a.out(4) a.out(4)
NAME [Toc] [Back]
a.out - assembler and link editor output
SYNOPSIS [Toc] [Back]
#include <elf.h> (for ELF files)
#include <a.out.h> (for SOM files)
DESCRIPTION [Toc] [Back]
ELF a.out
The file name a.out is the default output file name from the link
editor, ld(1). The link editor will make an a.out executable if there
were no errors in linking. The output file of the assembler, as(1),
also follows the format of the a.out file although its default file
name is different.
Programs that manipulate ELF files may use the library that elf(3E)
describes. An overview of the file format follows. For more complete
information, see the references given below.
Linking View Execution View
_______________________ _______________________
ELF header ELF header
|_____________________| |______________________|
|Program header table | | Program header table|
| optional | | |
|_____________________| | _____________________|
| Section 1 | | |
|_____________________| | |
| . . . | | Segment 1 |
| | | |
|_____________________| | _____________________|
| Section n | | |
|_____________________| | |
| . . . | | Segment 2 |
| | | |
|_____________________| | _____________________|
| . . . | | . . . |
|_____________________| | _____________________|
| Section header table| | Section header table|
| | | optional |
|_____________________| | _____________________|
An ELF header resides at the beginning and holds a ``road map''
describing the file's organization. Sections hold the bulk of object
file information for the linking view: instructions, data, symbol
table, relocation information, and so on. Segments hold the object
file information for the program execution view. As shown, a segment
may contain one or more sections.
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A program header table, if present, tells the system how to create a
process image. Files used to build a process image (execute a
program) must have a program header table; relocatable files do not
need one. A section header table contains information describing the
file's sections. Every section has an entry in the table; each entry
gives information such as the section name, the section size, and so
on. Files used during linking must have a section header table; other
object files may or may not have one.
Although the figure shows the program header table immediately after
the ELF header, and the section header table following the sections,
actual files may differ. Moreover, sections and segments have no
specified order. Only the ELF header has a fixed position in the
file.
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 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 maximal page boundary past the
last text address. (If the system supports more than one page size,
the ``maximal page'' is the largest supported size.) When the process
image is created, the part of the file holding the end of text and the
beginning of data may appear twice. 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 actual page size without having to realign the
beginning of the data section to a page boundary. Therefore, the
first data address is the sum of the next maximal page boundary past
the end of text plus the remainder of the last text address divided by
the maximal page size. If the last text address is a multiple of the
maximal page size, no duplication is necessary. The stack is
automatically extended as required. The data segment is extended as
requested by the brk(2) system call.
SOM a.out (PA-RISC Only) [Toc] [Back]
The file name a.out is the default file name for the output file from
the assembler (see as(1)), compilers, and the linker (see ld(1)). The
assembler and compilers create relocatable object files, ready for
input to the linker. The linker creates executable object files and
shared library files.
An object file consists of a file header, auxiliary headers, space
dictionary, subspace dictionary, symbol table, relocation information,
compiler records, space string table, symbol string table, and the
data for initialized code and data. Not all of these sections are
required for all object files. The file must begin with the file
header, but the remaining sections do not have to be in any particular
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order; the file header contains pointers to each of the other sections
of the file.
A relocatable object file, created by the assembler or compiler, must
contain at least the following sections: file header, space
dictionary, subspace dictionary, symbol table, relocation information,
space string table, symbol string table, and code and data. It may
also contain auxiliary headers and compiler records. Relocatable
files generally contain unresolved symbols. The linker combines
relocatable files and searches libraries to produce an executable
file. The linker can also be used to combine relocatable files and
produce a new relocatable file as output, suitable for input to a
subsequent linker run.
An executable file, created by the linker, typically contains the
following sections: file header, an HP-UX auxiliary header, space
dictionary, subspace dictionary, symbol table, space string table,
symbol string table, and code and data. The linker also copies any
auxiliary headers and compiler records from the input files to the
output file. If the file has been stripped (see strip(1)), it will
not contain a symbol table, symbol string table, or compiler records.
An executable file must not contain any unresolved symbols.
A shared library file, created by the linker, contains the same
sections found in an executable file, with additional information
added to the code section of the file. This additional information
contains a header, export table, import table, and dynamic relocation
records to be used by the dynamic loader.
Programs consist of two loadable spaces: a shared, non-writable, code
space named $TEXT$; and a private, writable, data space named
$PRIVATE$. A program may contain another loadable, private space
named $THREAD_SPECIFIC$. A program may contain other unloadable
spaces that contain data needed by development tools. For example,
symbolic debugging information is contained in a space named $DEBUG$
or $PINFO$. The linker treats loadable and unloadable spaces exactly
the same, so the full generality of symbol resolution and relocation
is available for the symbolic debugging information.
