library_18.html

Linux程序员的工作手册

which is <CODE>6.5</CODE> minus <CODE>6.9</CODE>.
<P>
The absolute value of the result is less than or equal to half the
absolute value of the <VAR>denominator</VAR>.  The difference between
<CODE>fmod (<VAR>numerator</VAR>, <VAR>denominator</VAR>)</CODE> and <CODE>drem
(<VAR>numerator</VAR>, <VAR>denominator</VAR>)</CODE> is always either
<VAR>denominator</VAR>, minus <VAR>denominator</VAR>, or zero.
<P>
If <VAR>denominator</VAR> is zero, <CODE>drem</CODE> fails and sets <CODE>errno</CODE> to
<CODE>EDOM</CODE>.
<P>
<A NAME="IDX1327"></A>
<H2><A NAME="SEC305" HREF="library_toc.html#SEC305" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC305">Integer Division</A></H2>
<P>
This section describes functions for performing integer division.  These
functions are redundant in the GNU C library, since in GNU C the <SAMP>`/'</SAMP>
operator always rounds towards zero.  But in other C implementations,
<SAMP>`/'</SAMP> may round differently with negative arguments.  <CODE>div</CODE> and
<CODE>ldiv</CODE> are useful because they specify how to round the quotient:
towards zero.  The remainder has the same sign as the numerator.
<P>
These functions are specified to return a result <VAR>r</VAR> such that
<CODE><VAR>r</VAR>.quot*<VAR>denominator</VAR> + <VAR>r</VAR>.rem</CODE> equals
<VAR>numerator</VAR>.
<A NAME="IDX1328"></A>
<P>
To use these facilities, you should include the header file
<TT>`stdlib.h'</TT> in your program.
<P>
<A NAME="IDX1329"></A>
<U>Data Type:</U> <B>div_t</B><P>
This is a structure type used to hold the result returned by the <CODE>div</CODE>
function.  It has the following members:
<P>
<DL COMPACT>
<DT><CODE>int quot</CODE>
<DD>The quotient from the division.
<P>
<DT><CODE>int rem</CODE>
<DD>The remainder from the division.
</DL>
<P>
<A NAME="IDX1330"></A>
<U>Function:</U> div_t <B>div</B> <I>(int <VAR>numerator</VAR>, int <VAR>denominator</VAR>)</I><P>
This function <CODE>div</CODE> computes the quotient and remainder from
the division of <VAR>numerator</VAR> by <VAR>denominator</VAR>, returning the
result in a structure of type <CODE>div_t</CODE>.
<P>
If the result cannot be represented (as in a division by zero), the
behavior is undefined.
<P>
Here is an example, albeit not a very useful one.
<P>
<PRE>
div_t result;
result = div (20, -6);
</PRE>
<P>
Now <CODE>result.quot</CODE> is <CODE>-3</CODE> and <CODE>result.rem</CODE> is <CODE>2</CODE>.
<P>
<A NAME="IDX1331"></A>
<U>Data Type:</U> <B>ldiv_t</B><P>
This is a structure type used to hold the result returned by the <CODE>ldiv</CODE>
function.  It has the following members:
<P>
<DL COMPACT>
<DT><CODE>long int quot</CODE>
<DD>The quotient from the division.
<P>
<DT><CODE>long int rem</CODE>
<DD>The remainder from the division.
</DL>
<P>
(This is identical to <CODE>div_t</CODE> except that the components are of
type <CODE>long int</CODE> rather than <CODE>int</CODE>.)
<P>
<A NAME="IDX1332"></A>
<U>Function:</U> ldiv_t <B>ldiv</B> <I>(long int <VAR>numerator</VAR>, long int <VAR>denominator</VAR>)</I><P>
The <CODE>ldiv</CODE> function is similar to <CODE>div</CODE>, except that the
arguments are of type <CODE>long int</CODE> and the result is returned as a
structure of type <CODE>ldiv</CODE>.
<P>
<A NAME="IDX1333"></A>
<A NAME="IDX1334"></A>
<A NAME="IDX1335"></A>
<A NAME="IDX1336"></A>
<H2><A NAME="SEC306" HREF="library_toc.html#SEC306" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC306">Parsing of Numbers</A></H2>
<P>
This section describes functions for "reading" integer and
floating-point numbers from a string.  It may be more convenient in some
cases to use <CODE>sscanf</CODE> or one of the related functions; see
section <A HREF="library_11.html#SEC145" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_11.html#SEC145">Formatted Input</A>.  But often you can make a program more robust by
finding the tokens in the string by hand, then converting the numbers
one by one.
<P>
<H3><A NAME="SEC307" HREF="library_toc.html#SEC307" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC307">Parsing of Integers</A></H3>
