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<a name="fupdate">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/fupdate.html"><!-- qtag -->Question 12.30</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm trying to update a file in place,
by using <TT>fopen</TT> mode <TT>"r+"</TT>,
reading a certain string,
and writing back a modified string,
but it's not working.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Be sure to call <TT>fseek</TT>

before you write,
both
to seek back
to the beginning of the string you're trying to overwrite,
and
because
an <TT>fseek</TT> or <TT>fflush</TT>

is always required between
reading and writing in the read/write <TT>"+"</TT> modes.

Also, remember that you can only overwrite characters
with the same number of replacement
characters;
there is no way to insert or delete characters in place.
Finally,
remember
that overwriting in text mode
may truncate the file at that point,
and that you may have to preserve line lengths.
See also question
<a href="faqcatea63.html?sec=osdep#insdelrec">19.14</a>.
</p>
<p>References:

ISO Sec. 7.9.5.3
<hr><hr><hr>
<a name="insdelrec">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/insdelrec.html"><!-- qtag -->Question 12.31</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I insert or delete a line
(or record)
in the middle of a file?
</p>
<p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcatea63.html?sec=osdep#insdelrec">19.14</a>.
<hr><hr><hr>
<a name="fdfilename">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/fdfilename.html"><!-- qtag -->Question 12.32</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I recover the file name given an open
stream?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcatea63.html?sec=osdep#fdfilename">19.15</a>.
<hr><hr><hr>
<a name="freopen">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/freopen.html"><!-- qtag -->Question 12.33</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I redirect <TT>stdin</TT> or <TT>stdout</TT>
to a file
from within a program?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Use <TT>freopen</TT>.
If you're calling a function <TT>f()</TT>
which writes to <TT>stdout</TT>,
and you want to send its output to a file,
and you don't have the option of rewriting <TT>f</TT>,
you can use a sequence like:
<pre>
	freopen(file, "w", stdout);
	f();
</pre>

See, however,
question <a href="faqcat1d60.html?sec=stdio#undofreopen">12.34</a>.
</p>

<p>References:

ISO Sec. 7.9.5.4
<br>
H&amp;S Sec. 15.2
<hr><hr><hr>
<a name="undofreopen">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/undofreopen.html"><!-- qtag -->Question 12.34</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Once I've used <TT>freopen</TT>, how can I get the original
<TT>stdout</TT>
(or <TT>stdin</TT>)
back?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There isn't a good way.
If you need to switch back,
the
best solution
is not to
have used
<TT>freopen</TT> in the
first place.
Try using your own explicit output (or input) stream variable,
which you can reassign at will, while leaving the original
<TT>stdout</TT> (or <TT>stdin</TT>) undisturbed.
For example, declare a global
<pre>
	FILE *ofp;
</pre>
and replace all calls to <TT>printf( </TT>...<TT> )</TT> with
<TT>fprintf(ofp, </TT>...<TT> )</TT>.
(Obviously, you'll have to check for calls to
<TT>putchar</TT> and <TT>puts</TT>, too.)
Then you can set <TT>ofp</TT> to <TT>stdout</TT>
or
to anything else.
</p><p>You might wonder if you could skip <TT>freopen</TT> entirely,
and do something like
<pre>
	FILE *savestdout = stdout;
	stdout = fopen(file, "w");	/* WRONG */
</pre>
leaving yourself able to restore <TT>stdout</TT> later by doing
<pre>
	stdout = savestdout;		/* WRONG */
</pre>
but code like this is not likely to work,


because <TT>stdout</TT>
(and <TT>stdin</TT> and <TT>stderr</TT>)
are

typically constants which cannot be reassigned

(which
is why <TT>freopen</TT> exists in the first place).
</p><p>It may be possible,
in a nonportable way,
to save away
information about a stream
before
calling <TT>freopen</TT>
to open some file in its place,
such that the original stream can later be restored.
The most straightforward and reliable way is to manipulate the
underlying file descriptors using 
a system-specific call such as
<TT>dup</TT>
or
<TT>dup2</TT>,
if available (<a href="../../stdio/rd.kirby.c">example</a>).
Another is to
copy or inspect the contents of the
<TT>FILE</TT>
structure,
but this is exceedingly
nonportable
and unreliable.
</p><p>Under some systems,
you might be able to reopen a special device file
(such as <TT>/dev/fd/1</TT> under modern versions of Unix)
which is still attached to (for example) the original standard output.
You can,
under some systems,
explicitly re-open the controlling terminal
(see question <a href="faqcat1d60.html?sec=stdio#devtty">12.36</a>),
but this isn't necessarily what you want,
since the
original
input or output
(i.e. what <TT>stdin</TT> or <TT>stdout</TT> had been
before you called <TT>freopen</TT>)
could have been redirected
from the command line.
</p><p>All of this pertains to stdio redirection
initially performed by <TT>freopen</TT>,
affecting the I/O calls
within the same program that called <TT>freopen</TT>.
If what you're trying to do is capture the result of a 
subprogram