Spaces have an addressing range of 4,294,967,296 (2^32) bytes. Each
loadable space is divided into four 1,073,741,824 (2^30) byte
quadrants. The HP-UX operating system places all code in the first
quadrant of the $TEXT$ space, all data in the second quadrant of the
$PRIVATE$ space, and all shared library code in the third quadrant of
shared memory space.
Each space is also divided into logical units called subspaces. When
the linker combines relocatable object files, it groups all subspaces
from the input files by name, then arranges the groups within the
space by a sort key associated with each subspace. Subspaces are not
architecturally significant; they merely provide a mechanism for
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combining individual parts of spaces independently from many input
files. Some typical subspaces in a program are shown in the following
table:
$SHLIB_INFO$ Information needed for dynamic loading
$MILLICODE$ Code for millicode routines
$LIT$ Sharable literals
$CODE$ Code
$UNWIND$ Stack unwind information
$GLOBAL$ Outer block declarations for Pascal
$DATA$ Static initialized data
$COMMON$ FORTRAN common
$BSS$ Uninitialized data
$TBSS$ Thread local storage
Subspaces can be initialized or uninitialized (although typically,
only $BSS$ and $TBSS$ are uninitialized). The subspace dictionary
entry for an initialized subspace contains a file pointer to the
initialization data, while the entry for an uninitialized subspace
contains only a 32-bit pattern used to initialize the entire area at
load time.
In a relocatable file, initialized code and data often contain
references to locations elsewhere in the file, and to unresolved
symbols defined in other files. These references are patched at link
time using the relocation information. Each entry in the relocation
information (a "fixup") specifies a location within the initialized
data for a subspace, and an expression that defines the actual value
that should be placed at that location, relative to one or two
symbols.
The linker summarizes the subspace dictionary in the HP-UX auxiliary
header when creating an executable file. HP-UX programs contain only
three separate sections: one for the code, one for initialized data,
and one for uninitialized data. By convention, this auxiliary header
is placed immediately following the file header.
When an a.out file is loaded into memory for execution, three areas of
memory are set up: the a.out code is loaded into the first quadrant of
a new, sharable space; the data (initialized followed by
uninitialized) is loaded into the second quadrant of a new, private
space; and a stack is created beginning at a fixed address near the
middle of the second quadrant of the data space.
If the a.out file uses shared libraries, then the dynamic loader
/usr/lib/dld.sl is loaded into memory and called to map into memory
all shared libraries requested by the program. The shared library
text is loaded into the third quadrant of the shared memory space, and
the shared library data is allocated in the second quadrant of the
data space.
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The file format described here is a common format for all operating
systems designed for HP's Precision Architecture. Therefore, there
are some fields and structures that are not used on HP-UX or have been
reserved for future use.
File Header [Toc] [Back]
The format of the file header is described by the following structure
declaration from <filehdr.h>.
struct header {
short int system_id; /* system id */
short int a_magic; /* magic number */
unsigned int version_id; /* a.out format version */
struct sys_clock file_time; /* timestamp */
unsigned int entry_space; /* index of space containing entry point */
unsigned int entry_subspace; /* subspace index of entry */
unsigned int entry_offset; /* offset of entry point */
unsigned int aux_header_location; /* file ptr to aux hdrs */
unsigned int aux_header_size; /* sizeof aux hdrs */
unsigned int som_length; /* length of object module */
unsigned int presumed_dp; /* DP value assumed during compilation */
unsigned int space_location; /* file ptr to space dict */
unsigned int space_total; /* # of spaces */
unsigned int subspace_location; /* file ptr to subsp dict */
unsigned int subspace_total; /* # of subspaces */
unsigned int loader_fixup_location; /* space reference array */
unsigned int loader_fixup_total; /* # of space reference recs */
unsigned int space_strings_location; /* file ptr to sp. strings */
unsigned int space_strings_size; /* sizeof sp. strings */
unsigned int init_array_location; /* location of init pointers */
unsigned int init_array_total; /* # of init pointers */
unsigned int compiler_location; /* file ptr to comp recs */
unsigned int compiler_total; /* # of compiler recs */
unsigned int symbol_location; /* file ptr to sym table */
unsigned int symbol_total; /* # of symbols */
unsigned int fixup_request_location; /* file ptr to fixups */
unsigned int fixup_request_total; /* # of fixups */
unsigned int symbol_strings_location; /* file ptr to sym strings */
unsigned int symbol_strings_size; /* sizeof sym strings */
unsigned int unloadable_sp_location; /* file ptr to debug info */
unsigned int unloadable_sp_size; /* size of debug info */
unsigned int checksum; /* header checksum */
};
The timestamp is a two-word structure as shown below. If unused, both
fields are zero.
