questions.html

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

or vice versa,
or for automatically generating prototypes?
</p><p><a href="ansi/extensions.html" rel=subdocument>11.32</a>
Why won't
the Frobozz Magic C Compiler,
which claims to be ANSI compliant,
accept this code?
I know that the code is ANSI, because <TT>gcc</TT> accepts it.
</p><p><a href="ansi/undef.html" rel=subdocument>11.33</a>
People seem to make a point of distinguishing
between
implementation-defined,
unspecified,
and
undefined
behavior.
What do these mean?
</p><p><a href="ansi/compliance.html" rel=subdocument>11.33b</a>
What does it really mean for a program to be
``legal''
or
``valid''
or
``conforming''?
</p><p><a href="ansi/appalled.html" rel=subdocument>11.34</a>
I'm appalled that the ANSI Standard leaves so many issues undefined.
Isn't a Standard's whole job to standardize these things?
</p><p><a href="ansi/experiment.html" rel=subdocument>11.35</a>
People keep saying that the behavior
of <TT>i&nbsp;=&nbsp;i++</TT>
is undefined,
but
I just tried
it
on an ANSI-conforming compiler,
and got the results I expected.
</p><hr>
<H4>12. Stdio</H4>
<p><a href="stdio/getcharc.html" rel=subdocument>12.1</a>
What's wrong with
this code?
<pre>
char c;
while((c = getchar()) != EOF) ...</pre>

</p><p><a href="stdio/eofval.html" rel=subdocument>12.1b</a>
I have a simple little program that reads characters until EOF,
but how do I actually <em>enter</em> that ``EOF'' value
from the keyboard?
I see that <TT>EOF</TT> is defined by <TT>&lt;stdio.h&gt;</TT> to be -1;
am I supposed to enter -1?
</p><p><a href="stdio/feof.html" rel=subdocument>12.2</a>
Why
does
the
simple line-copying loop
<TT>while(!feof(infp)) {
fgets(buf,&nbsp;MAXLINE,&nbsp;infp);
fputs(buf,&nbsp;outfp);
}
</TT>copy
the last line twice?

</p><p><a href="stdio/linebfdur.html" rel=subdocument>12.3</a>
I'm using <TT>fgets</TT> to read
lines from a file into an
array
of pointers.
Why do all the lines end up containing copies of the last line?
</p><p><a href="stdio/fflush.html" rel=subdocument>12.4</a>
My program's prompts and intermediate output don't always show up
on the screen,
especially when I pipe the output through another program.
</p><p><a href="stdio/charatatime.html" rel=subdocument>12.5</a>


How can I read one character at a time,
without waiting for the RETURN key?

</p><p><a href="stdio/printfpercent.html" rel=subdocument>12.6</a>
How can I print a <TT>'%'</TT> character
in a <TT>printf</TT> format string?
I tried <TT>\%</TT>, but it didn't work.
</p><p><a href="stdio/printftypes.html" rel=subdocument>12.7</a>
Why doesn't
<pre>
long int n = 123456;
printf("%d\n", n);</pre>

work?
</p><p><a href="stdio/varargproto.html" rel=subdocument>12.8</a>
I thought that ANSI function prototypes
were supposed to guard against argument type mismatches.
</p><p><a href="stdio/scanfvsprintf.html" rel=subdocument>12.9</a>
Someone told me it was wrong to use <TT>%lf</TT> with <TT>printf</TT>.
How can
<TT>printf</TT> use <TT>%f</TT> for type <TT>double</TT>,
if <TT>scanf</TT> requires <TT>%lf</TT>?
</p><p><a href="stdio/printftypedef.html" rel=subdocument>12.9b</a>
What <TT>printf</TT> format should I use for a typedef
like <TT>size_t</TT>
when I don't know
whether it's <TT>long</TT> or some other type?
</p><p><a href="stdio/printfvwid.html" rel=subdocument>12.10</a>
How can I implement a variable field width with <TT>printf</TT>?
That is, instead of
something like
<TT>%8d</TT>,
I want the width to be specified
at run time.
</p><p><a href="stdio/commaprint.html" rel=subdocument>12.11</a>
How can I print numbers with commas separating the thousands?
<br>What about currency formatted numbers?
</p><p><a href="stdio/scanf1.html" rel=subdocument>12.12</a>
Why doesn't the call
<TT>scanf("%d", i)</TT>
work?
</p><p><a href="stdio/scanf1a.html" rel=subdocument>12.12b</a>
Why <em>does</em> the call
<pre>
char s[30];
scanf("%s", s);</pre>

