This is Info file ld.info, produced by Makeinfo-1.55 from the input file ./ld.texinfo. START-INFO-DIR-ENTRY * Ld: (ld). The GNU linker. END-INFO-DIR-ENTRY This file documents the GNU linker LD. Copyright (C) 1991, 1992, 1993, 1994 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions.  File: ld.info, Node: Top, Next: Overview, Prev: (DIR), Up: (DIR) Using ld ******** This file documents the GNU linker ld. * Menu: * Overview:: Overview * Invocation:: Invocation * Commands:: Command Language * Machine Dependent:: Machine Dependent Features * BFD:: BFD * MRI:: MRI Compatible Script Files * Index:: Index  File: ld.info, Node: Overview, Next: Invocation, Prev: Top, Up: Top Overview ******** `ld' combines a number of object and archive files, relocates their data and ties up symbol references. Usually the last step in compiling a program is to run `ld'. `ld' accepts Linker Command Language files written in a superset of AT&T's Link Editor Command Language syntax, to provide explicit and total control over the linking process. This version of `ld' uses the general purpose BFD libraries to operate on object files. This allows `ld' to read, combine, and write object files in many different formats--for example, COFF or `a.out'. Different formats may be linked together to produce any available kind of object file. *Note BFD::, for more information. Aside from its flexibility, the GNU linker is more helpful than other linkers in providing diagnostic information. Many linkers abandon execution immediately upon encountering an error; whenever possible, `ld' continues executing, allowing you to identify other errors (or, in some cases, to get an output file in spite of the error).  File: ld.info, Node: Invocation, Next: Commands, Prev: Overview, Up: Top Invocation ********** The GNU linker `ld' is meant to cover a broad range of situations, and to be as compatible as possible with other linkers. As a result, you have many choices to control its behavior. * Menu: * Options:: Command Line Options * Environment:: Environment Variables  File: ld.info, Node: Options, Next: Environment, Up: Invocation Command Line Options ==================== Here is a summary of the options you can use on the `ld' command line: ld [ -o OUTPUT ] OBJFILE... [ -AARCHITECTURE ] [ -b INPUT-FORMAT ] [ -Bstatic ] [ -c MRI-COMMANDFILE ] [ -d | -dc | -dp ] [ -defsym SYMBOL=EXPRESSION ] [ -e ENTRY ] [ -F ] [ -F FORMAT ] [ -format INPUT-FORMAT ] [ -g ] [ -G SIZE ] [ -help ] [ -i ] [ -lARCHIVE ] [ -LSEARCHDIR ] [ -M ] [ -Map MAPFILE ] [ -m EMULATION ] [ -N | -n ] [ -noinhibit-exec ] [ -oformat OUTPUT-FORMAT ] [ -R FILENAME ] [ -relax ] [ -retain-symbols-file FILENAME ] [ -r | -Ur ] [ -S ] [ -s ] [ -sort-common ] [ -stats ] [ -T COMMANDFILE ] [ -Ttext ORG ] [ -Tdata ORG ] [ -Tbss ORG ] [ -t ] [ -u SYMBOL] [-V] [-v] [ -version ] [ -warn-common ] [ -y SYMBOL ] [ -X ] [-x ] This plethora of command-line options may seem intimidating, but in actual practice few of them are used in any particular context. For instance, a frequent use of `ld' is to link standard Unix object files on a standard, supported Unix system. On such a system, to link a file `hello.o': ld -o OUTPUT /lib/crt0.o hello.o -lc This tells `ld' to produce a file called OUTPUT as the result of linking the file `/lib/crt0.o' with `hello.o' and the library `libc.a', which will come from the standard search directories. (See the discussion of the `-l' option below.) The command-line options to `ld' may be specified in any order, and may be repeated at will. Repeating most options with a different argument will either have no further effect, or override prior occurrences (those further to the left on the command line) of that option. The exceptions--which may meaningfully be used more than once--are `-A', `-b' (or its synonym `-format'), `-defsym', `-L', `-l', `-R', and `-u'. The list of object files to be linked together, shown as OBJFILE..., may follow, precede, or be mixed in with command-line options, except that an OBJFILE argument may not be placed between an option and its argument. Usually the linker is invoked with at least one object file, but you can specify other forms of binary input files using `-l', `-R', and the script command language. If *no* binary input files at all are specified, the linker does not produce any output, and issues the message `No input files'. For options whose names are a single letter, option arguments must either follow the option letter without intervening whitespace, or be given as separate arguments immediately following the option that requires them. For options whose names are multiple letters, either one dash or two can precede the option name; for example, `--oformat' and `-oformat' are equivalent. Arguments to multiple-letter options must either be separated from the option