The atof 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 atof function is as follows:

double atof(const char *str);

Here, the argument str is a pointer to the null-terminated string to be converted.

The atof 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 atof 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";
   double value;
   value = atof(str);
   printf("The converted value is: %f\n", value);
   return 0;
}

In this example, the input string is the null-terminated string "3.14159". The atof function converts this string to the floating-point 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 atof function does not perform any error checking. If the input string is not a valid floating-point number, the function returns a result, but the result is undefined. It is the responsibility of the caller to ensure that the input string is a valid floating-point number before calling the atof function.