Why is absence of array index in "extern char name[]" not affecting strlen(name) but causing error for sizeof(name)?

Question

Now,from what I understand,the extern in the declaration of name[] tells the compiler that its definition is somewhere else (In my program,I have defined it below the part where I have used it).But why then there is different consequence for strlen() and sizeof?strlen() works fine but for sizeof() I get the error:

invalid application of 'sizeof' to incomplete type 'char[]' |

Here's my program:

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

extern char name[];

int main()
{
printf("%d,%d",strlen(name),sizeof(name));
}

char name[]="Bryan";
//char name[6]="Bryan"; //Neither of these make sizeof work

I tried to reason it out based on my understanding of the extern keyword,but I have hit a bottleneck.So please give your answers.

Answer 1

Because sizeof doesn't know the contents the variable at the point where sizeof is, but strlen doesn't care about the compile-time size, it simply walks from the beginning of the string until it finds a NUL character marking the end of the string.

For example:

 char name[40] = "Bryan";

 ... 

    size_t size = sizeof(name);
    size_t len = strlen(name)

    strcat(name, " Adams"); 

    printf("%s: len=%zd, size=%zd, strlen(name)=%zd\n", 
           name, len, size, strlen(name)); 

This will show

Bryan Adams: len=5, size=40, strlen(name)=11