struct sys_clock {
unsigned int secs;
unsigned int nanosecs;
};
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Auxiliary Headers [Toc] [Back]
The auxiliary headers are contained in a single contiguous area in the
file, and are located by a pointer in the file header. Auxiliary
headers are used for two purposes: to attach users' version and
copyright strings to an object file, and to contain the information
needed to load an executable program. In an executable program, the
HP-UX auxiliary header must precede all other auxiliary headers. The
following declarations are found in <aouthdr.h>.
struct aux_id {
unsigned int mandatory : 1; /* linker must understand aux hdr info */
unsigned int copy : 1; /* copy aux hdr without modification */
unsigned int append : 1; /* merge multiple entries of same type */
unsigned int ignore : 1; /* ignore aux hdr if type unknown */
unsigned int reserved : 12; /* reserved */
unsigned int type : 16; /* aux hdr type */
unsigned int length; /* sizeof rest of aux hdr */
};
/* Values for the aux_id.type field */
#define HPUX_AUX_ID 4
#define VERSION_AUX_ID 6
#define COPYRIGHT_AUX_ID 9
#define SHLIB_VERSION_AUX_ID 10
struct som_exec_auxhdr { /* HP-UX auxiliary header */
struct aux_id som_auxhdr; /* aux header id */
long exec_tsize; /* text size */
long exec_tmem; /* start address of text */
long exec_tfile; /* file ptr to text */
long exec_dsize; /* data size */
long exec_dmem; /* start address of data */
long exec_dfile; /* file ptr to data */
long exec_bsize; /* bss size */
long exec_entry; /* address of entry point */
long exec_flags; /* loader flags */
long exec_bfill; /* bss initialization value */
};
/* Values for exec_flags */
#define TRAP_NIL_PTRS 01
struct user_string_aux_hdr { /* Version string auxiliary header */
struct aux_id header_id; /* aux header id */
unsigned int string_length; /* strlen(user_string) */
char user_string[1]; /* user-defined string */
};
struct copyright_aux_hdr { /* Copyright string auxiliary header */
struct aux_id header_id; /* aux header id */
unsigned int string_length; /* strlen(user_string) */
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char copyright[1]; /* user-defined string */
};
struct shlib_version_aux_hdr {
struct aux_id header_id; /* aux header id */
short version; /* version number */
};
Space Dictionary [Toc] [Back]
The space dictionary consists of a sequence of space records, as
defined in <spacehdr.h>.
struct space_dictionary_record {
union name_pt name; /* index to space name */
unsigned int is_loadable: 1; /* space is loadable */
unsigned int is_defined: 1; /* space is defined within file */
unsigned int is_private: 1; /* space is not sharable */
unsigned int has_intermediate_code: 1; /* contains intermediate
code */
unsigned int is_tspecific: 1; /* space is $thread_specific$ */
unsigned int reserved: 11; /* reserved */
unsigned int sort_key: 8; /* sort key for space */
unsigned int reserved2: 8; /* reserved */
int space_number; /* space index */
int subspace_index; /* index to first subspace */
unsigned int subspace_quantity; /* # of subspaces in space */
int loader_fix_index; /* index into loader fixup array */
unsigned int loader_fix_quantity; /* # of loader fixups in space */
int init_pointer_index; /* index into init pointer array */
unsigned int init_pointer_quantity; /* # of init ptrs */
};
The strings for the space names are contained in the space strings
table, which is located by a pointer in the file header. Each entry
in the space strings table is preceded by a 4-byte integer that
defines the length of the string, and is terminated by one to five
null characters to pad the string out to a word boundary. Indices to
this table are relative to the start of the table, and point to the
first byte of the string (not the preceding length word). The union
defined below is used for all such string pointers; the character
pointer is defined for programs that read the string table into memory
and wish to relocate in-memory copies of space records.
union name_pt {
char *n_name;
unsigned int n_strx;
};
Subspace Dictionary [Toc] [Back]
The subspace dictionary consists of a sequence of subspace records, as
defined in <scnhdr.h>. Strings for subspace names are contained in
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the space strings table.
