绝对正宗的atoi源码，非自己实现

下面是atoi.c源文件

#include 
#include 

#undef	atoi


/* Convert a string to an int.  */
int
DEFUN(atoi, (nptr), CONST char *nptr)
{
     
  return((int) strtol(nptr, (char **) NULL, 10));
}

下面是strtol.c源文件

/* Copyright (C) 1991, 1992, 1994 Free Software Foundation, Inc.

This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.  */

#include 
#include 
#include 
#include 


#ifndef	UNSIGNED
#define	UNSIGNED	0
#endif

/* Convert NPTR to an `unsigned long int' or `long int' in base BASE.
   If BASE is 0 the base is determined by the presence of a leading
   zero, indicating octal or a leading "0x" or "0X", indicating hexadecimal.
   If BASE is < 2 or > 36, it is reset to 10.
   If ENDPTR is not NULL, a pointer to the character after the last
   one converted is stored in *ENDPTR.  */
#if	UNSIGNED
unsigned long int
#define	strtol	strtoul
#else
long int
#endif
strtol (nptr, endptr, base)
     const char *nptr;
     char **endptr;
     int base;
{
     
  int negative;
  register unsigned long int cutoff;
  register unsigned int cutlim;
  register unsigned long int i;
  register const char *s;
  register unsigned char c;
  const char *save;
  int overflow;

  if (base < 0 || base == 1 || base > 36)
    base = 10;

  s = nptr;

  /* Skip white space.  */
  while (isspace (*s))
    ++s;
  if (*s == '\0')
    goto noconv;

  /* Check for a sign.  */
  if (*s == '-')
    {
     
      negative = 1;
      ++s;
    }
  else if (*s == '+')
    {
     
      negative = 0;
      ++s;
    }
  else
    negative = 0;

  if (base == 16 && s[0] == '0' && toupper (s[1]) == 'X')
    s += 2;

  /* If BASE is zero, figure it out ourselves.  */
  if (base == 0)
    if (*s == '0')
      {
     
	if (toupper (s[1]) == 'X')
	  {
     
	    s += 2;
	    base = 16;
	  }
	else
	  base = 8;
      }
    else
      base = 10;

  /* Save the pointer so we can check later if anything happened.  */
  save = s;

  cutoff = ULONG_MAX / (unsigned long int) base;
  cutlim = ULONG_MAX % (unsigned long int) base;

  overflow = 0;
  i = 0;
  for (c = *s; c != '\0'; c = *++s)
    {
     
      if (isdigit (c))
	c -= '0';
      else if (isalpha (c))
	c = toupper (c) - 'A' + 10;
      else
	break;
      if (c >= base)
	break;
      /* Check for overflow.  */
      if (i > cutoff || (i == cutoff && c > cutlim))
	overflow = 1;
      else
	{
     
	  i *= (unsigned long int) base;
	  i += c;
	}
    }

  /* Check if anything actually happened.  */
  if (s == save)
    goto noconv;

  /* Store in ENDPTR the address of one character
     past the last character we converted.  */
  if (endptr != NULL)
    *endptr = (char *) s;

#if	!UNSIGNED
  /* Check for a value that is within the range of
     `unsigned long int', but outside the range of `long int'.  */
  if (i > (negative ?
	   -(unsigned long int) LONG_MIN : (unsigned long int) LONG_MAX))
    overflow = 1;
#endif

  if (overflow)
    {
     
      errno = ERANGE;
#if	UNSIGNED
      return ULONG_MAX;
#else
      return negative ? LONG_MIN : LONG_MAX;
#endif
    }

  /* Return the result of the appropriate sign.  */
  return (negative ? -i : i);

noconv:
  /* There was no number to convert.  */
  if (endptr != NULL)
    *endptr = (char *) nptr;
  return 0L;
}

二，更多的C库源码：

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