The C programming language provides a variety of string and character manipulation functions in its standard library, which is commonly referred to as the C Standard Library. Here are some commonly used string and character functions, along with sample code:

1. strlen (String Length)

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "Hello, World!";
    int length = strlen(str);

    printf("Length of the string: %d\n", length);

    return 0;
}

2. strcpy (String Copy)

#include <stdio.h>
#include <string.h>

int main() {
    char source[] = "Hello, World!";
    char destination[20];

    strcpy(destination, source);

    printf("Copied string: %s\n", destination);

    return 0;
}

3. strcat (String Concatenation)

#include <stdio.h>
#include <string.h>

int main() {
    char str1[] = "Hello, ";
    char str2[] = "World!";

    strcat(str1, str2);

    printf("Concatenated string: %s\n", str1);

    return 0;
}

4. strcmp (String Comparison)

#include <stdio.h>
#include <string.h>

int main() {
    char str1[] = "Hello";
    char str2[] = "World";

    int result = strcmp(str1, str2);

    if (result == 0)
        printf("Strings are equal\n");
    else if (result < 0)
        printf("String 1 is less than String 2\n");
    else
        printf("String 1 is greater than String 2\n");

    return 0;
}

5. strchr (Find Character in String)

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "Hello, World!";
    char *ptr = strchr(str, 'W');

    if (ptr != NULL)
        printf("Character found at position: %ld\n", ptr - str + 1);
    else
        printf("Character not found\n");

    return 0;
}

6. strstr (Find Substring in String)

#include <stdio.h>
#include <string.h>

int main() {
    char str[] = "Hello, World!";
    char *ptr = strstr(str, "World");

    if (ptr != NULL)
        printf("Substring found at position: %ld\n", ptr - str + 1);
    else
        printf("Substring not found\n");

    return 0;
}

7. gets (Get String)

#include <stdio.h>

int main() {
    char str[50];

    printf("Enter a string: ");
    gets(str);

    printf("You entered: %s\n", str);

    return 0;
}

8. puts (Put String)

#include <stdio.h>

int main() {
    char str[] = "Hello, World!";

    puts("String to be printed: ");
    puts(str);

    return 0;
}

9. getchar (Get Character)

#include <stdio.h>

int main() {
    char ch;

    printf("Enter a character: ");
    ch = getchar();

    printf("You entered: %c\n", ch);

    return 0;
}

10. putchar (Put Character)

#include <stdio.h>

int main() {
    char ch = 'A';

    printf("Character to be printed: ");
    putchar(ch);

    return 0;
}

11. isalpha (Is Alphabet)

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

int main() {
    char ch = 'A';

    if (isalpha(ch))
        printf("%c is an alphabet.\n", ch);
    else
        printf("%c is not an alphabet.\n", ch);

    return 0;
}

12. isdigit (Is Digit)

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

int main() {
    char ch = '5';

    if (isdigit(ch))
        printf("%c is a digit.\n", ch);
    else
        printf("%c is not a digit.\n", ch);

    return 0;
}

13. isalnum (Is Alphanumeric)

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

int main() {
    char ch = 'A';

    if (isalnum(ch))
        printf("%c is alphanumeric.\n", ch);
    else
        printf("%c is not alphanumeric.\n", ch);

    return 0;
}

14. toupper and tolower (Convert to Upper/Lower Case)

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

int main() {
    char ch = 'a';

    printf("Uppercase: %c\n", toupper(ch));
    printf("Lowercase: %c\n", tolower(ch));

    return 0;
}