struct subspace_dictionary_record {
int space_index; /* index into space dictionary */
unsigned int access_control_bits: 7; /* access and priv levels
of subsp */
unsigned int memory_resident: 1; /* lock in memory during exec */
unsigned int dup_common: 1; /* duplicate data symbols allowed */
unsigned int is_common: 1; /* initialized common block */
unsigned int is_loadable: 1; /* subspace is loadable */
unsigned int quadrant: 2; /* quadrant in space subsp
should reside in */
unsigned int initially_frozen: 1; /* lock in memory
when OS booted */
unsigned int is_first: 1; /* must be first subspace */
unsigned int code_only: 1; /* subspace contains only code */
unsigned int sort_key: 8; /* subspace sort key */
unsigned int replicate_init: 1; /* init values to be replicated
to fill subsp len */
unsigned int continuation: 1; /* subspace is a continuation */
unsigned int is_tspecific: 1; /* subspace contains TLS */
unsigned int reserved: 5; /* reserved */
int file_loc_init_value; /* file location or init value */
unsigned int initialization_length; /* length of initialization */
unsigned int subspace_start; /* starting offset */
unsigned int subspace_length; /* total subspace length */
unsigned int reserved2: 16; /* reserved */
unsigned int alignment: 16; /* alignment required */
union name_pt name; /* index of subspace name */
int fixup_request_index; /* index to first fixup */
unsigned int fixup_request_quantity; /* # of fixup requests */
};
Symbol Table [Toc] [Back]
The symbol table consists of a sequence of entries described by the
structure shown below, from <syms.h>. Strings for symbol and
qualifier names are contained in the symbol strings table, whose
structure is identical with the space strings table.
struct symbol_dictionary_record {
unsigned int hidden: 1; /* symbol not visible to loader */
unsigned int secondary_def: 1; /* secondary def symbol */
unsigned int symbol_type: 6; /* symbol type */
unsigned int symbol_scope: 4; /* symbol value */
unsigned int check_level: 3; /* type checking level */
unsigned int must_qualify: 1; /* qualifier required */
unsigned int initially_frozen: 1; /* lock in memory
when OS booted */
unsigned int memory_resident: 1; /* lock in memory during exec */
unsigned int is_common: 1; /* common block */
unsigned int dup_common: 1; /* duplicate data symbols allowed */
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unsigned int xleast: 2; /* MPE-only */
unsigned int arg_reloc: 10; /* parameter relocation bits */
union name_pt name; /* index to symbol name */
union name_pt qualifier_name; /* index to qual name */
unsigned int symbol_info; /* subspace index */
unsigned int symbol_value; /* symbol value */
};
/* Values for symbol_type */
#define ST_NULL 0 /* unused symbol entry */
#define ST_ABSOLUTE 1 /* non-relocatable symbol */
#define ST_DATA 2 /* initialized data symbol */
#define ST_CODE 3 /* generic code symbol */
#define ST_PRI_PROG 4 /* program entry point */
#define ST_SEC_PROG 5 /* secondary prog entry point*/
#define ST_ENTRY 6 /* procedure entry point */
#define ST_STORAGE 7 /* storage request */
#define ST_STUB 8 /* MPE-only */
#define ST_MODULE 9 /* Pascal module name */
#define ST_SYM_EXT 10 /* symbol extension record */
#define ST_ARG_EXT 11 /* argument extension record */
#define ST_MILLICODE 12 /* millicode entry point */
#define ST_PLABEL 13 /* MPE-only */
#define ST_OCT_DIS 14 /* Used by OCT only--ptr to translated code */
#define ST_MILLI_EXT 15 /* address of external millicode */
#define ST_TSTORAGE 16 /* TLS common symbol */
/* Values for symbol_scope */
#define SS_UNSAT 0 /* unsatisfied reference */
#define SS_EXTERNAL 1 /* import request to external symbol */
#define SS_LOCAL 2 /* local symbol */
#define SS_UNIVERSAL 3 /* global symbol */
The meaning of the symbol value depends on the symbol type. For the
code symbols (generic code, program entry points, procedure and
millicode entry points), the low-order two bits of the symbol value
encode the execution privilege level, which is not used on HP-UX, but
is generally set to 3. The symbol value with those bits masked out is
the address of the symbol (which is always a multiple of 4). For data
symbols, the symbol value is simply the address of the symbol. For
thread local storage symbols (not commons), the symbol value is the
thread local storage offset in a library or executable file, and is
the size of the symbol if in a relocatable object file. For storage
requests and thread local storage commons, the symbol value is the
number of bytes requested; the linker allocates space for the largest
request for each symbol in the $BSS$ or $TBSS$ subspaces, unless a
local or universal symbol is found for that symbol (in which case the
storage request is treated like an unsatisfied reference).
