COFF(5) FILE FORMATS COFF(5) NAME coff - common assembler and link editor output SYNOPSIS #include #include #include #include #include #include #include AVAILABILITY Available only on Sun 386i systems running a SunOS 4.0.x release or earlier. Not a SunOS 4.1 release feature. DESCRIPTION The output from the link editor and the assembler (named a.out by default) is in COFF format (Common Object File For- mat) on the Sun386i system. A common object file consists of a file header, a system header (if the file is link editor output), a table of sec- tion headers, a data section, relocation information, (optional) line numbers, a symbol table, and a string table. The general format looks like this: file-header system-header section-headers data relocation line-numbers symbol-table string-table section-headers contains a number of section headers: section 1 header ... section n header Similarly, data, relocation, and line-numbers are each divided into n sections. The last three parts of an object file (line numbers, symbol table and string table) may be missing if the program was linked with the -s option of ld(1) or if they were removed by strip(1). Also note that the relocation information will be absent after linking unless the -r option of ld(1) was used. The string table exists only if the symbol table con- tains symbols with names longer than eight characters. The sizes of each section (contained in the header, dis- cussed below) are in bytes. When an a.out file is loaded into memory for execution, three logical segments are set up: the text segment, the data segment (initialized data followed by uninitialized, the latter actually being initialized to all 0's), and a stack. The text segment starts at location 0x1000 by default. The a.out file produced by ld(1) has the magic number 0413 in the first field of the system header. The headers (file header, system header, and section headers) are loaded at the beginning of the text segment and the text immediately follows the headers in the user address space. The first text address will equal 0x1000 plus the size of the headers, and will vary depending upon the number of section headers in the a.out file. In an a.out file with three sections (.text, .data, .bss, and .comment), the first text address is at 0x000010D0. The text segment is not writable by the program; if other processes are executing the same a.out file, the processes will share a single text segment. The data segment starts at the next 4K boundary past the last text address. The first data address is determined by the following: If an a.out file were split into 4K chunks, one of the chunks would contain both the end of text and the beginning of data. When the a.out file is loaded into memory for execution, that chunk will appear twice; once at the end of text and once at the beginning of data (with some unused space in between). The duplicated chunk of text that appears at the beginning of data is never executed; it is duplicated so that the operating system may bring in pieces of the file in multiples of the page size without having to realign the beginning of the data section to a page boun- dary. Therefore the first data address is the sum of the next segment boundary past the end of text plus the remainder of the last text address divided by 4K. If the last text address is a multiple of 4K no duplication is necessary. On the Sun386i computer the stack begins at location 0xFBFFFFFF and grows toward lower addresses. The stack is automatically extended as required. The data segment is extended only as requested by the brk(2) system call. For relocatable files the value of a word in the text or data portions that is not a reference to an undefined exter- nal symbol is exactly the value that will appear in memory when the file is executed. If a word in the text involves a reference to an undefined external symbol, there will be a relocation entry for the word, the storage class of the symbol-table entry for the symbol will be marked as an "external symbol", and the value and section number of the symbol-table entry will be undefined. When the file is pro- cessed by the link editor and the external symbol becomes defined, the value of the symbol will be added to the word in the file. File Header The format of the file header is: struct filehdr { unsigned short f_magic; /* magic number */ unsigned short f_nscns; /* number of sections */ long f_timdat; /* time and date stamp */ long f_symptr; /* file ptr to symtab */ long f_nsyms; /* # symtab entries */ unsigned short