<A NAME="IDX1337"></A>
<P>
These functions are declared in <TT>`stdlib.h'</TT>.
<P>
<A NAME="IDX1338"></A>
<U>Function:</U> long int <B>strtol</B> <I>(const char *<VAR>string</VAR>, char **<VAR>tailptr</VAR>, int <VAR>base</VAR>)</I><P>
The <CODE>strtol</CODE> ("string-to-long") function converts the initial
part of <VAR>string</VAR> to a signed integer, which is returned as a value
of type <CODE>long int</CODE>.  
<P>
This function attempts to decompose <VAR>string</VAR> as follows:
<P>
<UL>
<LI>
A (possibly empty) sequence of whitespace characters.  Which characters
are whitespace is determined by the <CODE>isspace</CODE> function
(see section <A HREF="library_4.html#SEC55" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_4.html#SEC55">Classification of Characters</A>).  These are discarded.
<P>
<LI>
An optional plus or minus sign (<SAMP>`+'</SAMP> or <SAMP>`-'</SAMP>).
<P>
<LI>
A nonempty sequence of digits in the radix specified by <VAR>base</VAR>.
<P>
If <VAR>base</VAR> is zero, decimal radix is assumed unless the series of
digits begins with <SAMP>`0'</SAMP> (specifying octal radix), or <SAMP>`0x'</SAMP> or
<SAMP>`0X'</SAMP> (specifying hexadecimal radix); in other words, the same
syntax used for integer constants in C.
<P>
Otherwise <VAR>base</VAR> must have a value between <CODE>2</CODE> and <CODE>35</CODE>.
If <VAR>base</VAR> is <CODE>16</CODE>, the digits may optionally be preceeded by
<SAMP>`0x'</SAMP> or <SAMP>`0X'</SAMP>.
<P>
<LI>
Any remaining characters in the string.  If <VAR>tailptr</VAR> is not a null
pointer, <CODE>strtol</CODE> stores a pointer to this tail in
<CODE>*<VAR>tailptr</VAR></CODE>.
</UL>
<P>
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for an integer in the
specified <VAR>base</VAR>, no conversion is performed.  In this case,
<CODE>strtol</CODE> returns a value of zero and the value stored in
<CODE>*<VAR>tailptr</VAR></CODE> is the value of <VAR>string</VAR>.
<P>
In a locale other than the standard <CODE>"C"</CODE> locale, this function
may recognize additional implementation-dependent syntax.
<P>
If the string has valid syntax for an integer but the value is not
representable because of overflow, <CODE>strtol</CODE> returns either
<CODE>LONG_MAX</CODE> or <CODE>LONG_MIN</CODE> (see section <A HREF="library_28.html#SEC486" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_28.html#SEC486">Range of an Integer Type</A>), as
appropriate for the sign of the value.  It also sets <CODE>errno</CODE>
to <CODE>ERANGE</CODE> to indicate there was overflow.
<P>
There is an example at the end of this section.
<P>
<A NAME="IDX1339"></A>
<U>Function:</U> unsigned long int <B>strtoul</B> <I>(const char *<VAR>string</VAR>, char **<VAR>tailptr</VAR>, int <VAR>base</VAR>)</I><P>
The <CODE>strtoul</CODE> ("string-to-unsigned-long") function is like
<CODE>strtol</CODE> except that it returns its value with type <CODE>unsigned
long int</CODE>.  The value returned in case of overflow is <CODE>ULONG_MAX</CODE>
(see section <A HREF="library_28.html#SEC486" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_28.html#SEC486">Range of an Integer Type</A>).
<P>
<A NAME="IDX1340"></A>
<U>Function:</U> long int <B>atol</B> <I>(const char *<VAR>string</VAR>)</I><P>
This function is similar to the <CODE>strtol</CODE> function with a <VAR>base</VAR>
argument of <CODE>10</CODE>, except that it need not detect overflow errors.
The <CODE>atol</CODE> function is provided mostly for compatibility with
existing code; using <CODE>strtol</CODE> is more robust.
<P>
<A NAME="IDX1341"></A>
<U>Function:</U> int <B>atoi</B> <I>(const char *<VAR>string</VAR>)</I><P>
This function is like <CODE>atol</CODE>, except that it returns an <CODE>int</CODE>
value rather than <CODE>long int</CODE>.  The <CODE>atoi</CODE> function is also
considered obsolete; use <CODE>strtol</CODE> instead.
<P>
Here is a function which parses a string as a sequence of integers and
returns the sum of them:
<P>
<PRE>
sum_ints_from_string (char *string)
{
  int sum = 0;

  while (1) {
    char *tail;
    int next;

