Write a program to reverse a string without using theandfunctions defined in the string.h library.

Sure, here's a program written in C to reverse a string without using any predefined functions from string.h library:

#include<stdio.h>

void reverseString(char str[]);

int main()
{
    char str[100];

    printf("Enter a string: ");
    scanf("%s", str);

    reverseString(str);

    printf("Reversed string is: %s", str);

    return 0;
}

void reverseString(char str[])
{
    int length = 0;

    // Calculate the length of the string
    while (str[length] != '\0')
    {
        length++;
    }

    // Swap characters from beginning and end
    for (int i = 0; i < length / 2; i++)
    {
        char temp = str[i];
        str[i] = str[length - i - 1];
        str[length - i - 1] = temp;
    }
}

In this program, we first take input from the user and pass it to the `reverseString()` function. Inside this function, we calculate the length of the string by iterating through it till we find the null character `'\0'`. Then, we swap the first character with the last one, second with second last one, and so on till we reach the middle of the string. Finally, we print the reversed string.