COFF: Difference between revisions

The structure of this header can be found in filehdr.h, and is 20 bytes longs.

 

{

unsigned short f_magic; /* Magic number */

unsigned short f_flags; /* Flags */

}

 

The magic number varies from implementation to implementation, for example, DJGPP generates COFF files with the value 0x14C in this field.

The structure of this header can be found in aouthdr.h, and is 28 bytes long.

 

{

unsigned short magic; /* Magic Number */

unsigned long data_start; /* Base of Data used for this file */

}

 

===Section Header===

The structure of this header can be found in scnhdr.h, and is 40 bytes long.

 

{

char s_name[8]; /* Section Name */

long s_flags; /* Flags for this section */

}

 

The section name field stores the name of this section padded with NULL bytes if it is eight characters or fewer; otherwise, this field contains either a pointer into the String Table, or a special name that starts with `/` and is followed by the decimal digits representing the offset within the String Table where the section name may be found. See the entry on the String Table below.

The structure of this header can be found in reloc.h, and is 10 bytes long.

 

{

long r_vaddr; /* Reference Address */

unsigned short r_type; /* Type of relocation */

}

 

The Address field gives the offset within the Section's raw data where the address starts. The reference will almost always be a 32-bit value starting at the given address, but this is dependant on implementation and the r_type field.

The structure of this header can be found in linenum.h, and is 6 bytes long.

 

{

union

unsigned short l_lnno; /* Line Number */

}

 

In brief, The line number table will contain an entry with a line number of zero in which the l_symndx field will indicate the function name Symbol in the Symbol table. This entry will be followed by additional entries with incrementing line numbers which indicate, through the l_paddr field, the byte offset into the section where this line starts. Given this, an exception that occurs during processing of the COFF file can be traced back to a function and a line number within that function.

The structure of this header can be found in syms.h, and is 18 bytes long.

 

{

char n_name[8]; /* Symbol Name */

char n_numaux; /* Auxiliary Count */

}

 

The Symbol name field stores the name of this symbol if it is eight characters or fewer, otherwise this field contains a pointer into the String table. See the entry on the String table below.

===String Table===

 

 

<pre>

String Table Offset = File Header.f_symptr + File Header.f_nsyms * sizeof( Symbol Table Entry )

</pre>

 

 

{

union

}

}

 

 

If the name is more than eight characters, the "zeroes" field (the first four bytes of "name") will be zero. In this case the "offset" field should be used as an offset value into the String table to locate the Symbol or Section name.

 

==Example==

</pre>

 

int mGlobalInt;

short mGlobalShort;

return mTestValue;

}

 

As you can see, the static global variables are defined in either section 2 (.data) for the initialised ones, or section 3 (.bss) for the uninitialised, with n_value being an offset into the relevant section. The initialised global variables are also defined in section 2 (.data) with offsets.

=== External Links ===

* http://www.delorie.com/djgpp/doc/coff/

 

[[Category:Executable Formats]]