#pragmaDATA_SECTION(variable, section_name) hangs up the link

In CCS4, when use #pragma DATA_SECTION(variable, section_name), the variable can't be used out of the file. If you do it, the link will be hung up. Here is the example: 

File_a.c:

#pragma DATA_SECTION(a, "section_name")

unsigned short a = 0;

main()

{

a = 100;

 }

File_b.c

extern unsigned short a;

Function_a()

{

a = 200;

}

When you compile and link, the link will hang up. Please help, thanks,