f_opthdr; /* sizeof(opt hdr) */ unsigned short f_flags; /* flags */ }; System Header The format of the system header is: typedef struct aouthdr { short magic; /* magic number */ short vstamp; /* version stamp */ long tsize; /* text size in bytes, padded */ long dsize; /* initialized data (.data) */ long bsize; /* uninitialized data (.bss) */ long entry; /* entry point */ long text_start; /* base of text used for this file */ long data_start; /* base of data used for this file */ } AOUTHDR; Section Header The format of the section header is: struct scnhdr { char s_name[SYMNMLEN];/* section name */ long s_paddr; /* physical address */ long s_vaddr; /* virtual address */ long s_size; /* section size */ long s_scnptr; /* file ptr to raw data */ long s_relptr; /* file ptr to relocation */ long s_lnnoptr; /* file ptr to line numbers */ unsigned short s_nreloc; /* # reloc entries */ unsigned short s_nlnno; /* # line number entries */ long s_flags; /* flags */ }; Relocation Object files have one relocation entry for each relocatable reference in the text or data. If relocation information is present, it will be in the following format: struct reloc { long r_vaddr; /* (virtual) address of reference */ long r_symndx; /* index into symbol table */ ushort r_type; /* relocation type */ }; The start of the relocation information is s_relptr from the section header. If there is no relocation information, s_relptr is 0. Line Number The cc(1V) command generates an entry in the object file for each C source line on which a breakpoint is possible (when invoked with the -g option. Users can refer to line numbers when using the appropriate debugger, such as dbx(1)). The structure of these line number entries appears below. struct lineno { union { long l_symndx ; long l_paddr ; } l_addr ; unsigned short l_lnno ; } ; Numbering starts with one at the top of the source file and increments independent of transition between functions. The initial line number entry for a function has l_lnno equal to zero, and the symbol table index of the function's entry is in l_symndx. Otherwise, l_lnno is non-zero, and l_paddr is the physical address of the code for the referenced line. Thus the overall structure is the following: l_addr l_lnno function symtab index 0 physical address line physical address line ... function symtab index 0 physical address line physical address line ... Symbol Table The format of each symbol in the symbol table is described by the syment structure, shown below. This structure is com- patible with System V COFF, but has an added _n_dbx struc- ture which is needed by dbx(1). #define SYMNMLEN 8 #define FILNMLEN 14 #define DIMNUM 4 struct syment { union /* all ways to get a symbol name */ { char _n_name[SYMNMLEN]; /* name of symbol */ struct { long _n_zeroes; /* == 0L if in string table */ long _n_offset; /* location in string table */ } _n_n; char *_n_nptr[2]; /* allows overlaying */ struct { char _n_leading_zer/* null char */ char _n_dbx_type; /* stab type */ short _n_dbx_desc; /* value of desc field */ long _n_stab_ptr; /* table ptr */ } _n_dbx; } _n; long n_value; /* value of symbol */ short n_scnum; /* section number */ unsigned short n_type; /* type and derived type */ char n_sclass; /* storage class */ char n_numaux; /* number of aux entries */ }; #define n_name _n._n_name #define n_zeroes _n._n_n._n_zeroes #define n_offset _n._n_n._n_offset #define n_nptr _n._n_nptr[1] The storage class member (n_sclass) is set to one of the constants defined in . Some symbols require more information than a single entry; they are followed by auxiliary entries that are the same size as a symbol entry. The format follows: union auxent { struct { long x_tagndx; union { struct { unsigned short x_lnno; unsigned short x_size; } x_lnsz; long x_fsize; } x_misc; union { struct { long x_lnnoptr; long x_endndx; } x_fcn; struct { unsigned short x_dimen[DIMNUM]; } x_ary; } x_fcnary; unsigned short x_tvndx; } x_sym; struct { char x_fname[FILNMLEN]; } x_file; struct { long x_scnlen; unsigned short x_nreloc; unsigned short x_nlinno; } x_scn; struct { long x_tvfill; unsigned short x_tvlen; unsigned short x_tvran[2]; } x_tv; }; Indexes of symbol table entries begin at zero. The start of the symbol table is f_symptr (from the file header) bytes from the beginning of the file. If the symbol table is stripped, f_symptr is 0. The string table (if one exists) begins at f_symptr + (f_nsyms * SYMESZ) bytes from the beginning of the file. SEE ALSO as(1), cc(1V), ld(1), brk(2), ldfcn(3) Sun Release 4.1 Last change: 19 February 1988 6