COFF: Difference between revisions

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


<source lang="c">
<pre>
{
{
unsigned short f_magic; /* Magic number */
unsigned short f_magic; /* Magic number */
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unsigned short f_flags; /* Flags */
unsigned short f_flags; /* Flags */
}
}
</pre>
</source>


The magic number varies from implementation to implementation, for example, DJGPP generates COFF files with the value 0x14C in this field.
The magic number varies from implementation to implementation, for example, DJGPP generates COFF files with the value 0x14C in this field.
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The structure of this header can be found in aouthdr.h, and is 28 bytes long.
The structure of this header can be found in aouthdr.h, and is 28 bytes long.


<source lang="c">
<pre>
{
{
unsigned short magic; /* Magic Number */
unsigned short magic; /* Magic Number */
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unsigned long data_start; /* Base of Data used for this file */
unsigned long data_start; /* Base of Data used for this file */
}
}
</pre>
</source>


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


<source lang="c">
<pre>
{
{
char s_name[8]; /* Section Name */
char s_name[8]; /* Section Name */
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long s_flags; /* Flags for this section */
long s_flags; /* Flags for this section */
}
}
</pre>
</source>


The Section name field stores the name of this section 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.
The Section name field stores the name of this section 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.
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The structure of this header can be found in reloc.h, and is 10 bytes long.
The structure of this header can be found in reloc.h, and is 10 bytes long.


<source lang="c">
<pre>
{
{
long r_vaddr; /* Reference Address */
long r_vaddr; /* Reference Address */
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unsigned short r_type; /* Type of relocation */
unsigned short r_type; /* Type of relocation */
}
}
</pre>
</source>


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


<source lang="c">
<pre>
{
{
union
union
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unsigned short l_lnno; /* Line Number */
unsigned short l_lnno; /* Line Number */
}
}
</pre>
</source>


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


<source lang="c">
<pre>
{
{
char n_name[8]; /* Symbol Name */
char n_name[8]; /* Symbol Name */
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char n_numaux; /* Auxiliary Count */
char n_numaux; /* Auxiliary Count */
}
}
</pre>
</source>


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.
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.
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The Section table name field and the Symbol table name field are actaully more complicated than was detailed above, they in fact look more like this:
The Section table name field and the Symbol table name field are actaully more complicated than was detailed above, they in fact look more like this:
<source lang="c">
<pre>
{
{
union
union
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}
}
}
}
</pre>
</source>


If the name is eight characters or fewer, then the field "zeroes" will be non-zero, and "name" should be interpretted as a character array. Note that this field is not null-terminated unless it is fewer than eight characters in length.
If the name is eight characters or fewer, then the field "zeroes" will be non-zero, and "name" should be interpretted as a character array. Note that this field is not null-terminated unless it is fewer than eight characters in length.
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</pre>
</pre>


<source lang="c">
<pre>
int mGlobalInt;
int mGlobalInt;
short mGlobalShort;
short mGlobalShort;
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return mTestValue;
return mTestValue;
}
}
</pre>
</source>


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.
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.
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