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and a null pointer as the destination buffer.
Therefore, the call
<pre>
	nch = snprintf(NULL, 0, fmtstring, /* other arguments */ );
</pre>
computes
the number of characters required
for the fully-formatted
string.
With that number
(<TT>nch</TT>)
in hand,
you can then malloc a big-enough buffer
and make a second <TT>snprintf</TT> call to fill it.
</p><p>Yet another option is





the (nonstandard) <TT>asprintf</TT> function,
present in various C libraries including bsd's and GNU's,
which formats to

(and returns a pointer to)
a <TT>malloc</TT>'ed buffer,
like this:
<pre>
char *buf;
asprintf(&amp;buf, "%d = %s", 42, "forty-two");
/* now buf points to malloc'ed space containing formatted string */
</pre>
</p><p>Additional links:



<a href="../../stdio/asprintf.c">sample implementation of <TT>asprintf</TT>
</a></p>
<p>References:

C9X Sec. 7.13.6.6
<hr><hr><hr>
<a name="sprintfret">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/sprintfret.html"><!-- qtag -->Question 12.22</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What's the deal on <TT>sprintf</TT>'s return value?
Is it an <TT>int</TT> or a <TT>char&nbsp;*</TT>?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The Standard
says
that it returns an
<TT>int</TT>
(the number of characters written,
just like <TT>printf</TT> and <TT>fprintf</TT>).
Once upon a time,
in some C libraries,
<TT>sprintf</TT>
returned
the <TT>char&nbsp;*</TT> value of its first argument,
pointing
to the
completed
result
(i.e. analogous to <TT>strcpy</TT>'s return value).
</p>

<p>References:

ISO Sec. 7.9.6.5
<br>
PCS Sec. 11 p. 175
<hr><hr><hr>
<a name="getsvsfgets">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/getsvsfgets.html"><!-- qtag -->Question 12.23</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Why does everyone say not to use <TT>gets()</TT>?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Unlike <TT>fgets()</TT>,
<TT>gets()</TT>
cannot be told the size of the buffer it's to read into,
so it
cannot be prevented from overflowing
that
buffer
if an input line is longer than expected--and
Murphy's Law says that,
sooner or
later,
a larger-than-expected input line <em>will</em> occur.
<a href="../../stdio/rtmworm.html" rel=subdocument>[footnote]</a>






(It's possible to convince yourself that,
for some reason or another,
input lines longer than some maximum are impossible,
but it's also possible to be
mistaken,
<a href="../../stdio/oslinelen.html" rel=subdocument>[footnote]</a>
and in any case it's just
as easy to use <TT>fgets</TT>.)
</p><p>



The Standard <TT>fgets</TT> function
is a vast improvement over <TT>gets()</TT>,
although it's not perfect, either.
(If long lines are a real possibility,
their proper handling must be carefully considered.)
</p><p>One other difference between 
<TT>fgets()</TT> and <TT>gets()</TT>
is that <TT>fgets()</TT> retains the <TT>'\n'</TT>,
but it is straightforward to strip it out.
See question
<a href="faqcatbafd.html?sec=malloc#malloc1">7.1</a> 
for a code fragment illustrating
the replacement of <TT>gets()</TT> with <TT>fgets()</TT>.
</p>

<p>References:

Rationale Sec. 4.9.7.2
<br>
H&amp;S Sec. 15.7 p. 356
<hr><hr><hr>
<a name="printferrno">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/printferrno.html"><!-- qtag -->Question 12.24</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I thought I'd check <TT>errno</TT>
after a long string of <TT>printf</TT> calls,
to see if any of them had failed:
<pre>
	errno = 0;
	printf("This\n");
	printf("is\n");
	printf("a\n");
	printf("test.\n");
	if(errno != 0)
		fprintf(stderr, "printf failed: %s\n", strerror(errno));
</pre>

