String manipulation without using lib funcs

#include<stdio.h>
int len(char *x)
{
    int i=0;
    while(x[i]!='\0')
        i++;
    return i;
}
char * cpy(char *x, char *y)
{
    int i=0,l=len(y);
    while(i<l)
    {
        x[i]=y[i];
        i++;
    }
    x[i]='\0';
    return x;
}
char *cat(char *x, char* y)
{
    int i=len(x),j=0,l=len(y);
    while(j<l)
    {
        x[i]=y[j];
        i++;
        j++;
    }
    x[i]='\0';
    return x;
}
int cmp(char *x, char*y)
{
    int l1=len(x),l2=len(y),i=0,temp;
    temp=(l1<l2)?l1:l2;
    while(i<temp)
    {
        if(x[i]==y[i])
            ;
        else if(x[i]>y[i])
            return 1;
        else
            return -1;
        i++;
    }
    if(x[i]!='\0')
        return 1;
    else if(y[i]!='\0')
        return -1;
    else
        return 0;
}
int cmpi(char *x, char*y)
{
    int l1=len(x),l2=len(y),i=0,temp;
    temp=(l1<l2)?l1:l2;
    while(i<temp)
    {
        if(x[i]==y[i] || x[i]+32==y[i] || x[i]==y[i]+32)
            ;
        else if(x[i]>y[i])
            return 1;
        else
            return -1;
        i++;
    }
    if(x[i]!='\0')
        return 1;
    else if(y[i]!='\0')
        return -1;
    else
        return 0;
}
char *rev(char *x)
{
    int i=0,j=len(x)-1;
    char temp;
    while(i<len(x)/2)
    {
        temp=x[i];
        x[i]=x[j];
        x[j]=temp;
        i++;
        j--;
    }
    return x;
}
char *lwr(char *x)
{
    int i,l=len(x);
    for(i=0;i<l;i++)
    {
        if(x[i]>=65 && x[i]<=90)
            x[i]+=32;
    }
    return x;
}
char *upr(char *x)
{
    int i,l=len(x);
    for(i=0;i<l;i++)
    {
        if(x[i]>=97 && x[i]<=122)
            x[i]-=32;
    }
    return x;
}
void main()
{
    char str1[100],str2[100];
    printf("STRING MANIPULATION WITHOUT USING LIBRARY FUNCTIONS\n");
    int ch1=0,ch2,flag=0,temp;
    while(ch1<1 || ch1>6)
    {
        if(flag)
            printf("Invalid input!");
        printf("1.Length of string\n2.Copy a string\n3.Concatenate two strings\n4.Compare two strings\n5.Reverse a string\n6.Case conversion\nEnter your choice:");
        scanf("%d",&ch1);
        flag++;
    }
    switch(ch1)
    {
        case 1:
            printf("Enter a string:");
            scanf("%s",&str1);
            printf("Length of %s is %d.",str1,len(str1));
            break;
        case 2:
            printf("Enter source string and target string:");
            scanf("%s %s",&str1,&str2);
            cpy(str1,str2);
            printf("Contents of str2 after copying:%s",str2);
            break;
        case 3:
            printf("Enter two strings:");
            scanf("%s %s",&str1,&str2);
            printf("Before concatenation:\nstr1:%s\tstr2:%s",str1,str2);
            cat(str1,str2);
            printf("\nAfter concatenation:\nstr1:%s\tstr2:%s",str1,str2);
            break;
        case 4:
            flag=0;
            ch2=0;
            while(ch2!= 1 && ch2!=2)
            {
                if(flag)
                    printf("Invalid input!\n");
                printf("1.Case sensitive comparison\n2.Case insensitive comparison\nEnter your choice:");
                scanf("%d",&ch2);
                flag++;
            }
            switch(ch2)
            {
                case 1:
                    printf("Enter two strings:");
                    scanf("%s %s",&str1,&str2);
                    temp=cmp(str1,str2);
                    printf("Result of comparison:%d\n",temp);
                    if(temp==0)
                        printf("The strings are equal.");
                    else
                        printf("The strings are not equal.");
                    break;
                case 2:
                    printf("Enter two strings:");
                    scanf("%s %s",&str1,&str2);
                    temp=cmpi(str1,str2);
                    printf("Result of comparison:%d\n",temp);
                    if(temp==0)
                        printf("The strings are equal.");
                    else
                        printf("The strings are not equal.");
                    break;
            }
            break;
        case 5:
            printf("Enter a string:");
            scanf("%s",str1);
            printf("Reverse of %s is ",str1);
            printf("%s",rev(str1));
            break;
        case 6:
            flag=0;
            ch2=0;
            while(ch2!=1 && ch2!=2)
            {
                if(flag)
                    printf("Invalid input!\n");
                printf("1.Upper case to lower case\n2.Lower case to upper case\nEnter your chocie:");
                scanf("%d",&ch2);
                flag++;
            }
            switch(ch2)
            {
                case 1:
                    printf("Enter the string:");
                    scanf("%s",&str1);
                    printf("str1 before case-conversion:%s",str1);
                    lwr(str1);
                    printf("\nstr1 after case-conversion:%s",str1);
                    break;
                case 2:
                    printf("Enter the string:");
                    scanf("%s",&str1);
                    printf("str1 before case-conversion:%s",str1);
                    upr(str1);
                    printf("\nstr1 after case-conversion:%s",str1);
                    break;
            }
            break;
    }
}