Why does the atof() function not return 0 when I pass it a string with invalid characters after a number?

Question

I need to convert a string into a float. If the string is not a number I wish to return 0.

I have tried to test if the atof() function would work for this by using the following code:

printf("%f", atof("1a"));

From my understanding of atof, the value returned when atof cannot convert is 0, and yet this line prints 1.0.

Why does this happen? By the documentation I understood that atof is meant to return 0 whenever the input is not a number.

Answer 1

From doc. Emphasis mine

The function first discards as many whitespace characters (as in isspace) 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 (see below), and interprets them as a numerical value. The rest of the string after the last valid character is ignored and has no effect on the behavior of this function.