The isalnum() function in C is used to determine whether a character is an alphanumeric character (a letter or a digit). It returns an integer value that indicates the result of the check.

The return values of the isalnum() function are as follows:

1. Non-zero value: If the character is an alphanumeric character (a letter or a digit), the function returns a non-zero value. Typically, this value is 1 to indicate true or success.

0 (zero): If the character is not an alphanumeric character, the function returns zero to indicate false or failure.

The isalnum() function is part of the <ctype.h> header in C, which provides functions for character classification based on their types.