strtod.3p (9751B)
- '\" et
- .TH STRTOD "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- strtod,
- strtof,
- strtold
- \(em convert a string to a double-precision number
- .SH SYNOPSIS
- .LP
- .nf
- #include <stdlib.h>
- .P
- double strtod(const char *restrict \fInptr\fP, char **restrict \fIendptr\fP);
- float strtof(const char *restrict \fInptr\fP, char **restrict \fIendptr\fP);
- long double strtold(const char *restrict \fInptr\fP, char **restrict \fIendptr\fP);
- .fi
- .SH DESCRIPTION
- The functionality described on this reference page is aligned with the
- ISO\ C standard. Any conflict between the requirements described here and the
- ISO\ C standard is unintentional. This volume of POSIX.1\(hy2017 defers to the ISO\ C standard.
- .P
- These functions shall convert the initial portion of the string pointed
- to by
- .IR nptr
- to
- .BR double ,
- .BR float ,
- and
- .BR "long double"
- representation, respectively. First, they decompose the input string
- into three parts:
- .IP " 1." 4
- An initial, possibly empty, sequence of white-space characters (as
- specified by
- \fIisspace\fR())
- .IP " 2." 4
- A subject sequence interpreted as a floating-point constant or
- representing infinity or NaN
- .IP " 3." 4
- A final string of one or more unrecognized characters, including the
- terminating NUL character of the input string
- .P
- Then they shall attempt to convert the subject sequence to a
- floating-point number, and return the result.
- .P
- The expected form of the subject sequence is an optional
- .BR '+'
- or
- .BR '\-'
- sign, then one of the following:
- .IP " *" 4
- A non-empty sequence of decimal digits optionally containing a radix
- character; then an optional exponent part consisting of the character
- .BR 'e'
- or the character
- .BR 'E' ,
- optionally followed by a
- .BR '+'
- or
- .BR '\-'
- character, and then followed by one or more decimal digits
- .IP " *" 4
- A 0x or 0X, then a non-empty sequence of hexadecimal digits optionally
- containing a radix character; then an optional binary exponent part
- consisting of the character
- .BR 'p'
- or the character
- .BR 'P' ,
- optionally followed by a
- .BR '+'
- or
- .BR '\-'
- character, and then followed by one or more decimal digits
- .IP " *" 4
- One of INF or INFINITY, ignoring case
- .IP " *" 4
- One of NAN or NAN(\fIn-char-sequence\s-3\dopt\u\s+3\fR), ignoring case in
- the NAN part, where:
- .RS 4
- .sp
- .RS 4
- .nf
- n-char-sequence:
- digit
- nondigit
- n-char-sequence digit
- n-char-sequence nondigit
- .fi
- .P
- .RE
- .RE
- .P
- The subject sequence is defined as the longest initial subsequence of
- the input string, starting with the first non-white-space character,
- that is of the expected form. The subject sequence contains no
- characters if the input string is not of the expected form.
- .P
- If the subject sequence has the expected form for a floating-point
- number, the sequence of characters starting with the first digit or the
- decimal-point character (whichever occurs first) shall be interpreted
- as a floating constant of the C language, except that the radix
- character shall be used in place of a period, and that if neither an
- exponent part nor a radix character appears in a decimal floating-point
- number, or if a binary exponent part does not appear in a hexadecimal
- floating-point number, an exponent part of the appropriate type with
- value zero is assumed to follow the last digit in the string. If the
- subject sequence begins with a
- <hyphen-minus>,
- the sequence shall be
- interpreted as negated. A character sequence INF or INFINITY shall be
- interpreted as an infinity, if representable in the return type, else
- as if it were a floating constant that is too large for the range of
- the return type. A character sequence NAN or
- NAN(\fIn-char-sequence\s-3\dopt\u\s+3\fR) shall be interpreted as a
- quiet NaN, if supported in the return type, else as if it were a
- subject sequence part that does not have the expected form; the meaning
- of the \fIn\fR-char sequences is implementation-defined. A pointer to
- the final string is stored in the object pointed to by
- .IR endptr ,
- provided that
- .IR endptr
- is not a null pointer.
- .P
- If the subject sequence has the hexadecimal form and FLT_RADIX is a
- power of 2, the value resulting from the conversion is correctly
- rounded.
- .P
- The radix character is defined in the current locale (category
- .IR LC_NUMERIC ).
- In the POSIX locale, or in a locale where the radix character is not
- defined, the radix character shall default to a
- <period>
- (\c
- .BR '.' ).
- .P
- In other than the C
- or POSIX
- locale, additional locale-specific subject sequence forms may be
- accepted.
- .P
- If the subject sequence is empty or does not have the expected form, no
- conversion shall be performed; the value of
- .IR nptr
- is stored in the object pointed to by
- .IR endptr ,
- provided that
- .IR endptr
- is not a null pointer.
