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A.OUT(5) UNIX Programmer's Manual A.OUT(5) NNAAMMEE aa..oouutt - format of executable binary files SSYYNNOOPPSSIISS ##iinncclluuddee <<aa..oouutt..hh>> DDEESSCCRRIIPPTTIIOONN The include file <_a_._o_u_t_._h> declares three structures and several macros. The structures describe the format of executable machine code files (`binaries') on the system. A binary file consists of up to 7 sections. In order, these sections are: exec header Contains parameters used by the kernel to load a binary file into memory and execute it, and by the link editor ld(1) to combine a binary file with other binary files. This section is the only mandatory one. text segment Contains machine code and related data that are loaded into memory when a program executes. May be loaded read-only. data segment Contains initialized data; always loaded into writable memory. text relocations Contains records used by the link editor to update pointers in the text segment when combining binary files. data relocations Like the text relocation section, but for data segment pointers. symbol table Contains records used by the link editor to cross ref- erence the addresses of named variables and functions (`symbols') between binary files. string table Contains the character strings corresponding to the symbol names. Every binary file begins with an _e_x_e_c structure: struct exec { unsigned long a_midmag; unsigned long a_text; unsigned long a_data; unsigned long a_bss; unsigned long a_syms; unsigned long a_entry; unsigned long a_trsize; unsigned long a_drsize; }; The fields have the following functions: _a___m_i_d_m_a_g This field is stored in network byte-order so that binaries for for machines with alternate byte orders can be distinguished. It has a number of sub-components accessed by the macros N_GETFLAG(), N_GETMID(), and N_GETMAGIC(), and set by the macro N_SETMAGIC(). The macro N_GETFLAG() returns a few flags: EX_DYNAMIC indicates that the executable requires the services of the run-time link editor. EX_PIC indicates that the object contains position inde- pendent code. This flag is set by as(1) when given the `-k' flag and is preserved by ld(1) if neces- sary. If both EX_DYNAMIC and EX_PIC are set, the object file is a po- sition indendent executable image (eg. a shared library), which is to be loaded into the process address space by the run-time link editor. The macro N_GETMID() returns the machine-id. This indicates which machine(s) the binary is intended to run on. N_GETMAGIC() specifies the magic number, which uniquely identi- fies binary files and distinguishes different loading conven- tions. The field must contain one of the following values: OMAGIC The text and data segments immediately follow the head- er and are contiguous. The kernel loads both text and data segments into writable memory. NMAGIC As with OMAGIC, text and data segments immediately fol- low the header and are contiguous. However, the kernel loads the text into read-only memory and loads the data into writable memory at the next page boundary after the text. ZMAGIC The kernel loads individual pages on demand from the binary. The header, text segment and data segment are all padded by the link editor to a multiple of the page size. Pages that the kernel loads from the text seg- ment are read-only, while pages from the data segment are writable. _a___t_e_x_t Contains the size of the text segment in bytes. _a___d_a_t_a Contains the size of the data segment in bytes. _a___b_s_s Contains the number of bytes in the `bss segment' and is used by the kernel to set the initial break (brk(2)) after the data segment. The kernel loads the program so that this amount of writable memory appears to follow the data segment and initial- ly reads as zeroes. _a___s_y_m_s Contains the size in bytes of the symbol table section. _a___e_n_t_r_y Contains the address in memory of the entry point of the pro- gram after the kernel has loaded it; the kernel starts the exe- cution of the program from the machine instruction at this ad- dress. _a___t_r_s_i_z_e Contains the size in bytes of the text relocation table. _a___d_r_s_i_z_e Contains the size in bytes of the data relocation table. The _a_._o_u_t_._h include file defines several macros which use an _e_x_e_c struc- ture to test consistency or to locate section offsets in the binary file. NN__BBAADDMMAAGG(_e_x_e_c) Nonzero if the _a___m_a_g_i_c field does not contain a recog- nized value. NN__TTXXTTOOFFFF(_e_x_e_c) The byte offset in the binary file of the beginning of the text segment. NN__SSYYMMOOFFFF(_e_x_e_c) The byte offset of the beginning of the symbol table. NN__SSTTRROOFFFF(_e_x_e_c) The byte offset of the beginning of the string table. Relocation records have a standard format which is described by the _r_e_l_o_c_a_t_i_o_n___i_n_f_o structure: struct relocation_info { int r_address; unsigned int r_symbolnum : 24, r_pcrel : 1, r_length : 2, r_extern : 1, r_baserel : 1, r_jmptable : 1, r_relative : 1, r_copy : 1; }; The _r_e_l_o_c_a_t_i_o_n___i_n_f_o fields are used as follows: _r___a_d_d_r_e_s_s Contains the byte offset of a pointer that needs to be link- edited. Text relocation offsets are reckoned from the start of the text segment, and data relocation offsets from the start of the data segment. The link editor adds the value that is already stored at this offset into the new value that it computes using this relocation record. _r___s_y_m_b_o_l_n_u_m Contains the ordinal number of a symbol structure in the symbol table (it is _n_o_t a byte offset). After the link edi- tor resolves the absolute address for this symbol, it adds that address to the pointer that is undergoing relocation. (If the _r___e_x_t_e_r_n bit is clear, the situation is different; see below.) _r___p_c_r_e_l If this is set, the link editor assumes that it is updating a pointer that is part of a machine code instruction using pc-relative addressing. The address of the relocated point- er is implicitly added to its value when the running