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strtod (string to double) is a C language function that converts a character string to a double precision floating-point value. It is defined as:

double strtod ( const char * str, char ** endptr );[1]



The strtod function parses the C string str interpreting its content as a floating point number and returns its value as a double. If endptr is not a null pointer, the function also sets the value pointed by endptr to point to the first character after the number.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes as many characters as possible that are valid following a syntax resembling that of floating point literals, and interprets them as a numerical value. A pointer to the rest of the string after the last valid character is stored in the object pointed by endptr.

A valid floating point number for strtod is formed by a succession of:

  • An optional plus or minus sign
  • A sequence of digits, optionally containing a decimal-point character
  • An optional exponent part, which itself consists on an 'e' or 'E' character followed by an optional sign and a sequence of digits.

If the first sequence of non-whitespace characters in str does not form a valid floating-point number as just defined, or if no such sequence exists because either str is empty or contains only whitespace characters, no conversion is performed.



  • C string beginning with the representation of a floating-point number.


  • Reference to an already allocated object of type char*, whose value is set by the function to the next character in str after the numerical value. This parameter can also be a null pointer, in which case it is not used.

Return value

On success, the function returns the converted floating point number as a double value. If no valid conversion could be performed, a zero value (0.0) is returned. If the correct value is out of the range of representable values, a positive or negative HUGE_VAL is returned, and the global variable errno is set to ERANGE. If the correct value would cause underflow, zero is returned and errno is set to ERANGE.

The strtod function is included in the stdlib.h header file of the ISO/IEC standard C library.

See also


  1. "strtod - convert a string to a double-precision number". 

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