name by an equals sign, or be given as separate arguments immediately following the option that requires them. For example, `--oformat srec' and `--oformat=srec' are equivalent. Unique abbreviations of the names of multiple-letter options are accepted. `-AARCHITECTURE' In the current release of `ld', this option is useful only for the Intel 960 family of architectures. In that `ld' configuration, the ARCHITECTURE argument identifies the particular architecture in the 960 family, enabling some safeguards and modifying the archive-library search path. *Note `ld' and the Intel 960 family: i960, for details. Future releases of `ld' may support similar functionality for other architecture families. `-b INPUT-FORMAT' `ld' may be configured to support more than one kind of object file. If your `ld' is configured this way, you can use the `-b' option to specify the binary format for input object files that follow this option on the command line. Even when `ld' is configured to support alternative object formats, you don't usually need to specify this, as `ld' should be configured to expect as a default input format the most usual format on each machine. INPUT-FORMAT is a text string, the name of a particular format supported by the BFD libraries. (You can list the available binary formats with `objdump -i'.) `-format INPUT-FORMAT' has the same effect, as does the script command `TARGET'. *Note BFD::. You may want to use this option if you are linking files with an unusual binary format. You can also use `-b' to switch formats explicitly (when linking object files of different formats), by including `-b INPUT-FORMAT' before each group of object files in a particular format. The default format is taken from the environment variable `GNUTARGET'. *Note Environment::. You can also define the input format from a script, using the command `TARGET'; see *Note Option Commands::. `-Bstatic' Ignored. This option is accepted for command-line compatibility with the SunOS linker. `-c MRI-COMMANDFILE' For compatibility with linkers produced by MRI, `ld' accepts script files written in an alternate, restricted command language, described in *Note MRI Compatible Script Files: MRI. Introduce MRI script files with the option `-c'; use the `-T' option to run linker scripts written in the general-purpose `ld' scripting language. If MRI-CMDFILE does not exist, `ld' looks for it in the directories specified by any `-L' options. `-d' `-dc' `-dp' These three options are equivalent; multiple forms are supported for compatibility with other linkers. They assign space to common symbols even if a relocatable output file is specified (with `-r'). The script command `FORCE_COMMON_ALLOCATION' has the same effect. *Note Option Commands::. `-defsym SYMBOL=EXPRESSION' Create a global symbol in the output file, containing the absolute address given by EXPRESSION. You may use this option as many times as necessary to define multiple symbols in the command line. A limited form of arithmetic is supported for the EXPRESSION in this context: you may give a hexadecimal constant or the name of an existing symbol, or use `+' and `-' to add or subtract hexadecimal constants or symbols. If you need more elaborate expressions, consider using the linker command language from a script (*note Assignment: Symbol Definitions: Assignment.). *Note:* there should be no white space between SYMBOL, the equals sign ("="), and EXPRESSION. `-e ENTRY' Use ENTRY as the explicit symbol for beginning execution of your program, rather than the default entry point. *Note Entry Point::, for a discussion of defaults and other ways of specifying the entry point. `-F' `-FFORMAT' Ignored. Some older linkers used this option throughout a compilation toolchain for specifying object-file format for both input and output object files. The mechanisms `ld' uses for this purpose (the `-b' or `-format' options for input files, `-oformat' option or the `TARGET' command in linker scripts for output files, the `GNUTARGET' environment variable) are more flexible, but `ld' accepts the `-F' option for compatibility with scripts written to call the old linker. `-format INPUT-FORMAT' Synonym for `-b INPUT-FORMAT'. `-g' Ignored. Provided for compatibility with other tools. `-GVALUE' `-G VALUE' Set the maximum size of objects to be optimized using the GP register to SIZE under MIPS ECOFF. Ignored for other object file formats. `-help' Print a summary of the command-line options on the standard output and exit. `-i' Perform an incremental link (same as option `-r'). `-lAR' Add archive file ARCHIVE to the list of files to link. This option may be used any number of times. `ld' will search its path-list for occurrences of `libAR.a' for every ARCHIVE specified. `-LSEARCHDIR' `-L SEARCHDIR' Add path SEARCHDIR to the list of paths