Why is it printing
something strange like
``printf failed: Not a typewriter''
when I redirect the output to a file?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Many implementations of the stdio package adjust their
behavior slightly if <TT>stdout</TT> is a terminal.
To make the determination,
these
implementations perform
some
operation which
happens to
fail
(with
ENOTTY)
if <TT>stdout</TT> is not a terminal.
Although the output operation goes
on to complete successfully, <TT>errno</TT> still contains
ENOTTY.
This behavior can be mildly confusing, but it is not
strictly incorrect, because
it
is only meaningful for a
program to inspect the contents of <TT>errno</TT> after an error has
been reported.
(More
precisely,
<TT>errno</TT> is only meaningful
after a library function
that sets <TT>errno</TT> on error
has returned an error code.)
</p><p>In general,
it's best to detect errors by checking
a function's
return value.
To check for any
accumulated

error after a long string of stdio calls,
you can use <TT>ferror</TT>.
See also questions
<a href="faqcat1d60.html?sec=stdio#feof">12.2</a>
and
<a href="faqcat38c2.html?sec=misc#errno">20.4</a>.
</p>




<p>References:

ISO Sec. 7.1.4, Sec. 7.9.10.3
<br>
CT&amp;P Sec. 5.4 p. 73
<br>
PCS Sec. 14 p. 254
<hr><hr><hr>
<a name="fgetpos">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/fgetpos.html"><!-- qtag -->Question 12.25</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What's the difference between <TT>fgetpos</TT>/<TT>fsetpos</TT>
and <TT>ftell</TT>/<TT>fseek</TT>?
<br>What are <TT>fgetpos</TT> and <TT>fsetpos</TT> good for?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
<TT>ftell</TT> and <TT>fseek</TT>
use type <TT>long&nbsp;int</TT>
to represent
offsets
(positions)
in a file,
and
may therefore be
limited to offsets
which can be represented in a <TT>long&nbsp;int</TT>.
(Type <TT>long&nbsp;int</TT> is not guaranteed to hold values larger
than
2**31-1,




limiting the maximum
offset
to 2 gigabytes).
The newer
<TT>fgetpos</TT> and <TT>fsetpos</TT>
functions,
on the other hand,
use a
special
typedef,
<TT>fpos_t</TT>,
to represent the offsets.
The type behind this typedef,
if chosen appropriately,
can represent arbitrarily large offsets,
so <TT>fgetpos</TT> and <TT>fsetpos</TT> can be used with
arbitrarily huge files.
<TT>fgetpos</TT> and <TT>fsetpos</TT> also
record
the state associated with multibyte streams.
See
also question <a href="faqcatd3c2.html?sec=decl#octabyte">1.4</a>.
</p>



<p>References:

K&amp;R2 Sec. B1.6 p. 248
<br>
ISO Sec. 7.9.1, Secs. 7.9.9.1,7.9.9.3
<br>
H&amp;S Sec. 15.5 p. 252
<hr><hr><hr>
<a name="stdinflush">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/stdinflush.html"><!-- qtag -->Question 12.26a</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I flush pending input so that a user's typeahead
isn't read at the next prompt?
Will <TT>fflush(stdin)</TT>
work?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
<TT>fflush</TT> is defined only for output streams.
Since its definition of ``flush''
is to complete the writing of buffered characters
(not to discard them),
discarding unread input
would not be an analogous meaning
for <TT>fflush</TT> on input streams.
See also question <a href="faqcat1d60.html?sec=stdio#stdinflush2">12.26b</a>.
</p>

<p>References:

ISO Sec. 7.9.5.2
<br>
H&amp;S Sec. 15.2
<hr><hr><hr>
<a name="stdinflush2">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/stdinflush2.html"><!-- qtag -->Question 12.26b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
If <TT>fflush</TT> won't work,
what can I use to flush input?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It depends on what you're trying to do.
If you're trying to get rid of
an unread newline
or other unexpected input
after calling <TT>scanf</TT>
(see questions <a href="faqcat1d60.html?sec=stdio#scanfinterlace">12.18a</a>-<a href="faqcat1d60.html?sec=stdio#scanfjam">12.19</a>),
you really need to rewrite or replace
the call to <TT>scanf</TT>
(see question <a href="faqcat1d60.html?sec=stdio#scanfprobs">12.20</a>).
Alternatively, you can consume the rest of a partially-read line
with a simple code fragment like
<pre>
	while((c = getchar()) != '\n' &amp;&amp; c != EOF)
		/* discard */ ;
</pre>
</p><p>(You may also be able to
use the curses <TT>flushinp</TT> function.)
</p><p>There is no standard way to discard unread characters from a
stdio input
stream.
Some vendors do implement <TT>fflush</TT>
so that <TT>fflush(stdin)</TT> discards unread characters,
although portable programs
cannot depend on this.
(Some versions of
the stdio library
implement <TT>fpurge</TT>
or <TT>fabort</TT> calls
which do the same thing,
but these aren't standard, either.)
Note, too, that flushing stdio input buffers
is not necessarily
sufficient:
unread characters can also accumulate in other, OS-level input
buffers.
If you're trying to actively discard
input
(perhaps in anticipation of issuing
an unexpected
prompt
to confirm a destructive action,
for which an accidentally-typed ``y'' could be disastrous),
you'll have
to use a system-specific technique to detect the presence of typed-ahead input;
see
questions
<a href="faqcatea63.html?sec=osdep#cbreak">19.1</a>
and <a href="faqcatea63.html?sec=osdep#readavail">19.2</a>.