work?
I thought you always needed an <TT>&amp;</TT>
on each variable passed to <TT>scanf</TT>.
</p><p><a href="stdio/scanf2.html" rel=subdocument>12.13</a>
Why doesn't
this code:
<pre>
double d;
scanf("%f", &amp;d);</pre>

work?
</p><p><a href="stdio/scanfh.html" rel=subdocument>12.14</a>
Why doesn't the code
<pre>
short int s;
scanf("%d", &amp;s);</pre>

work?
</p><p><a href="stdio/scanfvwid.html" rel=subdocument>12.15</a>
How can I specify a variable width in a <TT>scanf</TT> format string?
</p><p><a href="stdio/datafmts.html" rel=subdocument>12.16</a>
How can I read data from
data files with particular formats?
<br>How can I read ten floats without having to use a jawbreaker 
<TT>scanf</TT> format
<br>like
<TT>"%f&nbsp;%f&nbsp;%f&nbsp;%f&nbsp;%f&nbsp;%f&nbsp;%f&nbsp;%f&nbsp;%f&nbsp;%f"</TT>?
<br>How can I read an arbitrary number of fields from a line into an array?
</p><p><a href="stdio/scanfhang.html" rel=subdocument>12.17</a>
When I read
numbers
from the keyboard with

<TT>scanf</TT>
and a
<TT>"%d\n"</TT>
format,
like this:
<pre>
	int n;
	scanf("%d\n", &amp;n);
	printf("you typed %d\n", n);
</pre>

it seems to hang until I type one extra line of input.
</p><p><a href="stdio/scanfinterlace.html" rel=subdocument>12.18a</a>
I'm reading a number with <TT>scanf</TT>
and
<TT>%d</TT>,
and then a string with
<TT>gets()</TT>:
<pre>
	int n;
	char str[80];

	printf("enter a number: ");
	scanf("%d", &amp;n);
	printf("enter a string: ");
	gets(str);
	printf("you typed %d and \"%s\"\n", n, str);
</pre>
but the compiler seems to be
skipping the call to <TT>gets()</TT>!
</p><p><a href="stdio/scanfc.html" rel=subdocument>12.18b</a>
I'm using <TT>scanf&nbsp;%c</TT> to read a Y/N response,
but later input gets skipped.
</p><p><a href="stdio/scanfjam.html" rel=subdocument>12.19</a>
I figured I could use <TT>scanf</TT> more safely
if I checked its return value
to make sure that the user
typed the numeric values I expect:
<pre>
	int n;

	while(1) {
		printf("enter a number: ");
		if(scanf("%d", &amp;n) == 1)
			break;
		printf("try again: ");
	}

	printf("you typed %d\n", n);
</pre>
but sometimes it seems to go into an infinite loop.
<a href="http://www.eskimo.com/notfound.html" rel=subdocument>[footnote]</a>
Why?
</p><p><a href="stdio/scanfprobs.html" rel=subdocument>12.20</a>
Why does everyone say not to use <TT>scanf</TT>?
What should I use instead?
</p><p><a href="stdio/sprintfsize.html" rel=subdocument>12.21</a>
How can I tell how much destination buffer space I'll need
for an arbitrary <TT>sprintf</TT> call?
How can I avoid
overflowing
the destination buffer with <TT>sprintf</TT>?
</p><p><a href="stdio/sprintfret.html" rel=subdocument>12.22</a>
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><a href="stdio/getsvsfgets.html" rel=subdocument>12.23</a>
Why does everyone say not to use <TT>gets()</TT>?
</p><p><a href="stdio/printferrno.html" rel=subdocument>12.24</a>
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><a href="stdio/fgetpos.html" rel=subdocument>12.25</a>
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><a href="stdio/stdinflush.html" rel=subdocument>12.26a</a>
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><a href="stdio/stdinflush2.html" rel=subdocument>12.26b</a>
If <TT>fflush</TT> won't work,
what can I use to flush input?
</p><p><a href="stdio/fopenfp.html" rel=subdocument>12.27</a>
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><a href="stdio/fopenmodec.html" rel=subdocument>12.28</a>
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><a href="stdio/constrpath.html" rel=subdocument>12.28b</a>
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><a href="stdio/fopenpath.html" rel=subdocument>12.29</a>
<TT>fopen</TT> is failing for certain pathnames.

</p><p><a href="stdio/fupdate.html" rel=subdocument>12.30</a>
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><a href="stdio/insdelrec.html" rel=subdocument>12.31</a>
How can I insert or delete a line
(or record)
in the middle of a file?