that `ld' will search for archive libraries and `ld' control scripts. You may use this option any number of times. The default set of paths searched (without being specified with `-L') depends on which emulation mode `ld' is using, and in some cases also on how it was configured. *Note Environment::. The paths can also be specified in a link script with the `SEARCH_DIR' command. `-M' Print (to the standard output) a link map--diagnostic information about where symbols are mapped by `ld', and information on global common storage allocation. `-Map MAPFILE' Print to the file MAPFILE a link map--diagnostic information about where symbols are mapped by `ld', and information on global common storage allocation. `-mEMULATION' `-m EMULATION' Emulate the EMULATION linker. You can list the available emulations with the `-V' option. The default depends on how your `ld' was configured. `-N' Set the text and data sections to be readable and writable. Also, do not page-align the data segment. If the output format supports Unix style magic numbers, mark the output as `OMAGIC'. `-n' Set the text segment to be read only, and mark the output as `NMAGIC' if possible. `-noinhibit-exec' Retain the executable output file whenever it is still usable. Normally, the linker will not produce an output file if it encounters errors during the link process; it exits without writing an output file when it issues any error whatsoever. `-o OUTPUT' Use OUTPUT as the name for the program produced by `ld'; if this option is not specified, the name `a.out' is used by default. The script command `OUTPUT' can also specify the output file name. `-oformat OUTPUT-FORMAT' `ld' may be configured to support more than one kind of object file. If your `ld' is configured this way, you can use the `-oformat' option to specify the binary format for the output object file. Even when `ld' is configured to support alternative object formats, you don't usually need to specify this, as `ld' should be configured to produce as a default output format the most usual format on each machine. OUTPUT-FORMAT is a text string, the name of a particular format supported by the BFD libraries. (You can list the available binary formats with `objdump -i'.) The script command `OUTPUT_FORMAT' can also specify the output format, but this option overrides it. *Note BFD::. `-R FILENAME' Read symbol names and their addresses from FILENAME, but do not relocate it or include it in the output. This allows your output file to refer symbolically to absolute locations of memory defined in other programs. `-relax' An option with machine dependent effects. Currently this option is only supported on the H8/300 and the Intel 960. *Note `ld' and the H8/300: H8/300. *Note `ld' and the Intel 960 family: i960. On some platforms, the `-relax' option performs global optimizations that become possible when the linker resolves addressing in the program, such as relaxing address modes and synthesizing new instructions in the output object file. On platforms where this is not supported, `-relax' is accepted, but ignored. `-retain-symbols-file FILENAME' Retain *only* the symbols listed in the file FILENAME, discarding all others. FILENAME is simply a flat file, with one symbol name per line. This option is especially useful in environments (such as VxWorks) where a large global symbol table is accumulated gradually, to conserve run-time memory. `-retain-symbols-file' does *not* discard undefined symbols, or symbols needed for relocations. You may only specify `-retain-symbols-file' once in the command line. It overrides `-s' and `-S'. `-r' Generate relocatable output--i.e., generate an output file that can in turn serve as input to `ld'. This is often called "partial linking". As a side effect, in environments that support standard Unix magic numbers, this option also sets the output file's magic number to `OMAGIC'. If this option is not specified, an absolute file is produced. When linking C++ programs, this option *will not* resolve references to constructors; to do that, use `-Ur'. This option does the same thing as `-i'. `-S' Omit debugger symbol information (but not all symbols) from the output file. `-s' Omit all symbol information from the output file. `-sort-common' Normally, when `ld' places the global common symbols in the appropriate output sections, it sorts them by size. First come all the one byte symbols, then all the two bytes, then all the four bytes, and then everything else. This is to prevent gaps between symbols due to alignment constraints. This option disables that sorting. `-stats' Compute and display statistics about the operation of the linker, such as execution time and memory usage. `-Tbss ORG' `-Tdata ORG' `-Ttext ORG' Use ORG as the starting address for--respectively--the `bss', `data', or the `text' segment of the output file. ORG must be a single hexadecimal integer; for compatibility with other linkers, you may omit the leading `0x' usually associated with hexadecimal values. `-T COMMANDFILE' `-TCOMMANDFILE' Read link commands from the file COMMANDFILE. These commands replace `ld''s default link script (rather than adding to it), so COMMANDFILE must specify everything necessary to describe the target format. *Note Commands::. If COMMANDFILE does not exist, `ld' looks for it in the directories specified by any preceding `-L' options. Multiple `-T' options accumulate. `-t' Print the names of the input files as `ld' processes them. `-u SYMBOL' Force SYMBOL to be entered in the output file as an undefined symbol. Doing this may, for example, trigger linking of additional modules from standard libraries. `-u' may be repeated with different option arguments to enter additional undefined symbols. `-Ur' For anything other than C++ programs, this option is equivalent to `-r': it generates relocatable output--i.e., an output file that can in turn serve as input to `ld'. When linking C++ programs, `-Ur' *does* resolve references to constructors, unlike `-r'. It does not work to use `-Ur' on files that were themselves linked with `-Ur'; once the constructor table has been built, it cannot be added to. Use `-Ur' only for the last partial link, and `-r' for the others. `-V' Display the version number for `ld' and list the linker emulations supported. Display which input files can and cannot be opened. `-v' Display the version number for `ld'. `-version' Display the version number for `ld' and exit. `-warn-common' Warn when a common symbol is combined with another common symbol or with a symbol definition. Unix linkers allow this somewhat sloppy practice, but linkers on some other operating systems do not. This option allows you to find potential problems from combining global symbols. Unfortunately, some C libraries use this practice, so you may get some warnings about symbols in the libraries as well as in your programs. There are three kinds of global symbols, illustrated here by C examples: `int i = 1;' A definition, which goes in the initialized data section of the output file. `extern int i;' An undefined reference, which does not allocate space. There must be either a definition or a common symbol for the variable somewhere. `int i;' A common symbol. If there are only (one or more) common symbols for a variable, it goes in the uninitialized data area of the output file. The linker merges multiple common symbols for the same variable into a single symbol. If they are of different sizes, it picks the largest size. The linker turns a common symbol into a declaration, if there is a definition of the same variable. The `-warn-common' option can produce five kinds of warnings. Each warning consists of a pair of lines: the first describes the symbol just encountered, and the second describes the previous symbol encountered with the same name. One or both of the two symbols will be a common symbol. 1. Turning a common symbol into a reference, because there is already a definition for the symbol. FILE(SECTION): warning: common of `SYMBOL' overridden by definition FILE(SECTION): warning: defined here 2. Turning a common symbol into a reference, because a later definition for the symbol is encountered. This is the same as the previous case, except that the symbols are encountered in a different order. FILE(SECTION): warning: definition of `SYMBOL' overriding common FILE(SECTION): warning: common is here 3. Merging a common symbol with a previous same-sized common symbol. FILE(SECTION): warning: multiple common of `SYMBOL' FILE(SECTION): warning: previous common is here 4. Merging a common symbol with a previous larger common symbol. FILE(SECTION): warning: common of `SYMBOL' overridden by larger common FILE(SECTION): warning: larger common is here 5. Merging a common symbol with a previous smaller common symbol. This is the same as the previous case, except that the symbols are encountered in a different order. FILE(SECTION): warning: common of `SYMBOL' overriding smaller common FILE(SECTION): warning: smaller common is here `-X' If `-s' or `-S' is also specified, delete only local symbols beginning with `L'. `-x' If `-s' or `-S' is also specified, delete all local symbols, not just those beginning with `L'. `-y SYMBOL' Print the name of each linked file in which SYMBOL appears. This option may be given any number of times. On many systems it is necessary to prepend an underscore. This option is useful when you have an undefined symbol in your link but don't know where the reference is coming from.  