Keep in mind that users can become frustrated
if you discard input that happened to be typed
too quickly.


</p>

<p>References:

ISO Sec. 7.9.5.2
<br>
H&amp;S Sec. 15.2
<hr><hr><hr>
<a name="fopenfp">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/fopenfp.html"><!-- qtag -->Question 12.27</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I wrote this routine which is supposed to open a file:
<pre>
	myfopen(char *filename, FILE *fp)
	{
		fp = fopen(filename, "r");
	}
</pre>
But when I call it like this:
<pre>
		FILE *infp;
		myfopen("filename.dat", infp);
</pre>
the <TT>infp</TT> variable
in the caller doesn't get set properly.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Functions in C always receive copies of their arguments,
so a function can never ``return'' a value to the 
caller by assigning to an argument.
See question <a href="faqcatabdc.html?sec=ptrs#passptrinit">4.8</a>.
</p><p>For this example,
one fix is to
change <TT>myfopen</TT> to return a <TT>FILE&nbsp;*</TT>:
<pre>
	FILE *myfopen(char *filename)
	{
		FILE *fp = fopen(filename, "r");
		return fp;
	}
</pre>
and call it like this:
<pre>
		FILE *infp;
		infp = myfopen("filename.dat");
</pre>
Alternatively,
have <TT>myfopen</TT> accept a <em>pointer</em> to a <TT>FILE&nbsp;*</TT>
(a pointer-to-pointer-to-<TT>FILE</TT>):
<pre>
	myfopen(char *filename, FILE **fpp)
	{
		FILE *fp = fopen(filename, "r");
		*fpp = fp;
	}
</pre>
and call it like this:
<pre>
		FILE *infp;
		myfopen("filename.dat", &amp;infp);
</pre>
<hr><hr><hr>
<a name="fopenmodec">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/fopenmodec.html"><!-- qtag -->Question 12.28</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I can't even get a simple <TT>fopen</TT> call to work!
What's wrong with
this call?
<pre>
	FILE *fp = fopen(filename, 'r');
</pre>
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
<TT>fopen</TT>'s mode argument must be a string,
like <TT>"r"</TT>,
not a character
like <TT>'r'</TT>.
See also question <a href="faqcate107.html?sec=charstring#strvschar">8.1</a>.
<hr><hr><hr>
<a name="constrpath">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/constrpath.html"><!-- qtag -->Question 12.28b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I open files with names like
``<TT>file1</TT>'',
``<TT>file2</TT>'',
``<TT>file3</TT>'',
etc.,
where the numeric part is controlled by a variable?
Basically I want ``<TT>file%d</TT>'',
like <TT>printf</TT>.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
You want <TT>printf</TT>'s close cousin <TT>sprintf</TT>,
which ``prints'' to a string:
<pre>
	char filename[FILENAME_MAX];
	sprintf(filename, "file%d", i);
	fp = fopen(filename, "r");
</pre>
<hr><hr><hr>
<a name="fopenpath">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/fopenpath.html"><!-- qtag -->Question 12.29</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
<TT>fopen</TT> is failing for certain pathnames.

</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See questions <a href="faqcatea63.html?sec=osdep#dospath">19.17</a> and <a href="faqcatea63.html?sec=osdep#fopenenv">19.17b</a>.
<hr><hr><hr>