fgets and strlen in c - c

Question

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
main()
{
char str[5]={'\0'};
printf("Initial length before passing = %ld\n",strlen(str));
input(str);
printf("Received string = %s\n",str);
printf("Length after getting input(calling function) = %ld\n",sizeof(str));
}
input(char * buffer)
{
puts("Enter something: ");
printf("Initial length after passing = %ld\n",strlen(buffer));
if ( fgets(buffer, sizeof(buffer), stdin) == NULL )
return -1;
else
{
printf("Length after getting input(called function)= %ld\n",strlen(buffer));
return 0;
}
}
Output 1
Initial length before passing = 0
Enter something:
Initial length after passing = 0
hello
Length after getting input(called function)= 6
Received string = hello
Length after getting input(calling function) = 5
Output 2
Initial length before passing = 0
Enter something:
Initial length after passing = 0
helloooooo
Length after getting input(called function)= 7
Received string = hellooo
Length after getting input(calling function) = 5
Why is it printing different lengths in when I gave different input?
In output 1 & 2 why the initial length is 6 when I allocated space for only 5 characters?
Why is the length of string different before passing and after passing in both output 1 and output 2?
In output 2 why "Length after getting input(called function)= 7" when I allocated only less space?

Answer 1

strlen was supposed to be working with C strings, that is, char arrays that ends with \0. When you define:
char str[5];
str contains garbage, you are lucky calling strlen didn't cause segmentation fault.

Answer 2

1) In output 1 & 2 why the initial length is 6 when i allocated space for only 5 characters?
Your first strlen(str) call isn't really even defined since you declared char str[5] but didn't put anything into it. So the contents are who-knows-what. That fact that strlen(str) returned a 6 just means that there happened to be 6 non-zero characters in memory, starting at address str before it encountered a 0.
2) why the length of string is different before passing and after passing in both output 1 and output 2?
After getting a length of 6 for random memory contents, you loaded something into the string buffer and zero terminated it. So the length changed to something real.
3) In output 2 why "Length after getting input(called function)= 7" when i allocated only less space?
Because you actually overran your allocated space with a longer string (of length 7). You were lucky the program didn't crash in that case.
When you declare a buffer in C, such as:
char str[5];
All it does is tell the compiler you're reserving 5 bytes of space to do something and it entitles you to use that space and only that space for str. It doesn't necessarily have anything in the buffer to start unless you put something there, and it doesn't prevent you from writing more than you declared.
Note that str[5] isn't big enough to hold "hello" since strings in C are zero-terminated. So you need a character buffer of size N+1 to hold a string of size N. When you overflow buffers in C, your results will become erratic and unpredictable.

Answer 3

char str[5];
printf("Initial length before passing = %ld\n",strlen(str));
strlen returns the length of specified string, not the size of the buffer. You didn't initialize str to anything, so it's full of garbage. strlen will return the "length" of that garbage, which could be anything.