File: ld.info, Node: Environment, Prev: Options, Up: Invocation Environment Variables ===================== You can change the behavior of `ld' with the environment variable `GNUTARGET'. `GNUTARGET' determines the input-file object format if you don't use `-b' (or its synonym `-format'). Its value should be one of the BFD names for an input format (*note BFD::.). If there is no `GNUTARGET' in the environment, `ld' uses the natural format of the target. If `GNUTARGET' is set to `default' then BFD attempts to discover the input format by examining binary input files; this method often succeeds, but there are potential ambiguities, since there is no method of ensuring that the magic number used to specify object-file formats is unique. However, the configuration procedure for BFD on each system places the conventional format for that system first in the search-list, so ambiguities are resolved in favor of convention.  File: ld.info, Node: Commands, Next: Machine Dependent, Prev: Invocation, Up: Top Command Language **************** The command language provides explicit control over the link process, allowing complete specification of the mapping between the linker's input files and its output. It controls: * input files * file formats * output file layout * addresses of sections * placement of common blocks You may supply a command file (also known as a link script) to the linker either explicitly through the `-T' option, or implicitly as an ordinary file. If the linker opens a file which it cannot recognize as a supported object or archive format, it reports an error. * Menu: * Scripts:: Linker Scripts * Expressions:: Expressions * MEMORY:: MEMORY Command * SECTIONS:: SECTIONS Command * Entry Point:: The Entry Point * Option Commands:: Option Commands  File: ld.info, Node: Scripts, Next: Expressions, Up: Commands Linker Scripts ============== The `ld' command language is a collection of statements; some are simple keywords setting a particular option, some are used to select and group input files or name output files; and two statement types have a fundamental and pervasive impact on the linking process. The most fundamental command of the `ld' command language is the `SECTIONS' command (*note SECTIONS::.). Every meaningful command script must have a `SECTIONS' command: it specifies a "picture" of the output file's layout, in varying degrees of detail. No other command is required in all cases. The `MEMORY' command complements `SECTIONS' by describing the available memory in the target architecture. This command is optional; if you don't use a `MEMORY' command, `ld' assumes sufficient memory is available in a contiguous block for all output. *Note MEMORY::. You may include comments in linker scripts just as in C: delimited by `/*' and `*/'. As in C, comments are syntactically equivalent to whitespace.  File: ld.info, Node: Expressions, Next: MEMORY, Prev: Scripts, Up: Commands Expressions =========== Many useful commands involve arithmetic expressions. The syntax for expressions in the command language is identical to that of C expressions, with the following features: * All expressions evaluated as integers and are of "long" or "unsigned long" type. * All constants are integers. * All of the C arithmetic operators are provided. * You may reference, define, and create global variables. * You may call special purpose built-in functions. * Menu: * Integers:: Integers * Symbols:: Symbol Names * Location Counter:: The Location Counter * Operators:: Operators * Evaluation:: Evaluation * Assignment:: Assignment: Defining Symbols * Arithmetic Functions:: Built-In Functions  File: ld.info, Node: Integers, Next: Symbols, Up: Expressions Integers -------- An octal integer is `0' followed by zero or more of the octal digits (`01234567'). _as_octal = 0157255; A decimal integer starts with a non-zero digit followed by zero or more digits (`0123456789'). _as_decimal = 57005; A hexadecimal integer is `0x' or `0X' followed by one or more hexadecimal digits chosen from `0123456789abcdefABCDEF'. _as_hex = 0xdead; To write a negative integer, use the prefix operator `-'; *note Operators::.. _as_neg = -57005; Additionally the suffixes `K' and `M' may be used to scale a constant by `1024' or `1024*1024' respectively. For example, the following all refer to the same quantity: _fourk_1 = 4K; _fourk_2 = 4096; _fourk_3 = 0x1000;  File: ld.info, Node: Symbols, Next: Location Counter, Prev: Integers, Up: Expressions Symbol Names ------------ Unless quoted, symbol names start with a letter, underscore, or point and may include any letters, underscores, digits, points, and hyphens. Unquoted symbol names must not conflict with any keywords. You can specify a symbol which contains odd characters or has the same name as a keyword, by surrounding the symbol name in double quotes: "SECTION" = 9; "with a space" = "also with a space" + 10; Since symbols can contain many non-alphabetic characters, it is safest to delimit symbols with spaces. For example, `A-B' is one symbol, whereas `A - B' is an expression involving subtraction.  