coff.h" -- Header File" "

Format for COFF objects 

#include <coff.h>
coff.h describes the Common Object File Format (COFF), which is the object format used by COHERENT 386.

What Is COFF?

In brief, COFF is the UNIX System V standard for file formats. It defines the formats for relocatable object modules, for executable files, and for archives.

A COFF file is built around three sections, or _s_e_g_m_e_n_t_s:

tteexxtt
This holds executable machine code. It is write protected -- the operating system is forbidden to overwrite it. (This is why operating systems that use COFF or similar formats are said to run in ``protected mode.'')

ddaattaa
This holds initialized data, that is, the data that the program finds when it begins execution. The program can read and write into this segment.

bbssss
This segment holds unitialized data. It is simply a mass of space that is initialized to zeroes. It is contiguous with the ddaattaa segment. The term bbssss from the old IBM mainframe days, and stands for ``block started by symbol''.

Not all segments have to be included in every COFF file. Further, some implementations of COFF define their own segments that manipulate special features of the operating system or hardware.

The following describes the structure of a COFF file. The areas within the file are described in the order in which they appear.

11.. _f_i_l_e _h_e_a_d_e_r
This holds information set when the file was created, such as the date and time it was created, the number of segments in the file, a pointer to the symbol table, and status flags.

22.. _o_p_t_i_o_n_a_l _h_e_a_d_e_r
This gives information set at run-time, such as the address of the program entry point, and the size of the code and data segments.

33.. _s_e_g_m_e_n_t _h_e_a_d_e_r_s
The next area holds a header for each segment in the file. Each header describes its segment's characteristics and contains pointers to the segment's contents, relocation information, line-number information, and other useful addresses.

44.. _s_e_g_m_e_n_t _c_o_n_t_e_n_t_s
The next area holds the contents of the segments used in this file.

55.. _r_e_l_o_c_a_t_i_o_n _i_n_f_o_r_m_a_t_i_o_n
The fifth area gives relocation information, one set of information for each segment in the file. The linker ld uses this information to generate the executable file at link time.

66.. _l_i_n_e_-_n_u_m_b_e_r _i_n_f_o_r_m_a_t_i_o_n
This area holds debug information, one set of information for each segment. This area is optional.

77.. _s_y_m_b_o_l _t_a_b_l_e
This area holds information used by both the linker and the debugger.

88.. _s_t_r_i_n_g _t_a_b_l_e
This table holds very long names of variables.

Most of this information is irrelevant to the average user, or even the average developer of software. To the average user, COFF is ``a machine that would go of itself''; you can run or compile programs without worrying what the linker puts where, or why. These details, however, can be very important if you are writing tools that manipulate the internals of files, such as archivers or debuggers. If you need detailed information on COFF and how to manipulate it, see _U_n_d_e_r_s_t_a_n_d_i_n_g _a_n_d _U_s_i_n_g _C_O_F_F (citation appears below).

For more information on how the COFF format affects COHERENT's language tools, see the Lexicon articles for ar, as, cc, db, and ld.

See Also