffer of the function overlap, > +results are undefined. > .TP > .B C > (Not in C99, but in SUSv2.) Thanks for the report. I agree that there's a problem with the page, but I don't think this patch is sufficient, nor does is it made in quite the right place. (This isn't a point that relates specifically to "%s", but rather to the *s*printf() functions.) I made a much bigger change that goes into more detail. The patch, destined for 3.16, is shown below. Does it look okay to you? Cheers, Michael --- a/man3/printf.3 +++ b/man3/printf.3 @@ -133,6 +133,17 @@ string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities of .BR stdarg (3)) are converted for output. + +C99 and POSIX.1-2001 specify that the results are undefined if a call to +.BR sprintf (), +.BR snprintf (), +.BR vsprintf (), +or +.BR vsnprintf () +would cause to copying to take place between objects that overlap +(e.g., if the target string array and one of the supplied input arguments +refer to the same buffer). +See NOTES. .SS "Return value" Upon successful return, these functions return the number of characters printed (not including the @@ -851,6 +862,26 @@ and conversion characters \fBa\fP and \fBA\fP. glibc 2.2 adds the conversion character \fBF\fP with C99 semantics, and the flag character \fBI\fP. .SH NOTES +Some programs imprudently rely on code such as the following + + sprintf(buf, "%s some further text", buf); + +to append text to +.IR buf . +However, the standards explicitly note that the results are undefined +if source and destination buffers overlap when calling +.BR sprintf (), +.BR snprintf (), +.BR vsprintf (), +and +.BR vsnprintf (). +.\" http://sourceware.org/bugzilla/show_bug.cgi?id=7075 +Depending on the version of +.BR gcc (1) +used, and the compiler options employed, calls such as the above will +.B not +produce the expected results. + The glibc implementation of the functions .BR snprintf () and -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
