The int isxdigit(int c) function in the ctype.h library of C programming is used to determine whether a given character is a hexadecimal digit.

The isxdigit() function takes a single integer argument c, which is the character to be checked. The function returns an integer value, which is nonzero if the character is a hexadecimal digit (0-9, A-F, or a-f), and zero otherwise.

Here's an example of how to use the isxdigit() function in C:

#include <stdio.h>
#include <ctype.h>

int main() {
    char c = 'E';
    if (isxdigit(c)) {
        printf("%c is a hexadecimal digit\n", c);
    } else {
        printf("%c is not a hexadecimal digit\n", c);
    }
    return 0;
}

In this example, the isxdigit() function is used to check whether the character 'E' is a hexadecimal digit. Since 'E' is a hexadecimal digit, the function returns a nonzero value, and the output will be: 'E' is a hexadecimal digit'.

If the character had been a non-hexadecimal digit such as 'g', the isxdigit() function would return zero, and the output would be: 'g' is not a hexadecimal digit'.