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this is a mirrored site c-faq. thought might need offline

(If you have the choice,
the best compromise
is probably one of the <TT>stat</TT> functions.)
</p><p>Rather than trying to predict in advance
whether an operation
such as opening a file
will succeed,
it's often better to

try it,
check the return value,
and complain if it
fails.
(Obviously, this approach won't work
if you're trying to avoid overwriting an
existing file, unless you've got something like the
<TT>O_EXCL</TT>
file opening
option available,
which does just what you want
in
this
case.)
</p>



<p>References:

PCS Sec. 12 pp. 189,213
<br>
POSIX Sec. 5.3.1, Sec. 5.6.2, Sec. 5.6.3
<hr><hr><hr>
<a name="filesize">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/filesize.html"><!-- qtag -->Question 19.12</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I find out the size of a file,
prior to reading it in?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>

If the ``size of a file''
is the number of characters you'll be able to read from it
in C
(or which were written to it by a previous program),
it
can be
difficult
or
impossible
to determine this number
exactly



(other than
by reading the whole file).
</p><p>Under
Unix,
the <TT>stat</TT> call
(specifically,
the <TT>st_size</TT> field of the stat structure)
will give you an exact answer.
<a href="../../osdep/concur2.html" rel=subdocument>[footnote]</a>
Several other systems supply a
Unix-like
<TT>stat</TT>
call,
but the sizes reported for text files
may be approximate
(due to differing end-of-line representations;
see question <a href="faqcat1d60.html?sec=stdio#textvsbinary">12.40</a>).
You can
open the file and use <TT>fstat</TT>,
or
<TT>fseek</TT> to the end
of the file
and then use <TT>ftell</TT>,
but

these tend to have
the same
problems:
<TT>fstat</TT> is not portable,
and generally tells you the same thing <TT>stat</TT> tells you;
<TT>ftell</TT> is not guaranteed to return a byte count
except for binary files









(but,
strictly speaking,
binary files don't necessarily support <TT>fseek</TT> to <TT>SEEK_END</TT>
at all).



Some systems
provide functions called
<TT>filesize</TT>
or <TT>filelength</TT>,
but these are obviously not portable, either.
</p><p>Are you sure you have to determine the file's size in advance?
Since the most accurate way of determining the size of a file as
a C program will see it is to
open the file
and read it,
perhaps you can rearrange the code to learn the size as it reads.
(In general, your program
should behave
gracefully
if
the
number of characters
actually
read
does not
match
prior expectations,
since any advance determination of the size might be approximate.)
See also questions
<a href="faqcatbafd.html?sec=malloc#realloc">7.29</a>,
<a href="faqcatbafd.html?sec=malloc#reallocnull">7.30</a>,
and <a href="faqcat38c2.html?sec=misc#ragged">20.2</a>.
</p><p>Additional links:
<a href="../../osdep/sd23.html" rel=subdocument>further reading</a>
</p>



<p>References:

ISO Sec. 7.9.9.4
<br>
H&amp;S Sec. 15.5.1
<br>
PCS Sec. 12 p. 213
<br>
POSIX Sec. 5.6.2
<hr><hr><hr>
<a name="modtime">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/modtime.html"><!-- qtag -->Question 19.12b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I find the modification date
and time
of a file?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The Unix and POSIX function is <TT>stat</TT>,
which several other systems supply as well.
(See also question <a href="faqcatea63.html?sec=osdep#filesize">19.12</a>.)
<hr><hr><hr>
<a name="ftruncate">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/ftruncate.html"><!-- qtag -->Question 19.13</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can a file be shortened in-place without completely clearing
or rewriting it?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
BSD systems provide <TT>ftruncate</TT>,
several others supply <TT>chsize</TT>,
and a few may provide a
(possibly undocumented)
<TT>fcntl</TT> option
F_FREESP.
Under MS-DOS,
you can sometimes use <TT>write(fd,&nbsp;"",&nbsp;0)</TT>.
However,
there is no portable solution,
nor a way to delete blocks at the beginning
or in the middle.
See also question <a href="faqcatea63.html?sec=osdep#insdelrec">19.14</a>.
<hr><hr><hr>
<a name="insdelrec">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/insdelrec.html"><!-- qtag -->Question 19.14</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>
In general, there is no way to
do this.
<a href="../../osdep/nonseq.html" rel=subdocument>[footnote]</a>
The usual solution is simply to rewrite the file.
</p><p>When you find yourself needing to insert data
into an existing file,
here are a few alternatives you can try:

</p><UL><li>Rearrange the data file so that you can append the new 
information at the end.
<li>Put the information in a second file.

<li>Leave some blank space
(e.g. a line of 80 spaces,
or a field like <TT>0000000000</TT>)
in the file when it is first written,
and overwrite it
later
with the final
information
(see also question <a href="faqcat1d60.html?sec=stdio#fupdate">12.30</a>).

(This technique is most portable in binary mode;
on some systems,
overwriting a text file may truncate it.)
<li>Use a database

instead of a flat file.
</UL><p>Instead of actually deleting records,
you might consider
just marking them as deleted,
and having the code which reads the file ignore them.

(You could run a separate
coalescion
program once in a while
to rewrite the file,
finally
discarding
the deleted records.