If a relocatable file is compiled with parameter type checking,
extension records follow symbols that define and reference procedure
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entry points and global variables. The first extension record, the
symbol extension record, defines the type of the return value or
global variable, and (if a procedure or function) the number of
parameters and the types of the first three parameters. If more
parameter type descriptors are needed, one or more argument extension
records follow, each containing four more descriptors. A check level
of 0 specifies no type checking; no extension records follow. A check
level of 1 or more specifies checking of the return value or global
variable type. A check level of 2 or more specifies checking of the
number of parameters, and a check level of 3 specifies checking the
types of each individual parameter. The linker performs the requested
level of type checking between unsatisfied symbols and local or
universal symbols as it resolves symbol references.
union arg_descriptor {
struct {
unsigned int reserved: 3; /* reserved */
unsigned int packing: 1; /* packing algorithm used */
unsigned int alignment: 4; /* byte alignment */
unsigned int mode: 4; /* type of descriptor and its use */
unsigned int structure: 4; /* structure of symbol */
unsigned int hash: 1; /* set if arg_type is hashed */
int arg_type: 15; /* data type */
} arg_desc;
unsigned int word;
};
struct symbol_extension_record {
unsigned int type: 8; /* always ST_SYM_EXT */
unsigned int max_num_args: 8; /* max # of parameters */
unsigned int min_num_args: 8; /* min # of parameters */
unsigned int num_args: 8; /* actual # of parameters */
union arg_descriptor symbol_desc; /* symbol type desc. */
union arg_descriptor argument_desc[3]; /* first 3 parameters */
};
struct argument_desc_array {
unsigned int type: 8; /* always ST_ARG_EXT */
unsigned int reserved: 24; /* reserved */
union arg_descriptor argument_desc[4]; /* next 4 parameters */
};
The alignment field in arg_descriptor indicates the minimum alignment
of the data, where a value of n represents 2^n byte alignment. The
values for the mode, structure, and arg_type (when the data type is
not hashed) fields in arg_descriptor are given in the following table.
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Value mode structure arg_type
___________________________________________________________
0 any any any
1 value parm scalar void
2 reference parm array signed byte
3 value-result struct unsigned byte
4 name pointer signed short
5 variable long ptr unsigned short
6 function return C string signed long
7 procedure Pascal string unsigned long
8 long ref parm procedure signed dbl word
9 function unsigned dbl word
10 label short real
11 real
12 long real
13 short complex
14 complex
15 long complex
16 packed decimal
17 struct/array
For procedure entry points, the parameter relocation bits define the
locations of the formal parameters and the return value. Normally,
the first four words of the parameter list are passed in general
registers (r26-r23) instead of on the stack, and the return value is
returned in r29. Floating-point parameters in this range are passed
instead in floating-point registers (fr4-fr7) and a floating-point
value is returned in fr4. The parameter relocation bits consist of
five pairs of bits that describe the first four words of the parameter
list and the return value. The leftmost pair of bits describes the
first parameter word, and the rightmost pair of bits describes the
return value. The meanings of these bits are shown in the following
table.
Bits | Meaning
_____|_____________________________________________________
00 | No parameter or return value
01 | Parameter or return value in general register
10 | Parameter or return value in floating-point register
11 | Double-precision floating-point value
For double-precision floating-point parameters, the odd-numbered
parameter word should be marked 11 and the even-numbered parameter
word should be marked 10. Double-precision return values are simply
marked 11.
Every procedure call is tagged with a similar set of bits (see
"Relocation Information" below), so that the linker can match each
call with the expectations of the procedure entry point. If the call
and entry point mismatch, the linker creates a stub that relocates the
parameters and return value as appropriate.
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Relocation Information [Toc] [Back]
Each initialized subspace defines a range of fixups that apply to the
data in that subspace. A fixup request is associated with every word
that requires relocation or that contains a reference to an
unsatisfied symbol. In relocatable object files created prior to HPUX
Release 3.0 on Series 800 systems, each fixup request is a fiveword
structure describing a code or data word to be patched at link
time. Object files created on Release 3.0 or later contain variablelength
fixup requests that describe every byte of the subspace. The
version_id field in the file header distinguishes these two formats;
the constant VERSION_ID is found in older object files, and the
constant NEW_VERSION_ID is found in newer ones.