execution,
<TT>freopen</TT> probably won't work
anyway;
see question <a href="faqcatea63.html?sec=osdep#popen">19.30</a> instead.
</p><p>Additional links:
<a href="../../stdio/sd13.html" rel=subdocument>examples</a>
<hr><hr><hr>
<a name="redirp">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/redirp.html"><!-- qtag -->Question 12.35</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I tell if standard input or output is redirected
(i.e. whether ``<TT>&lt;</TT>'' or ``<TT>&gt;</TT>''
was used on the invocation command line)?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
You can't tell directly,
but you can usually look at a few other things
to make whatever decision you need to.

If
you want
your program
to
take input
from <TT>stdin</TT> when not given any input files,
you can do so if <TT>argv</TT> doesn't mention any input files
(see question <a href="faqcat38c2.html?sec=misc#argv">20.3</a>),
or perhaps if you're given a placeholder like
``<TT>-</TT>''

instead of
a filename.
If you want to suppress prompts
if input is not coming from an interactive terminal,
on some systems
(e.g. Unix, and usually MS-DOS)
you can use <TT>isatty(0)</TT> or <TT>isatty(fileno(stdin))</TT>
to make the determination.
<hr><hr><hr>
<a name="devtty">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/devtty.html"><!-- qtag -->Question 12.36</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm trying to write a program like ``<TT>more</TT>.''

How can I
get
back
to
the interactive keyboard if <TT>stdin</TT> is redirected?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There is no portable way of doing this.

Under Unix,
you can open the special file <TT>/dev/tty</TT>.
Under MS-DOS,
you can try opening
the ``file'' <TT>CON</TT>,
or use routines
or BIOS calls
such as <TT>getch</TT>
which
may go to the keyboard
whether or not input is redirected.
<hr><hr><hr>
<a name="multiway">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/multiway.html"><!-- qtag -->Question 12.36b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I arrange to have output go two places at once,
e.g. to the screen and to a file?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
You can't do this directly,
but you
could write your own <TT>printf</TT> variant
which printed everything twice.
Here is a sample <TT>logprintf</TT> function
which
prints to both <TT>stdout</TT> and a preopened log file:
<pre>
#include &lt;stdio.h&gt;
#include &lt;stdarg.h&gt;

extern FILE *logfp;

void logprintf(char *fmt, ...)
{
	va_list argp;
	va_start(argp, fmt);
	vfprintf(stdout, fmt, argp);
	va_end(argp);
	va_start(argp, fmt);
	vfprintf(logfp, fmt, argp);
	va_end(argp);
}
</pre>
Now, whenever you call <TT>logprintf</TT>
(which you can call with format strings just like <TT>printf</TT>),
it prints both to <TT>stdout</TT> and to <TT>logfp</TT>,
which you have presumably opened to your desired log file.
Another way to arrange this would be
<pre>
void f2printf(FILE *fp1, FILE *fp2, char *fmt, ...)
{
	va_list argp;
	va_start(argp, fmt); vfprintf(fp1, fmt, argp); va_end(argp);
	va_start(argp, fmt); vfprintf(fp2, fmt, argp); va_end(argp);
}
</pre>
where <TT>f2printf</TT> is just like <TT>fprintf</TT>
except that you give it two file pointers
(e.g. <TT>stdout</TT> and <TT>logfp</TT>)
and it prints to both of them.
</p><p>Both of these techniques obviously require you to use explicit
calls to <TT>logprintf</TT> or <TT>f2printf</TT>.
There is no known way
in Standard C to arrange implicitly
(i.e. via some call analogous to <TT>freopen</TT>)
that one stream,
which you print to once with a normal call like <TT>fprintf</TT>,
print to two places at once.
<a href="../../stdio/functional.html" rel=subdocument>[footnote]</a>
</p><p>See
also
question <a href="faqcat744e.html?sec=varargs#vprintf">15.5</a>.
<hr><hr><hr>
<a name="binaryio">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/binaryio.html"><!-- qtag -->Question 12.37</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I want to
read and write numbers
between files and memory
in a byte-at-a-time way,
not as formatted characters
the way <TT>fprintf</TT> and <TT>fscanf</TT> do.
How
can I do this?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
What you're trying to do is usually called ``binary'' I/O.
First,
make sure that you are calling <TT>fopen</TT> with the 
<TT>"b"</TT> modifier
(<TT>"rb"</TT>, <TT>"wb"</TT>, etc.;
see question <a href="faqcat1d60.html?sec=stdio#fopenbinary">12.38</a>).
Then,
use the <TT>&amp;</TT> and <TT>sizeof</TT> operators
to
get a handle on