File: ld.info, Node: Location Counter, Next: Operators, Prev: Symbols, Up: Expressions The Location Counter -------------------- The special linker variable "dot" `.' always contains the current output location counter. Since the `.' always refers to a location in an output section, it must always appear in an expression within a `SECTIONS' command. The `.' symbol may appear anywhere that an ordinary symbol is allowed in an expression, but its assignments have a side effect. Assigning a value to the `.' symbol will cause the location counter to be moved. This may be used to create holes in the output section. The location counter may never be moved backwards. SECTIONS { output : { file1(.text) . = . + 1000; file2(.text) . += 1000; file3(.text) } = 0x1234; } In the previous example, `file1' is located at the beginning of the output section, then there is a 1000 byte gap. Then `file2' appears, also with a 1000 byte gap following before `file3' is loaded. The notation `= 0x1234' specifies what data to write in the gaps (*note Section Options::.).  File: ld.info, Node: Operators, Next: Evaluation, Prev: Location Counter, Up: Expressions Operators --------- The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels: precedence associativity Operators Notes (highest) 1 left ! - ~ (1) 2 left * / % 3 left + - 4 left >> << 5 left == != > < <= >= 6 left & 7 left | 8 left && 9 left || 10 right ? : 11 right &= += -= *= /= (2) (lowest) Notes: (1) Prefix operators (2) *Note Assignment::  File: ld.info, Node: Evaluation, Next: Assignment, Prev: Operators, Up: Expressions Evaluation ---------- The linker uses "lazy evaluation" for expressions; it only calculates an expression when absolutely necessary. The linker needs the value of the start address, and the lengths of memory regions, in order to do any linking at all; these values are computed as soon as possible when the linker reads in the command file. However, other values (such as symbol values) are not known or needed until after storage allocation. Such values are evaluated later, when other information (such as the sizes of output sections) is available for use in the symbol assignment expression.  File: ld.info, Node: Assignment, Next: Arithmetic Functions, Prev: Evaluation, Up: Expressions Assignment: Defining Symbols ---------------------------- You may create global symbols, and assign values (addresses) to global symbols, using any of the C assignment operators: `SYMBOL = EXPRESSION ;' `SYMBOL &= EXPRESSION ;' `SYMBOL += EXPRESSION ;' `SYMBOL -= EXPRESSION ;' `SYMBOL *= EXPRESSION ;' `SYMBOL /= EXPRESSION ;' Two things distinguish assignment from other operators in `ld' expressions. * Assignment may only be used at the root of an expression; `a=b+3;' is allowed, but `a+b=3;' is an error. * You must place a trailing semicolon (";") at the end of an assignment statement. Assignment statements may appear: * as commands in their own right in an `ld' script; or * as independent statements within a `SECTIONS' command; or * as part of the contents of a section definition in a `SECTIONS' command. The first two cases are equivalent in effect--both define a symbol with an absolute address. The last case defines a symbol whose address is relative to a particular section (*note SECTIONS::.). When a linker expression is evaluated and assigned to a variable, it is given either an absolute or a relocatable type. An absolute expression type is one in which the symbol contains the value that it will have in the output file; a relocatable expression type is one in which the value is expressed as a fixed offset from the base of a section. The type of the expression is controlled by its position in the script file. A symbol assigned within a section definition is created relative to the base of the section; a symbol assigned in any other place is created as an absolute symbol. Since a symbol created within a section definition is relative to the base of the section, it will remain relocatable if relocatable output is requested. A symbol may be created with an absolute value even when assigned to within a section definition by using the absolute assignment function `ABSOLUTE'. For example, to create an absolute symbol whose address is the last byte of an output section named `.data': SECTIONS{ ... .data : { *(.data) _edata = ABSOLUTE(.) ; } ... } The linker tries to put off the evaluation of an assignment until all the terms in the source expression are known (*note Evaluation::.). For instance, the sizes of sections cannot be known until after allocation, so assignments dependent upon these are not performed until after allocation. Some expressions, such as those depending upon the location counter "dot", `.' must be evaluated during allocation. If the result of an expression is required, but the value is not available, then an error results. For example, a script like the following SECTIONS { ... text 9+this_isnt_constant : { ... } ... } will cause the error message "`Non constant expression for initial address'".  