In older object files, fixups can compute an expression involving
zero, one, or two symbols and a constant, then extract a field of bits
from that result and deposit those bits in any of several different
formats (corresponding to the Precision Architecture instruction set).
The fixup_request_index field in the subspace dictionary entry indexes
into the fixup request area defined by the file header and the
fixup_request_quantity field refers to the number of fixup requests
used for that subspace. The structure of a fixup request is contained
in <reloc.h>.
struct fixup_request_record {
unsigned int need_data_ref: 1; /* reserved */
unsigned int arg_reloc: 10; /* parameter relocation bits */
unsigned int expression_type: 5; /* how to compute value */
unsigned int exec_level: 2; /* reserved */
unsigned int fixup_format: 6; /* how to deposit bits */
unsigned int fixup_field: 8; /* field to extract */
unsigned int subspace_offset; /* subspace offset of word */
unsigned int symbol_index_one; /* index of first symbol */
unsigned int symbol_index_two; /* index of second symbol */
int fixup_constant; /* constant */
};
/* Values for expression_type */
#define e_one 0 /* symbol1 + constant */
#define e_two 1 /* symbol1 - symbol2 + constant */
#define e_pcrel 2 /* symbol1 - pc + constant */
#define e_con 3 /* constant */
#define e_plabel 7 /* symbol1 + constant */
#define e_abs 18 /* absolute, 1st sym index is address */
/* Values for fixup_field (assembler mnemonics shown) */
#define e_fsel 0 /* F': no change */
#define e_lssel 1 /* LS': inverse of RS' */
#define e_rssel 2 /* RS': rightmost 11 bits, signed */
#define e_lsel 3 /* L': leftmost 21 bits */
#define e_rsel 4 /* R': rightmost 11 bits */
#define e_ldsel 5 /* LD': inverse of RD' */
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#define e_rdsel 6 /* RD': rightmost 11 bits, filled left with ones */
#define e_lrsel 7 /* LR': L' with "rounded" constant */
#define e_rrsel 8 /* RR': R' with "rounded" constant */
#define e_nsel 9 /* N1': set all bits to zero: for id of 3-inst
code gen sequence */
/* Values for fixup_format (typical instructions shown) */
#define i_exp14 0 /* 14-bit immediate (LDW, STW) */
#define i_exp21 1 /* 21-bit immediate (LDIL, ADDIL) */
#define i_exp11 2 /* 11-bit immediate (ADDI, SUBI) */
#define i_rel17 3 /* 17-bit pc-relative (BL) */
#define i_rel12 4 /* 12 bit pc-relative (COMBT, COMBF, etc.) */
#define i_data 5 /* whole word */
#define i_none 6
#define i_abs17 7 /* 17-bit absolute (BE, BLE) */
#define i_milli 8 /* 17-bit millicode call (BLE) */
#define i_break 9 /* reserved (no effect on HP-UX) */
In newer object files, relocation entries consist of a stream of
bytes. The fixup_request_index field in the subspace dictionary entry
is a byte offset into the fixup dictionary defined by the file header,
and the fixup_request_quantity field defines the length of the fixup
request stream, in bytes, for that subspace. The first byte of each
fixup request (the opcode) identifies the request and determines the
length of the request.
In general, the fixup stream is a series of linker instructions that
governs how the linker places data in the a.out file. Certain fixup
requests cause the linker to copy one or more bytes from the input
subspace to the output subspace without change, while others direct
the linker to relocate words or resolve external references. Still
others direct the linker to insert zeroes in the output subspace or to
leave areas uninitialized without copying any data from the input
subspace, and others describe points in the code without contributing
any new data to the output file.
The include file <reloc.h> defines constants for each major opcode.
Many fixup requests use a range of opcodes; only a constant for the
beginning of the range is defined. The meaning of each fixup request
is described below. The opcode ranges and parameters for each fixup
are described in the table further below.
R_NO_RELOCATION Copy L bytes with no relocation.
R_ZEROES Insert L zero bytes into the output subspace.
R_UNINIT Skip L bytes in the output subspace.
R_RELOCATION Copy one data word with relocation. The word is
assumed to contain a 32-bit pointer relative to its
own subspace.
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R_DATA_ONE_SYMBOL Copy one data word with relocation relative to an
external symbol whose symbol index is S.
R_DATA_PLABEL Copy one data word as a 32-bit procedure label,
referring to the symbol S. The original contents
of the word should be 0 (no static link) or 2
(static link required).
R_SPACE_REF Copy one data word as a space reference. This
fixup request is not currently supported.