the sequences of bytes
you are trying to transfer.
Usually,
the <TT>fread</TT>
and <TT>fwrite</TT> functions
are what you 
want to use;
see question <a href="faqcat6b6b.html?sec=struct#io">2.11</a>
for an example.
</p><p>Note, though,
that <TT>fread</TT> and <TT>fwrite</TT> do not necessarily imply
binary I/O.
If you've opened a file in binary mode,
you can use any I/O calls on it
(see
for example
the
examples in
question <a href="faqcat1d60.html?sec=stdio#extconform">12.42</a>);
if you've opened it in text mode,
you can
use <TT>fread</TT> or <TT>fwrite</TT> if they're 
convenient.
</p><p>Finally,
note that binary data files are not very portable;
see question
<a href="faqcat38c2.html?sec=misc#binaryfiles">20.5</a>.
</p><p>See also question <a href="faqcat1d60.html?sec=stdio#textvsbinary">12.40</a>.
<hr><hr><hr>
<a name="fopenbinary">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/fopenbinary.html"><!-- qtag -->Question 12.38</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I read a binary data file properly?
I'm occasionally seeing <TT>0x0a</TT> and <TT>0x0d</TT> values getting garbled,
and
I seem
to hit EOF prematurely if the data contains the value <TT>0x1a</TT>.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
When you're reading a binary data file,
you should

specify <TT>"rb"</TT> mode
when calling <TT>fopen</TT>,
to make sure that text file translations do not occur.
Similarly,
when writing binary data files, use <TT>"wb"</TT>.
(Under operating systems
such as Unix
which don't distinguish between text and binary files,
<TT>"b"</TT>
may not be required,
but is harmless.)
</p><p>Note that
the text/binary distinction is made when you open the file:
once a file is open,
it doesn't matter which I/O calls you use on it.
See also
questions <a href="faqcat1d60.html?sec=stdio#binaryio">12.37</a>, <a href="faqcat1d60.html?sec=stdio#textvsbinary">12.40</a>, <a href="faqcat1d60.html?sec=stdio#extconform">12.42</a>, and <a href="faqcat38c2.html?sec=misc#binaryfiles">20.5</a>.


</p>

<p>References:

ISO Sec. 7.9.5.3
<br>
H&amp;S Sec. 15.2.1 p. 348
<hr><hr><hr>
<a name="stdinbinary">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/stdinbinary.html"><!-- qtag -->Question 12.39</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm writing a
``filter''
for binary files,
but <TT>stdin</TT> and <TT>stdout</TT> are preopened
as text streams.
How can I change
their mode
to binary?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There is no standard way to do this.
On Unix-like systems,
there is no text/binary distinction,
so
there is no need to change the mode.
Some MS-DOS compilers supply a <TT>setmode</TT> call.
Otherwise, you're on your own.
<hr><hr><hr>
<a name="textvsbinary">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../stdio/textvsbinary.html"><!-- qtag -->Question 12.40</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What's the difference between text and binary I/O?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
In text mode,
a file is assumed to consist of
lines of printable characters
(perhaps including tabs).
The routines in the stdio library
(<TT>getc</TT>, <TT>putc</TT>, and all the rest)
translate between
the underlying system's
end-of-line representation
and the single <TT>\n</TT> used in C programs.
C programs
which simply read and write text
therefore don't have to worry about
the underlying system's
newline conventio