program uses it. _r___l_e_n_g_t_h Contains the log base 2 of the length of the pointer in bytes; 0 for 1-byte displacements, 1 for 2-byte displace- ments, 2 for 4-byte displacements. _r___e_x_t_e_r_n Set if this relocation requires an external reference; the link editor must use a symbol address to update the pointer. When the _r___e_x_t_e_r_n bit is clear, the relocation is `local'; the link editor updates the pointer to reflect changes in the load addresses of the various segments, rather than changes in the value of a symbol (except when _r___b_a_s_e_r_e_l is also set (see below). In this case, the content of the _r___s_y_m_b_o_l_n_u_m field is an _n___t_y_p_e value (see below); this type field tells the link editor what segment the relocated pointer points into. _r___b_a_s_e_r_e_l If set, the symbol, as identified by the _r___s_y_m_b_o_l_n_u_m field, is to be relocated to an offset into the Global Offset Table. At run-time, the entry in the Global Offset Table at this offset is set to be the address of the symbol. _r___j_m_p_t_a_b_l_e If set, the symbol, as identified by the _r___s_y_m_b_o_l_n_u_m field, is to be relocated to an offset into the Procedure Linkage Table. _r___r_e_l_a_t_i_v_e If set, this relocation is relative to the (run-time) load address of the image this object file is going to be a part of. This type of relocation only occurs in shared objects. _r___c_o_p_y If set, this relocation record identifies a symbol whose contents should be copied to the location given in _r___a_d_d_r_e_s_s_. The copying is done by the run-time link-editor from a suitable data item in a shared object. Symbols map names to addresses (or more generally, strings to values). Since the link-editor adjusts addresses, a symbol's name must be used to stand for its address until an absolute value has been assigned. Symbols consist of a fixed-length record in the symbol table and a variable- length name in the string table. The symbol table is an array of _n_l_i_s_t structures: struct nlist { union { char *n_name; long n_strx; } n_un; unsigned char n_type; char n_other; short n_desc; unsigned long n_value; }; The fields are used as follows: _n___u_n_._n___s_t_r_x Contains a byte offset into the string table for the name of this symbol. When a program accesses a symbol table with the nlist(3) function, this field is replaced with the _n___u_n_._n___n_a_m_e field, which is a pointer to the string in memo- ry. _n___t_y_p_e Used by the link editor to determine how to update the sym- bol's value. The _n___t_y_p_e field is broken down into three sub-fields using bitmasks. The link editor treats symbols with the N_EXT type bit set as `external' symbols and per- mits references to them from other binary files. The N_TYPE mask selects bits of interest to the link editor: N_UNDF An undefined symbol. The link editor must locate an external symbol with the same name in another binary file to determine the absolute value of this symbol. As a special case, if the _n___v_a_l_u_e field is nonzero and no binary file in the link-edit defines this symbol, the link-editor will resolve this symbol to an address in the bss segment, reserving an amount of bytes equal to _n___v_a_l_u_e. If this symbol is unde- fined in more than one binary file and the binary files do not agree on the size, the link editor chooses the greatest size found across all binaries. N_ABS An absolute symbol. The link editor does not update an absolute symbol. N_TEXT A text symbol. This symbol's value is a text ad- dress and the link editor will update it when it merges binary files. N_DATA A data symbol; similar to N_TEXT but for data ad- dresses. The values for text and data symbols are not file offsets but addresses; to recover the file offsets, it is necessary to identify the loaded ad- dress of the beginning of the corresponding section and subtract it, then add the offset of the section. N_BSS A bss symbol; like text or data symbols but has no corresponding offset in the binary file. N_FN A filename symbol. The link editor inserts this symbol before the other symbols from a binary file when merging binary files. The name of the symbol is the filename given to the link editor, and its value is the first text address from that binary file. Filename symbols are not needed for link- editing or loading, but are useful for debuggers. The N_STAB mask selects bits of interest to symbolic debug- gers such as gdb(1); the values are described in stab(5). _n___o_t_h_e_r This field provides information on the nature of the symbol independent of the symbol's location in terms of segments as determined by the _n___t_y_p_e field. Currently, the lower 4 bits of the _n___o_t_h_e_r field hold one of two values: AUX_FUNC and AUX_OBJECT (see <_l_i_n_k_._h> for their definitions). AUX_FUNC associates the symbol with a callable function, while AUX_OBJECT associates the symbol with data, irrespective of their locations in either the text or the data segment. This field is intended to be used by ld(1) for the construc- tion of dynamic executables. _n___d_e_s_c Reserved for use by debuggers; passed untouched by the link editor. Different debuggers use this field for different purposes. _n___v_a_l_u_e Contains the value of the symbol. For text, data and bss symbols, this is an address; for other symbols (such as de- bugger symbols), the value may be arbitrary. The string table consists of an _u_n_s_i_g_n_e_d _l_o_n_g length followed by null- terminated symbol strings. The length represents the size of the entire table in bytes, so its minimum value (or the offset of the first string) is always 4 on 32-bit machines. SSEEEE AALLSSOO as(1), gdb(1), ld(1), brk(2), execve(2), nlist(3), core(5), dbx(5), stab(5), link(5) HHIISSTTOORRYY The _a_._o_u_t_._h include file appeared in Version 7 AT&T UNIX. BBUUGGSS Nobody seems to agree on what _b_s_s stands for. New binary file formats may be supported in the future, and they probably will not be compatible at any level with this ancient format. BSD Experimental June 5, 1993 5