atof, atoi, _atoi64, atol

Convert strings to double (atof), integer (atoi, _atoi64), or long (atol).



__int64 _atoi64(const char\string);*


Routine Required Header Compatibility
atof <math.h> and <stdlib.h> ANSI, Win 95, Win NT
atoi <stdlib.h> ANSI, Win 95, Win NT
_atoi64 <stdlib.h> Win 95, Win NT
atol <stdlib.h> ANSI, Win 95, Win NT

For additional compatibility information, see Compatibility in the Introduction.


LIBC.LIB Single thread static library, retail version
LIBCMT.LIB Multithread static library, retail version
MSVCRT.LIB Import library for MSVCRT.DLL, retail version

Return Value

Each function returns the double, int, __int64 or long value produced by interpreting the input characters as a number. The return value is 0 (for atoi and _atoi64), 0L (for atol), or 0.0 (for atof) if the input cannot be converted to a value of that type. The return value is undefined in case of overflow.



String to be converted


These functions convert a character string to a double-precision floating-point value (atof), an integer value (atoi and _atoi64), or a long integer value (atol). The input string is a sequence of characters that can be interpreted as a numerical value of the specified type. The output value is affected by the setting of the LC_NUMERIC category in the current locale. For more information on the LC_NUMERIC category, see setlocale. The longest string size that atof can handle is 100 characters. The function stops reading the input string at the first character that it cannot recognize as part of a number. This character may be the null character ('\0') terminating the string.

The string argument to atof has the following form:

[whitespace] [sign] [digits] [.digits] [ {d | D | e | E }[sign]digits]

A whitespace consists of space and/or tab characters, which are ignored; sign is either plus (+) or minus ( – ); and digits are one or more decimal digits. If no digits appear before the decimal point, at least one must appear after the decimal point. The decimal digits may be followed by an exponent, which consists of an introductory letter ( d, D, e, or E) and an optionally signed decimal integer.

atoi, _atoi64, and atol do not recognize decimal points or exponents. The string argument for these functions has the form:

[whitespace] [sign]digits

where whitespace, sign, and digits are exactly as described above for atof.

Generic-Text Routine Mappings

TCHAR.H Routine _UNICODE & _MBCS Not Defined _MBCS Defined _UNICODE Defined
_ttoi atoi atoi _wtoi
_ttol atol atol _wtol


/* ATOF.C: This program shows how numbers stored
 * as strings can be converted to numeric values
 * using the atof, atoi, and atol functions.

#include <stdlib.h>
#include <stdio.h>

void main( void )
   char *s; double x; int i; long l;

   s = "  -2309.12E-15";    /* Test of atof */
   x = atof( s );
   printf( "atof test: ASCII string: %s\tfloat:  %e\n", s, x );

   s = "7.8912654773d210";  /* Test of atof */
   x = atof( s );
   printf( "atof test: ASCII string: %s\tfloat:  %e\n", s, x );

   s = "  -9885 pigs";      /* Test of atoi */
   i = atoi( s );
   printf( "atoi test: ASCII string: %s\t\tinteger: %d\n", s, i );

   s = "98854 dollars";     /* Test of atol */
   l = atol( s );
   printf( "atol test: ASCII string: %s\t\tlong: %ld\n", s, l );


atof test: ASCII string:   -2309.12E-15   float:  -2.309120e-012
atof test: ASCII string: 7.8912654773d210   float:  7.891265e+210
atoi test: ASCII string:   -9885 pigs      integer: -9885
atol test: ASCII string: 98854 dollars      long: 98854

Data Conversion Routines | Floating-Point Support Routines | Locale Routines

See Also _ecvt, _fcvt, _gcvt, setlocale, strtod, wcstol, strtoul