The strtod function in the stdlib.h library of the C programming language is used to convert a string that represents a floating-point number into a double value. The syntax of the strtod function is as follows:

double strtod(const char *str, char **endptr);

Here, the argument str is a pointer to the null-terminated string to be converted, and endptr is a pointer to a char * variable. If endptr is not NULL, the strtod function sets *endptr to the first character in str that is not part of the number being converted.

The strtod function scans the input string pointed to by str, and converts it to a double value. The function stops scanning the input string when it encounters the first character that cannot be part of a floating-point number. The function returns the converted value as a double value.

For example, the following code uses the strtod function to convert a string representing a floating-point number into a double value:

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

int main() {
   char str[] = "3.14159";
   char *endptr;
   double value;
   value = strtod(str, &endptr);
   printf("The converted value is: %f\n", value);
   return 0;
}

In this example, the input string is the null-terminated string "3.14159". The strtod function converts this string to the double value 3.14159, which is stored in the value variable. The program then uses the printf function to print the converted value. The output of the program is:

The converted value is: 3.141590

Note that the strtod function returns 0.0 if the input string is not a valid floating-point number. If endptr is not NULL, the strtod function sets *endptr to the first character in str that is not part of the number being converted. If the entire input string is part of the number being converted, *endptr is set to the null character ('\0'). It is the responsibility of the caller to check the value of *endptr to determine if the conversion was successful.