strtof, strtod, strtold

 

C

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Technical Specifications

Symbol index

 

Strings library

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Null-terminated byte strings

Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C99)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to and from numeric formats
atoiatolatoll (C99) atof strtolstrtoll (C99) strtoulstrtoull (C99)	strtoimaxstrtoumax (C99)(C99) strtofstrtodstrtold (C99)(C99) strfromfstrfromdstrfroml (C23)(C23)(C23)
String manipulation
strcpystrcpy_s (C11) strncpystrncpy_s (C11) strcatstrcat_s (C11)	strncatstrncat_s (C11) strxfrm strdup (C23) strndup (C23)
String examination
strlenstrnlen_s (C11) strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtokstrtok_s (C11)
Memory manipulation
memchr memcmp memsetmemset_explicitmemset_s (C23)(C11)	memcpymemcpy_s (C11) memmovememmove_s (C11) memccpy (C23)
Miscellaneous
strerrorstrerror_sstrerrorlen_s (C11)(C11)

 

Defined in header <stdlib.h>
float       strtof( const char *restrict str, char **restrict str_end );		(since C99)
double      strtod( const char          *str, char          **str_end );		(until C99)
double      strtod( const char *restrict str, char **restrict str_end );		(since C99)
long double strtold( const char *restrict str, char **restrict str_end );		(since C99)

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

Function discards any whitespace characters (as determined by 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 to base 10)

hexadecimal 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 to base 2) 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 digits, Latin letters, and underscores. The result is a quiet NaN floating-point value.

(since C99)

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

The functions sets the pointer pointed to by str_end to point to the character past the last character interpreted. If str_end is a null pointer, it is ignored.

Parameters

str	-	pointer to the null-terminated byte string to be interpreted
str_end	-	pointer to a pointer to character.

Return value

Floating-point value corresponding to the contents of str on success. If the converted value falls out of range of corresponding return type, range error occurs (errno is set to ERANGE) and HUGE_VAL, HUGE_VALF or HUGE_VALL is returned. If no conversion can be performed, ​0​ is returned.

Example

Run this code

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
 
int main(void)
{
    // parsing with error handling
    const char *p = "111.11 -2.22 Nan nan(2) inF 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz";
    printf("Parsing '%s':\n", p);
    char *end;
    for (double f = strtod(p, &end); p != end; f = strtod(p, &end))
    {
        printf("'%.*s' -> ", (int)(end-p), p);
        p = end;
        if (errno == ERANGE){
            printf("range error, got ");
            errno = 0;
        }
        printf("%f\n", f);
    }
 
    // parsing without error handling
    printf("\"  -0.0000000123junk\"  -->  %g\n", strtod("  -0.0000000123junk", NULL));
    printf("\"junk\"                 -->  %g\n", strtod("junk", NULL));
}

Possible output:

Parsing '111.11 -2.22 Nan nan(2) inF 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz':
'111.11' -> 111.110000
' -2.22' -> -2.220000
' Nan' -> nan
' nan(2)' -> nan
' inF' -> inf
' 0X1.BC70A3D70A3D7P+6' -> 111.110000
'  1.18973e+4932' -> range error, got inf
"  -0.0000000123junk"  -->  -1.23e-08
"junk"                 -->  0

References

• C17 standard (ISO/IEC 9899:2018):

• 7.22.1.3 The strtod, strtof, and strtold functions (p: 249-251)

• C11 standard (ISO/IEC 9899:2011):

• 7.22.1.3 The strtod, strtof, and strtold functions (p: 342-344)

• C99 standard (ISO/IEC 9899:1999):

• 7.20.1.3 The strtod, strtof, and strtold functions (p: 308-310)

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

• 4.10.1.4 The strtod function

See also

atof	converts a byte string to a floating-point value (function)
wcstofwcstodwcstold (C99)(C95)(C99)	converts a wide string to a floating-point value (function)
C++ documentation for strtof, strtod, strtold