</p><p><a href="stdio/fdfilename.html" rel=subdocument>12.32</a>
How can I recover the file name given an open
stream?
</p><p><a href="stdio/freopen.html" rel=subdocument>12.33</a>
How can I redirect <TT>stdin</TT> or <TT>stdout</TT>
to a file
from within a program?
</p><p><a href="stdio/undofreopen.html" rel=subdocument>12.34</a>
Once I've used <TT>freopen</TT>, how can I get the original
<TT>stdout</TT>
(or <TT>stdin</TT>)
back?
</p><p><a href="stdio/redirp.html" rel=subdocument>12.35</a>
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><a href="stdio/devtty.html" rel=subdocument>12.36</a>
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><a href="stdio/multiway.html" rel=subdocument>12.36b</a>
How can I arrange to have output go two places at once,
e.g. to the screen and to a file?
</p><p><a href="stdio/binaryio.html" rel=subdocument>12.37</a>
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><a href="stdio/fopenbinary.html" rel=subdocument>12.38</a>
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><a href="stdio/stdinbinary.html" rel=subdocument>12.39</a>
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><a href="stdio/textvsbinary.html" rel=subdocument>12.40</a>
What's the difference between text and binary I/O?
</p><p><a href="stdio/structio.html" rel=subdocument>12.41</a>
How can I read/write structures from/to

data files?


</p><p><a href="stdio/extconform.html" rel=subdocument>12.42</a>
How can I write code to conform to these old, binary data file formats?
</p><p><a href="stdio/runtimesc2.html" rel=subdocument>12.43</a>
I'm reading
strings
typed by
the user
into an array,
and then printing them out later.
When the user types
a sequence like <TT>\n</TT>,
why
isn't it being
handled properly?

</p><hr>
<H4>13. Library Functions</H4>
<p><a href="lib/itoa.html" rel=subdocument>13.1</a>
How can I convert numbers to strings
(the opposite of <TT>atoi</TT>)?
Is there an <TT>itoa</TT> function?

</p><p><a href="lib/strncpy.html" rel=subdocument>13.2</a>
Why does <TT>strncpy</TT> not always
place a <TT>'\0'</TT> terminator
in the destination string?
</p><p><a href="lib/substr.html" rel=subdocument>13.3</a>
Does C have anything like the ``substr''
(extract substring)
routine present in other languages?
</p><p><a href="lib/strupper.html" rel=subdocument>13.4</a>
How do I convert a string to all upper or lower case?

</p><p><a href="lib/toupper.html" rel=subdocument>13.5</a>
Why
do some versions of
<TT>toupper</TT> act strangely if given an upper-case letter?
<br>Why does some code call <TT>islower</TT> before <TT>toupper</TT>?
</p><p><a href="lib/strtok.html" rel=subdocument>13.6</a>
How can I split up a
string
into
whitespace-separated fields?
<br>How can I duplicate the process
by which <TT>main()</TT>
is handed <TT>argc</TT> and <TT>argv</TT>?
</p><p><a href="lib/regex.html" rel=subdocument>13.7</a>
I need some code to do
regular expression
and
wildcard matching.
</p><p><a href="lib/qsort1.html" rel=subdocument>13.8</a>
I'm trying to sort an array of strings with <TT>qsort</TT>,
using
<TT>strcmp</TT>
as the comparison function,
but it's not working.
</p><p><a href="lib/qsort2.html" rel=subdocument>13.9</a>
Now I'm trying to sort an array of structures
with <TT>qsort</TT>.
My comparison function takes pointers to structures,
but the compiler
complains
that the function is
of the wrong type for <TT>qsort</TT>.
How can I cast the function pointer to shut off the warning?
</p><p><a href="lib/listsort.html" rel=subdocument>13.10</a>
How can I sort a linked list?
</p><p><a href="lib/extsort.html" rel=subdocument>13.11</a>
How can I sort more data than will fit in memory?
</p><p><a href="lib/curtime.html" rel=subdocument>13.12</a>
How can I get the
current date
or
time of day in a C program?
</p><p><a href="lib/mktime.html" rel=subdocument>13.13</a>
I know that the library function <TT>localtime</TT>
will convert a <TT>time_t</TT> into a broken-down <TT>struct&nbsp;tm</TT>,
and that <TT>ctime</TT>
will convert a
<TT>time_t</TT> to a printable string.
How can I
perform the inverse operatio