- .P
- These functions shall not change the setting of
- .IR errno
- if successful.
- .P
- Since 0 is returned on error and is also a valid return on success,
- an application wishing to check for error situations should set
- .IR errno
- to 0, then call
- \fIstrtod\fR(),
- \fIstrtof\fR(),
- or
- \fIstrtold\fR(),
- then check
- .IR errno .
- .SH "RETURN VALUE"
- Upon successful completion, these functions shall return the converted
- value. If no conversion could be performed, 0 shall be returned, and
- .IR errno
- may be set to
- .BR [EINVAL] .
- .P
- If the correct value is outside the range of representable values,
- \(+-HUGE_VAL, \(+-HUGE_VALF, or \(+-HUGE_VALL shall be returned
- (according to the sign of the value), and
- .IR errno
- shall be set to
- .BR [ERANGE] .
- .P
- If the correct value would cause an underflow, a value whose magnitude
- is no greater than the smallest normalized positive number in the
- return type shall be returned and
- .IR errno
- set to
- .BR [ERANGE] .
- .SH ERRORS
- These functions shall fail if:
- .TP
- .BR ERANGE
- The value to be returned would cause overflow
- or underflow.
- .P
- These functions may fail if:
- .TP
- .BR EINVAL
- No conversion could be performed.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- None.
- .SH "APPLICATION USAGE"
- If the subject sequence has the hexadecimal form and FLT_RADIX is not a
- power of 2, and the result is not exactly representable, the result
- should be one of the two numbers in the appropriate internal format
- that are adjacent to the hexadecimal floating source value, with the
- extra stipulation that the error should have a correct sign for the
- current rounding direction.
- .P
- If the subject sequence has the decimal form and at most DECIMAL_DIG
- (defined in
- .IR <float.h> )
- significant digits, the result should be correctly rounded. If the
- subject sequence
- .IR D
- has the decimal form and more than DECIMAL_DIG significant digits,
- consider the two bounding, adjacent decimal strings
- .IR L
- and
- .IR U ,
- both having DECIMAL_DIG significant digits, such that the values of
- .IR L ,
- .IR D ,
- and
- .IR U
- satisfy
- .IR L
- <=
- .IR D
- <=
- .IR U .
- The result should be one of the (equal or adjacent) values that would
- be obtained by correctly rounding
- .IR L
- and
- .IR U
- according to the current rounding direction, with the extra stipulation
- that the error with respect to
- .IR D
- should have a correct sign for the current rounding direction.
- .P
- The changes to
- \fIstrtod\fR()
- introduced by the ISO/IEC\ 9899:\|1999 standard can alter the behavior of well-formed
- applications complying with the ISO/IEC\ 9899:\|1990 standard and thus earlier versions of
- this standard. One such example would be:
- .sp
- .RS 4
- .nf
- int
- what_kind_of_number (char *s)
- {
- char *endp;
- double d;
- long l;
- .P
- d = strtod(s, &endp);
- if (s != endp && *endp == `\e0\(aq)
- printf("It\(aqs a float with value %g\en", d);
- else
- {
- l = strtol(s, &endp, 0);
- if (s != endp && *endp == `\e0\(aq)
- printf("It\(aqs an integer with value %ld\en", 1);
- else
- return 1;
- }
- return 0;
- }
- .fi
- .P
- .RE
- .P
- If the function is called with:
- .sp
- .RS 4
- .nf
- what_kind_of_number ("0x10")
- .fi
- .P
- .RE
- .P
- an ISO/IEC\ 9899:\|1990 standard-compliant library will result in the function printing:
- .sp
- .RS 4
- .nf
- It\(aqs an integer with value 16
- .fi
- .P
- .RE
- .P
- With the ISO/IEC\ 9899:\|1999 standard, the result is:
- .sp
- .RS 4
- .nf
- It\(aqs a float with value 16
- .fi
- .P
- .RE
- .P
- The change in behavior is due to the inclusion of floating-point
- numbers in hexadecimal notation without requiring that either a decimal
- point or the binary exponent be present.
- .SH RATIONALE
- None.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .IR "\fIfscanf\fR\^(\|)",
- .IR "\fIisspace\fR\^(\|)",
- .IR "\fIlocaleconv\fR\^(\|)",
- .IR "\fIsetlocale\fR\^(\|)",
- .IR "\fIstrtol\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "Chapter 7" ", " "Locale",
- .IR "\fB<float.h>\fP",
- .IR "\fB<stdlib.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 by the Institute of
- Electrical and Electronics Engineers, Inc and The Open Group.
- In the event of any discrepancy between this version and the original IEEE and
- The Open Group Standard, the original IEEE and The Open Group Standard
- is the referee document. The original Standard can be obtained online at
- http://www.opengroup.org/unix/online.html .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .