Section number field

Section numbers are listed in Table 7-22, ``Section number''.

Table 7-22 Section number

 Mnemonic   Section Number   Meaning
 -----------------------------------------------------
 N_DEBUG             -2      Special symbolic
                             debugging symbol
 N_ABS               -1      Absolute symbol
 N_UNDEF              0      Undefined external symbol
 N_SCNUM       1-077777      Section number where
                             symbol is defined

A special section number (-2) marks symbolic debugging symbols, including structure/union/enumeration tag names, typedefs, and the name of the file. A section number of -1 indicates that the symbol has a value but is not relocatable. Examples of absolute-valued symbols include automatic and register variables, function arguments, and .eos symbols.

With one exception, a section number of 0 indicates a relocatable external symbol that is not defined in the current file. The one exception is a multiply-defined external symbol (i.e., FORTRAN common or an uninitialized variable-defined external to a function in C). In the symbol table of each file where the symbol is defined, the section number of the symbol is 0, and the value of the symbol is a positive number giving the size of the symbol. When the files are combined to form an executable object file, the link editor combines all the input symbols of the same name into one symbol with the section number of the .bss section. The maximum size of all the input symbols with the same name is used to allocate space for the symbol and the value becomes the address of the symbol. This is the only case where a symbol has a section number of 0 and a non-zero value.