The C programming exp function is defined in the math.h header file and is used to calculate the exponential value of a given argument x, which is e raised to the power of x, where e is the mathematical constant approximately equal to 2.71828.

The exp function takes a single argument of type double, which represents the exponent to be calculated. The function returns a double value, which represents the calculated exponential value.

Here's an example usage of the exp function to calculate the exponential value of a given input:

#include <stdio.h>
#include <math.h>

int main() {
    double x = 2.5;
    double exp_value = exp(x);
    
    printf("The exponential value of %f is %f\n", x, exp_value);
    
    return 0;
}

In this example, the exp function is used to calculate the exponential value of 2.5, which is stored in the x variable. The calculated exponential value is then stored in the exp_value variable, and printed to the console using the printf function.

Note that the exp function can also be used to calculate the exponential value of negative numbers, which results in a value between 0 and 1. For example, exp(-2.5) would return a value of approximately 0.0821.