R_REPEATED_INIT Copy L bytes from the input subspace, replicating
the data to fill M bytes in the output subspace.
R_PCREL_CALL Copy one instruction word with relocation. The
word is assumed to be a pc-relative procedure call
instruction (for example, BL). The target
procedure is identified by symbol S, and the
parameter relocation bits are R.
R_ABS_CALL Copy one instruction word with relocation. The
word is assumed to be an absolute procedure call
instruction (for example, BLE). The target
procedure is identified by symbol S, and the
parameter relocation bits are R.
R_DP_RELATIVE Copy one instruction word with relocation. The
word is assumed to be a dp-relative load or store
instruction (for example, ADDIL, LDW, STW). The
target symbol is identified by symbol S. The
linker forms the difference between the value of
the symbol S and the value of the symbol $global$.
By convention, the value of $global$ is always
contained in register 27. Instructions may have a
small constant in the displacement field of the
instruction.
R_DLT_REL Copy one instruction word with relocation. The
word is assumed to be a register-18-relative load
or store instruction (for example, LDW, LDO, STW).
The target symbol is identified by symbol S. The
linker computes a linkage table offset relative to
register 18 (reserved for a linkage table pointer
in position-independent code) for the symbol S.
R_CODE_ONE_SYMBOL Copy one instruction word with relocation. The
word is assumed to be an instruction referring to
symbol S (for example, LDIL, LDW, BE).
Instructions may have a small constant in the
displacement field of the instruction.
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R_MILLI_REL Copy one instruction word with relocation. The
word is assumed to be a short millicode call
instruction (for example, BLE). The linker forms
the difference between the value of the target
symbol S and the value of symbol 1 in the module's
symbol table. By convention, the value of symbol 1
should have been previously loaded into the base
register used in the BLE instruction. The
instruction may have a small constant in the
displacement field of the instruction.
R_CODE_PLABEL Copy one instruction word with relocation. The
word is assumed to be part of a code sequence
forming a procedure label (for example, LDIL, LDO),
referring to symbol S. The LDO instruction should
contain the value 0 (no static link) or 2 (static
link required) in its displacement field.
R_BREAKPOINT Copy one instruction word conditionally. On HP-UX,
the linker always replaces the word with a NOP
instruction.
R_ENTRY Define a procedure entry point. The stack unwind
bits, U, and the frame size, F, are recorded in a
stack unwind descriptor.
R_ALT_ENTRY Define an alternate procedure entry point.
R_EXIT Define a procedure exit point.
R_BEGIN_TRY Define the beginning of a try/recover region.
R_END_TRY Define the end of a try/recover region. The offset
R defines the distance in bytes from the end of the
region to the beginning of the recover block.
R_BEGIN_BRTAB Define the beginning of a branch table.
R_END_BRTAB Define the end of a branch table.
R_AUX_UNWIND Define an auxiliary unwind table. CN is a symbol
index of the symbol that labels the beginning of
the compilation unit string table. SN is the
offset, relative to the CN symbol, of the scope
name string. SK is an integer specifying the scope
kind.
R_STATEMENT Define the beginning of statement number N.
R_SEC_STATEMENT Define the beginning of a secondary statement
number N.
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R_DATA_EXPR Pop one word from the expression stack and copy one
data word from the input subspace to the output
subspace, adding the popped value to it.
R_CODE_EXPR Pop one word from the expression stack, and copy
one instruction word from the input subspace to the
output subspace, adding the popped value to the
displacement field of the instruction.
R_FSEL Use an F' field selector for the next fixup request
instead of the default appropriate for the
instruction.
R_LSEL Use an L-class field selector for the next fixup
request instead of the default appropriate for the
instruction. Depending on the current rounding
mode, L', LS', LD', or LR' may be used.
R_RSEL Use an R-class field selector for the next fixup
request instead of the default appropriate for the
instruction. Depending on the current rounding
mode, R', RS', RD', or RR' may be used.
R_N_MODE Select round-down mode (L'/R'). This is the
default mode at the beginning of each subspace.
This setting remains in effect until explicitly
changed or until the end of the subspace.
R_S_MODE Select round-to-nearest-page mode (LS'/RS'). This
setting remains in effect until explicitly changed
or until the end of the subspace.
R_D_MODE Select round-up mode (LD'/RD'). This setting
remains in effect until explicitly changed or until
the end of the subspace.
R_R_MODE Select round-down-with-adjusted-constant mode
(LR'/RR'). This setting remains in effect until
explicitly changed or until the end of the
subspace.