Or,
if the records are all the same length,
you could take the last record
and use it to overwrite the record to be deleted,
then truncate the file.)
</p><p>See also questions
<a href="faqcat1d60.html?sec=stdio#fupdate">12.30</a>
and
<a href="faqcatea63.html?sec=osdep#ftruncate">19.13</a>.
</p><p>Additional links:
<a href="../../osdep/insertf.20000216.html">further reading</a>
<hr><hr><hr>
<a name="fdfilename">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/fdfilename.html"><!-- qtag -->Question 19.15</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
or
file descriptor?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
This problem is, in general, insoluble.
Under
Unix,
for instance,
a scan of the entire disk
(perhaps involving special permissions)
would theoretically be required,
and would fail if the
descriptor
were
connected to a pipe
or referred to a deleted file
(and
could give a misleading answer for a file with multiple links).
It is best to remember
the names
of files yourself
as
you open them

(perhaps with

a wrapper function around
<TT>fopen</TT>).
<hr><hr><hr>
<a name="delete">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/delete.html"><!-- qtag -->Question 19.16</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I delete a file?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The Standard C Library function is
<TT>remove</TT>.
(This is therefore one of the few questions in this section
for which the answer is <em>not</em>
``It's system-dependent.'')
On older,
pre-ANSI
Unix systems,
<TT>remove</TT> may not exist,
in which case you can try
<TT>unlink</TT>.
<a href="../../osdep/unlink.html" rel=subdocument>[footnote]</a>
</p>





<p>References:

K&amp;R2 Sec. B1.1 p. 242
<br>
ISO Sec. 7.9.4.1
<br>
H&amp;S Sec. 15.15 p. 382
<br>
PCS Sec. 12 pp. 208,220-221
<br>
POSIX Sec. 5.5.1, Sec. 8.2.4
<hr><hr><hr>
<a name="copy">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/copy.html"><!-- qtag -->Question 19.16b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How do I copy files?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Either use <TT>system()</TT>
to invoke your operating system's copy utility
(see question <a href="faqcatea63.html?sec=osdep#system">19.27</a>),
or open
the source and destination files
(using <TT>fopen</TT> or some lower-level file-opening system call),
read characters or blocks of characters from the source file,

and write them to the destination file.
Here is a simple example:
<pre>
#include &lt;stdio.h&gt;

int copyfile(char *fromfile, char *tofile)
{
	FILE *ifp, *ofp;
	int c;

	if((ifp = fopen(fromfile, "r")) == NULL) return -1;
	if((ofp = fopen(tofile, "w")) == NULL) { fclose(ifp); return -1; }

	while((c = getc(ifp)) != EOF)
		putc(c, ofp);

	fclose(ifp);
	fclose(ofp);

	return 0;
}
</pre>
To copy a block at a time, rewrite the inner loop as
<pre>
	while((r = fread(buf, 1, sizeof(buf), ifp)) &gt; 0)
		fwrite(buf, 1, r, ofp);
</pre>
where <TT>r</TT> is an <TT>int</TT>
and <TT>buf</TT> is a suitably-sized array of <TT>char</TT>.
</p>

<p>References:

K&amp;R Sec. 1, Sec. 7
<hr><hr><hr>
<a name="dospath">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/dospath.html"><!-- qtag -->Question 19.17</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Why can't I open a file by its explicit path?
The
call
<pre>
fopen("c:\newdir\file.dat",&nbsp;"r")</pre>

is failing.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The file you actually requested--with
the characters <TT>\n</TT> and <TT>\f</TT> in its
name--probably
doesn't exist,
and isn't what you thought you were trying to open.
</p><p>In character constants and string literals,
the backslash <TT>\</TT> is an escape character,
giving special meaning to the character following it.
In order
for
literal backslashes in
a pathname to
be
passed through to <TT>fopen</TT>
(or any other function)
correctly,
they have to be doubled,
so that the first
backslash in each pair
quotes the second one:
<pre>
	fopen("c:\\newdir\\file.dat", "r")
</pre>
Alternatively,
under MS-DOS,
it turns out that
forward slashes are also 
accepted as directory separators,
so you could use
<pre>
	fopen("c:/newdir/file.dat", "r")
</pre>
(Note, by the way, that
header file names
mentioned in
preprocessor <TT>#include</TT> directives

are <em>not</em> string 
literals,
so you may not have to worry about backslashes there.)
<hr><hr><hr>
<a name="fopenenv">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/fopenenv.html"><!-- qtag -->Question 19.17b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
<TT>fopen</TT> isn't letting me open files like
<TT>"$HOME/.profile"</TT>
and
<TT>"~/.myrcfile"</TT>.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Under Unix, at least,
environment variables like <TT>$HOME</TT>,
along with the home-directory notation involving the <TT>~</TT> character,
are expanded by the shell,
and there's no mechanism to perform these expansions
automatically when you call <TT>fopen</TT>.

<hr><hr><hr>
<a name="abrtretrign">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/abrtretrign.html"><!-- qtag -->Question 19.17c</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I suppress the
dreaded MS-DOS
``Abort, Retry, Ignore?'' message?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>




Among other things,
you need to intercept the DOS Critical Error Interrupt,
interrupt 24H.
See the comp.os.msdos.programmer FAQ list

for more details.

<hr><hr><hr>
<a name="nfile">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/nfile.html"><!-- qtag -->Question 19.18</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm getting an error,
``Too many open files''.
How can I increase the allowable number of simultaneously open files?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There are typically
at least
two resource limitations on the number of simultaneously open files:

the number of low-level ``file descriptors'' or 
``file handles'' available in the operating system, 
and the number of <TT>FILE</TT> structures available in the stdio 
library.
Both must be sufficient.
Under MS-DOS systems,
you can control the number of operating 
system file handles with a line in CONFIG.SYS.

Some compilers come with instructions (and perhaps a source 
file or two) for increasing the number of stdio <TT>FILE</TT> 
structures.
</p><p>Additional links:
<a href="../../osdep/sd24.html" rel=subdocument>further reading</a>
<hr><hr><hr>
<a name="diskfree">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../osdep/diskfree.html"><!-- qtag -->Question 19.19</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I find out how much free space
is available on disk?
</p><p><hr>