File header

The file header contains the 20 bytes of information shown in ``File header contents''. The last 2 bytes are flags that are used by ld and object file utilities.

File header contents

Bytes	Declaration	Name	Description
0-1	unsigned short	f_magic	Magic number
2-3	unsigned short	f_nscns	Number of sections
4-7	long int	f_timdat	Time and date stamp indicating when the file was created, expressed as the number of elapsed seconds since 00:00:00 GMT, January 1, 1970
8-11	long int	f_symptr	File pointer containing the starting address of the symbol table
12-15	long int	f_nsyms	Number of entries in the symbol table
16-17	unsigned short	f_opthdr	Number of bytes in the optional header
18-19	unsigned short	f_flags	Flags (see ``File header flags'')

Magic numbers

The magic number specifies the target machine on which the object file is executable.

Flags

The last 2 bytes of the file header are flags that describe the type of the object file. Currently defined flags are found in the header file filehdr.h and are shown in ``File header flags''.

File header flags

Mnemonic	Flag	Meaning
F_RELFLG	00001	Relocation information stripped from the file
F_EXEC	00002	File is executable (i.e., no unresolved external references)
F_LNNO	00004	Line numbers stripped from the file
F_LSYMS	00010	Local symbols stripped from the file
F_AR16WR	0000200	16-bit byte reversed word
F_AR32WR	0000400	32-bit byte reversed word

File header declaration

The C structure declaration for the file header is shown below. This declaration may be found in the header file filehdr.h.

   struct filehdr
   {
     unsigned short   f_magic;  /* magic number */
     unsigned short   f_nscns;  /* number of section */
   

     long             f_timdat; /* time and date stamp */
   

     long             f_symptr; /* file ptr to symbol table */
   

     long             f_nsyms;  /* number entries in the symbol table */
   

     unsigned short   f_opthdr; /* size of optional header */
   

     unsigned short   f_flags;  /* flags */
   };
   

   #define FILHDR struct filehdr
   #define FILHSZ sizeof(FILHDR)