R_DATA_OVERRIDE Use the constant V for the next fixup request in
place of the constant from the data word or
instruction in the input subspace.
R_TRANSLATED Toggle "translated" mode. This fixup request is
generated only by the linker during a relocatable
link to indicate a subspace that was originally
read from an old-format relocatable object file.
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R_COMP1 Stack operations. The second byte of this fixup
request contains a secondary opcode. In the
descriptions below, A refers to the top of the
stack and B refers to the next item on the stack.
All items on the stack are considered signed 32-bit
integers.
R_PUSH_PCON1 Push the (positive) constant V.
R_PUSH_DOT Push the current virtual address.
R_MAX Pop A and B, then push max(A, B).
R_MIN Pop A and B, then push min(A, B).
R_ADD Pop A and B, then push A + B.
R_SUB Pop A and B, then push B - A.
R_MULT Pop A and B, then push A * B.
R_DIV Pop A and B, then push B / A.
R_MOD Pop A and B, then push B % A.
R_AND Pop A and B, then push A & B.
R_OR Pop A and B, then push A | B.
R_XOR Pop A and B, then push A XOR B.
R_NOT Replace A with its complement.
R_LSHIFT If C = 0, pop A and B, then push B
<< A. Otherwise, replace A with A
<< C.
R_ARITH_RSHIFT If C = 0, pop A and B, then push B
>> A. Otherwise, replace A with A
>> C. The shifting is done with
sign extension.
R_LOGIC_RSHIFT If C = 0, pop A and B, then push B
>> A. Otherwise, replace A with A
>> C. The shifting is done with
zero fill.
R_PUSH_NCON1 Push the (negative) constant V.
R_COMP2 More stack operations.
R_PUSH_PCON2 Push the (positive) constant V.
R_PUSH_SYM Push the value of the symbol S.
R_PUSH_PLABEL Push the value of a procedure label
for symbol S. The static link bit
is L.
R_PUSH_NCON2 Push the (negative) constant V.
R_COMP3 More stack operations.
R_PUSH_PROC Push the value of the procedure
entry point S. The parameter
relocation bits are R.
R_PUSH_CONST Push the constant V.
R_PREV_FIXUP The linker keeps a queue of the last four unique
multi-byte fixup requests. This is an abbreviation
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for a fixup request identical to one on the queue.
The queue index X references one of the four; X = 0
refers to the most recent. As a side effect of
this fixup request, the referenced fixup is moved
to the front of the queue.
R_N0SEL Indicates that the following fixup is applied to
the first of a three-instruction sequence to access
data, generated by the compilers to enable the
importing of shared library data.
R_N1SEL Uses a (N') field selector for the next fixup
request. This indicates that zero bits are to be
used for the displacement on the instruction. This
fixup is used to identify three-instruction
sequences to access data (for importing shared
library data).
R_LINETAB Defines the beginning of a line table. CU is a
symbol index of the symbol that labels the
beginning of the line table. SM is the offset
relative to the CU symbol. ES designates the
version information for the current line table.
R_LINETAB_ESC Defines an escape entry to be entered into the line
table. ES designates the escape entry entered in
the table. M designates the number of R_STATEMENT
fixups to be interpreted as raw 8-bit table data.
R_LTP_OVERRIDE Override the following fixup, which is expected to
be a R_DATA_ONE_SYMBOL fixup to copy one data word
without relocation when building a shared library.
The absolute byte offset of the symbol relative to
the linkage table pointer is copied. If the linker
is building a complete executable, the absolute
virtual address is copied.
R_COMMENT Fixup used to pass comment information from the
compiler to the linker. This fixup has a 5 byte
argument that can be skipped and ignored by
applications.
R_TP_OVERRIDE Override the next one of these fixups seen:
R_DP_RELATIVE, R_DLT_REL, or R_DATA_ONE_SYMBOL, to
use the thread local storage offset when fixing the
instruction. This fixup is also used to catch
thread local storage symbol mismatches.
R_RESERVED Fixups in this range are reserved for internal use
by the compilers and linker.
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The following table shows the mnemonic fixup request type and length
and parameter information for each range of opcodes. In the
parameters column, the symbol D refers to the difference between the
opcode and the beginning of the range described by that table entry;
the symbols B1, B2, B3, and B4 refer to the value of the next one,
two, three, or four bytes of the fixup request, respectively.
Mnemonic Opcodes Length Parameters
____________________________________________________________________________
R_NO_RELOCATION 0-23 1 L = (D+1) * 4
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