atof

 

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Technical Specifications

 

Strings library

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Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C99)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to numeric formats
atof atoiatolatoll strtolstrtoll	strtoulstrtoull strtofstrtodstrtold strtoimaxstrtoumax (C99)(C99)
String manipulation
strcpystrcpy_s (C11)
strncpystrncpy_s (C11)
strcatstrcat_s (C11)
strncatstrncat_s (C11)
strxfrm
String examination
strlenstrnlen_s (C11) strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtokstrtok_s (C11)
Memory manipulation
memchr
memcmp
memsetmemset_s (C11)
memcpymemcpy_s (C11)
memmovememmove_s (C11)
Miscellaneous
strerrorstrerror_sstrerrorlen_s (C11)(C11)

 

Defined in header <stdlib.h>
double atof( const char* str );

Interprets a floating-point value in a byte string pointed to by str.

Function discards any whitespace characters (as determined by std::isspace()) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating-point representation and converts them to a floating-point value. The valid floating-point value can be one of the following:

• decimal floating-point expression. It consists of the following parts:

• (optional) plus or minus sign
• nonempty sequence of decimal digits optionally containing decimal-point character (as determined by the current C locale) (defines significand)
• (optional) e or E followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)

• binary floating-point expression. It consists of the following parts:

• (optional) plus or minus sign
• 0x or 0X
• nonempty sequence of hexadecimal digits optionally containing a decimal-point character (as determined by the current C locale) (defines significand)
• (optional) p or P followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)

• infinity expression. It consists of the following parts:

• (optional) plus or minus sign
• INF or INFINITY ignoring case

• not-a-number expression. It consists of the following parts:

• (optional) plus or minus sign
• NAN or NAN(char_sequence) ignoring case of the NAN part. char_sequence can only contain alphanumeric characters. The result is a quiet NaN floating-point value.

• any other expression that may be accepted by the currently installed C locale

Parameters

str

-

pointer to the null-terminated byte string to be interpreted

Return value

double value corresponding to the contents of str on success. If the converted value falls out of range of the return type, the return value is undefined. If no conversion can be performed, 0.0 is returned.

Example

Run this code

#include <stdlib.h>
#include <stdio.h>
 
int main(void)
{
    printf("%g\n", atof("  -0.0000000123junk"));
    printf("%g\n", atof("0.012"));
    printf("%g\n", atof("15e16"));
    printf("%g\n", atof("-0x1afp-2"));
    printf("%g\n", atof("inF"));
    printf("%g\n", atof("Nan"));
    printf("%g\n", atof("1.0e+309"));   // UB: out of range of double
    printf("%g\n", atof("0.0"));
    printf("%g\n", atof("junk"));       // no conversion can be performed
}

Possible output:

-1.23e-08
0.012
1.5e+17
-107.75
inf
nan
inf
0
0

References

• C11 standard (ISO/IEC 9899:2011):

• 7.22.1.1 The atof function (p: 341)

• C99 standard (ISO/IEC 9899:1999):

• 7.20.1.1 The atof function (p: 307)

• C89/C90 standard (ISO/IEC 9899:1990):

• 4.10.1.1 The atof function

See also

strtofstrtodstrtold (C99)(C99)	converts a byte string to a floating point value (function)
C++ documentation for atof