    /* Skip whitespace by hand, to detect the end.  */
    while (isspace (*string)) string++;
    if (*string == 0)
      break;

    /* There is more nonwhitespace,  */
    /* so it ought to be another number.  */
    errno = 0;
    /* Parse it.  */
    next = strtol (string, &#38;tail, 0);
    /* Add it in, if not overflow.  */
    if (errno)
      printf ("Overflow\n");
    else
      sum += next;
    /* Advance past it.  */
    string = tail;
  }

  return sum;
}
</PRE>
<P>
<H3><A NAME="SEC308" HREF="library_toc.html#SEC308" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC308">Parsing of Floats</A></H3>
<A NAME="IDX1342"></A>
<P>
These functions are declared in <TT>`stdlib.h'</TT>.
<P>
<A NAME="IDX1343"></A>
<U>Function:</U> double <B>strtod</B> <I>(const char *<VAR>string</VAR>, char **<VAR>tailptr</VAR>)</I><P>
The <CODE>strtod</CODE> ("string-to-double") function converts the initial
part of <VAR>string</VAR> to a floating-point number, which is returned as a
value of type <CODE>double</CODE>.  
<P>
This function attempts to decompose <VAR>string</VAR> as follows:
<P>
<UL>
<LI>
A (possibly empty) sequence of whitespace characters.  Which characters
are whitespace is determined by the <CODE>isspace</CODE> function
(see section <A HREF="library_4.html#SEC55" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_4.html#SEC55">Classification of Characters</A>).  These are discarded.
<P>
<LI>
An optional plus or minus sign (<SAMP>`+'</SAMP> or <SAMP>`-'</SAMP>).
<P>
<LI>
A nonempty sequence of digits optionally containing a decimal-point
character--normally <SAMP>`.'</SAMP>, but it depends on the locale
(see section <A HREF="library_7.html#SEC82" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_7.html#SEC82">Numeric Formatting</A>).
<P>
<LI>
An optional exponent part, consisting of a character <SAMP>`e'</SAMP> or
<SAMP>`E'</SAMP>, an optional sign, and a sequence of digits.
<P>
<LI>
Any remaining characters in the string.  If <VAR>tailptr</VAR> is not a null
pointer, a pointer to this tail of the string is stored in
<CODE>*<VAR>tailptr</VAR></CODE>.
</UL>
<P>
If the string is empty, contains only whitespace, or does not contain an
initial substring that has the expected syntax for a floating-point
number, no conversion is performed.  In this case, <CODE>strtod</CODE> returns
a value of zero and the value returned in <CODE>*<VAR>tailptr</VAR></CODE> is the
value of <VAR>string</VAR>.
<P>
In a locale other than the standard <CODE>"C"</CODE> locale, this function may
recognize additional locale-dependent syntax.
<P>
If the string has valid syntax for a floating-point number but the value
is not representable because of overflow, <CODE>strtod</CODE> returns either
positive or negative <CODE>HUGE_VAL</CODE> (see section <A HREF="library_17.html#SEC290" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_17.html#SEC290">Mathematics</A>), depending on
the sign of the value.  Similarly, if the value is not representable
because of underflow, <CODE>strtod</CODE> returns zero.  It also sets <CODE>errno</CODE>
to <CODE>ERANGE</CODE> if there was overflow or underflow.
<P>
<A NAME="IDX1344"></A>
<U>Function:</U> double <B>atof</B> <I>(const char *<VAR>string</VAR>)</I><P>
This function is similar to the <CODE>strtod</CODE> function, except that it
need not detect overflow and underflow errors.  The <CODE>atof</CODE> function
is provided mostly for compatibility with existing code; using
<CODE>strtod</CODE> is more robust.
<P>
<P>Go to the <A HREF="library_17.html" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_17.html">previous</A>, <A HREF="library_19.html" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_19.html">next</A> section.<P>