Difference between strlen and sizeof

strlen()

• strlen() is a predefined function in C.
• strlen() is used to get the length of an array of chars/string.

sizeof()

• sizeof is a unary operator in C.
• sizeof() is used to get the actual size of any type of data in bytes.

Some example to determine difference between strlen() and sizeof().

• sizeof() calculate the length of string including ‘\0’. Whereas strlen() calculate the length of string excluding ‘\0’.
  
  char arr[5] = "john"; printf("%d\n", strlen(arr)); //4 printf("%d\n", sizeof(arr)); //5

  1 2 3

  char arr[5] = "john"; printf("%d\n", strlen(arr));   //4 printf("%d\n", sizeof(arr));   //5

• .
  
  char arr[] = "jemjohn\0"; printf("%d\n", strlen(arr)); //7 printf("%d\n", sizeof(arr)); //9

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  char arr[] = "jemjohn\0"; printf("%d\n", strlen(arr));   //7 printf("%d\n", sizeof(arr));   //9

• strlen() stops calculating a length of string when the pointer points to ‘\0’, even though ‘\0’ is in the string.
  
  char arr[]="jem\0john"; printf("%d\n",strlen(arr)); //3 printf("%d\n",sizeof(arr)); //9

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  char arr[]="jem\0john"; printf("%d\n",strlen(arr));        //3 printf("%d\n",sizeof(arr));        //9

• .
  
  char arr[]="johnjohny"; printf("%d\n",sizeof(&arr[1])); //8 printf("%d\n",sizeof(arr)); //10 printf("%d\n",strlen(&arr[1])); //8 printf("%d\n",strlen(arr)); //9

  1 2 3 4 5 6

  char arr[]="johnjohny"; printf("%d\n",sizeof(&arr[1]));     //8 printf("%d\n",sizeof(arr));         //10 printf("%d\n",strlen(&arr[1]));     //8 printf("%d\n",strlen(arr));         //9

  Here in first printf statement, the address of array elements is always an integer. So that it return value of integer data type i.e, 4 bytes in the second printf statement, it returns the number of elements in arr including ‘\0’.

  strlen(&arr[1]) – This calculate length from arr[1] upto end of string excluding ‘\0’. strlen(arr) – it return the number of elements in arr excluding ‘\0’