str2int.c

Many C samples. It is a good sample for students to learn C language.

/*  File   : str2int.c
    Author : Richard A. O'Keefe
    Updated: 27 April 1984
    Defines: str2int(), atoi(), atol()

    str2int(src, radix, lower, upper, &val)
    converts the string pointed to by src to an integer and stores it in
    val.  It skips leading spaces and tabs (but not newlines, formfeeds,
    backspaces), then it accepts an optional sign and a sequence of digits
    in the specified radix.  The result should satisfy lower <= *val <= upper.
    The result is a pointer to the first character after the number;
    trailing spaces will NOT be skipped.

    If an error is detected, the result will be NullS, the value put
    in val will be 0, and errno will be set to
	EDOM	if there are no digits
	ERANGE	if the result would overflow or otherwise fail to lie
		within the specified bounds.
    Check that the bounds are right for your machine.
    This looks amazingly complicated for what you probably thought was an
    easy task.  Coping with integer overflow and the asymmetric range of
    twos complement machines is anything but easy.

    So that users of atoi and atol can check whether an error occured,
    I have taken a wholly unprecedented step: errno is CLEARED if this
    call has no problems.
*/

#include "strings.h"
#include "ctypes.h"
#include <errno.h>
extern int errno;

/*	CHECK THESE CONSTANTS FOR YOUR MACHINE!!!	*/

#if	pdp11
#   define	MaxInt      0x7fffL	/* int  = 16 bits */
#   define	MinInt      0x8000L
#   define	MaxLong 0x7fffffffL	/* long = 32 bits */
#   define	MinLong 0x80000000L
#else  !pdp11
#   define	MaxInt  0x7fffffffL	/* int  = 32 bits */
#   define	MinInt  0x80000000L
#   define	MaxLong 0x7fffffffL	/* long = 32 bits */
#   define	MinLong 0x80000000L
#endif	pdp11


char *str2int(src, radix, lower, upper, val)
    register char *src;
    register int radix;
    long lower, upper, *val;
    {
	int sign;		/* is number negative (+1) or positive (-1) */
	int n;			/* number of digits yet to be converted */
	long limit;		/* "largest" possible valid input */
	long scale;		/* the amount to multiply next digit by */
	long sofar;		/* the running value */
	register int d;		/* (negative of) next digit */
	char *answer;		

	/*  Make sure *val is sensible in case of error  */

	*val = 0;

	/*  Check that the radix is in the range 2..36  */

	if (radix < 2 || radix > 36) {
	    errno = EDOM;
	    return NullS;
	}

	/*  The basic problem is: how do we handle the conversion of
	    a number without resorting to machine-specific code to
	    check for overflow?  Obviously, we have to ensure that
	    no calculation can overflow.  We are guaranteed that the
	    "lower" and "upper" arguments are valid machine integers.
	    On sign-and-magnitude, twos-complement, and ones-complement
	    machines all, if +|n| is representable, so is -|n|, but on
	    twos complement machines the converse is not true.  So the
	    "maximum" representable number has a negative representative.
	    Limit is set to min(-|lower|,-|upper|); this is the "largest"
	    number we are concerned with.	*/

	/*  Calculate Limit using Scale as a scratch variable  */

	if ((limit = lower) > 0) limit = -limit;
	if ((scale = upper) > 0) scale = -scale;
	if (scale < limit) limit = scale;

	/*  Skip leading spaces and check for a sign.
	    Note: because on a 2s complement machine MinLong is a valid
	    integer but |MinLong| is not, we have to keep the current
	    converted value (and the scale!) as *negative* numbers,
	    so the sign is the opposite of what you might expect.
	    Should the test in the loop be isspace(*src)?
	*/
	while (*src == ' ' || *src == '\t') src++;
	sign = -1;
	if (*src == '+') src++; else
	if (*src == '-') src++, sign = 1;

	/*  Check that there is at least one digit  */

	if (_c2type[1+ *src] >= radix) {
	    errno = EDOM;
	    return NullS;
	}

	/*  Skip leading zeros so that we never compute a power of radix
	    in scale that we won't have a need for.  Otherwise sticking
	    enough 0s in front of a number could cause the multiplication
	    to overflow when it neededn't.
	*/
	while (*src == '0') src++;

	/*  Move over the remaining digits.  We have to convert from left
	    to left in order to avoid overflow.  Answer is after last digit.
	*/
	for (n = 0; _c2type[1+ *src++] < radix; n++) ;
	answer = --src;

	/*  The invariant we want to maintain is that src is just
	    to the right of n digits, we've converted k digits to
	    sofar, scale = -radix**k, and scale < sofar < 0.  Now
	    if the final number is to be within the original
	    Limit, we must have (to the left)*scale+sofar >= Limit,
	    or (to the left)*scale >= Limit-sofar, i.e. the digits
	    to the left of src must form an integer <= (Limit-sofar)/(scale).
	    In particular, this is true of the next digit.  In our
	    incremental calculation of Limit,

		IT IS VITAL that (-|N|)/(-|D|) = |N|/|D|
	*/
	
	for (sofar = 0, scale = -1; --n >= 0; ) {
	    d = _c2type[1+ *--src];
	    if (-d < limit) {
		errno = ERANGE;
		return NullS;
	    }
	    limit = (limit+d)/radix, sofar += d*scale;
	    if (n != 0) scale *= radix;	/* watch out for overflow!!! */
	}    
	/*  Now it might still happen that sofar = -32768 or its equivalent,
	    so we can't just multiply by the sign and check that the result
	    is in the range lower..upper.  All of this caution is a right
	    pain in the neck.  If only there were a standard routine which
	    says generate thus and such a signal on integer overflow...
	    But not enough machines can do it *SIGH*.
	*/
	if (sign < 0 && sofar < -MaxLong /* twos-complement problem */
	||  (sofar*=sign) < lower || sofar > upper) {
	    errno = ERANGE;
	    return NullS;
	}
	*val = sofar;
	errno = 0;		/* indicate that all went well */
	return answer;
    }


int atoi(src)
    char *src;
    {
	long val;
	str2int(src, 10, MinInt, MaxInt, &val);
	return (int)val;
    }


long atol(src)
    char *src;
    {
	long val;
	str2int(src, 10, MinLong, MaxLong, &val);
	return val;
    }