File: ld.info, Node: Arithmetic Functions, Prev: Assignment, Up: Expressions Arithmetic Functions -------------------- The command language includes a number of built-in functions for use in link script expressions. `ABSOLUTE(EXP)' Return the absolute (non-relocatable, as opposed to non-negative) value of the expression EXP. Primarily useful to assign an absolute value to a symbol within a section definition, where symbol values are normally section-relative. `ADDR(SECTION)' Return the absolute address of the named SECTION. Your script must previously have defined the location of that section. In the following example, `symbol_1' and `symbol_2' are assigned identical values: SECTIONS{ ... .output1 : { start_of_output_1 = ABSOLUTE(.); ... } .output : { symbol_1 = ADDR(.output1); symbol_2 = start_of_output_1; } ... } `ALIGN(EXP)' Return the result of the current location counter (`.') aligned to the next EXP boundary. EXP must be an expression whose value is a power of two. This is equivalent to (. + EXP - 1) & ~(EXP - 1) `ALIGN' doesn't change the value of the location counter--it just does arithmetic on it. As an example, to align the output `.data' section to the next `0x2000' byte boundary after the preceding section and to set a variable within the section to the next `0x8000' boundary after the input sections: SECTIONS{ ... .data ALIGN(0x2000): { *(.data) variable = ALIGN(0x8000); } ... } The first use of `ALIGN' in this example specifies the location of a section because it is used as the optional START attribute of a section definition (*note Section Options::.). The second use simply defines the value of a variable. The built-in `NEXT' is closely related to `ALIGN'. `DEFINED(SYMBOL)' Return 1 if SYMBOL is in the linker global symbol table and is defined, otherwise return 0. You can use this function to provide default values for symbols. For example, the following command-file fragment shows how to set a global symbol `begin' to the first location in the `.text' section--but if a symbol called `begin' already existed, its value is preserved: SECTIONS{ ... .text : { begin = DEFINED(begin) ? begin : . ; ... } ... } `NEXT(EXP)' Return the next unallocated address that is a multiple of EXP. This function is closely related to `ALIGN(EXP)'; unless you use the `MEMORY' command to define discontinuous memory for the output file, the two functions are equivalent. `SIZEOF(SECTION)' Return the size in bytes of the named SECTION, if that section has been allocated. In the following example, `symbol_1' and `symbol_2' are assigned identical values: SECTIONS{ ... .output { .start = . ; ... .end = . ; } symbol_1 = .end - .start ; symbol_2 = SIZEOF(.output); ... } `SIZEOF_HEADERS' `sizeof_headers' Return the size in bytes of the output file's headers. You can use this number as the start address of the first section, if you choose, to facilitate paging.  File: ld.info, Node: MEMORY, Next: SECTIONS, Prev: Expressions, Up: Commands Memory Layout ============= The linker's default configuration permits allocation of all available memory. You can override this configuration by using the `MEMORY' command. The `MEMORY' command describes the location and size of blocks of memory in the target. By using it carefully, you can describe which memory regions may be used by the linker, and which memory regions it must avoid. The linker does not shuffle sections to fit into the available regions, but does move the requested sections into the correct regions and issue errors when the regions become too full. A command file may contain at most one use of the `MEMORY' command; however, you can define as many blocks of memory within it as you wish. The syntax is: MEMORY { NAME (ATTR) : ORIGIN = ORIGIN, LENGTH = LEN ... } `NAME' is a name used internally by the linker to refer to the region. Any symbol name may be used. The region names are stored in a separate name space, and will not conflict with symbols, file names or section names. Use distinct names to specify multiple regions. `(ATTR)' is an optional list of attributes, permitted for compatibility with the AT&T linker but not used by `ld' beyond checking that the attribute list is valid. Valid attribute lists must be made up of the characters "`LIRWX'". If you omit the attribute list, you may omit the parentheses around it as well. `ORIGIN' is the start address of the region in physical memory. It is an expression that must evaluate to a constant before memory allocation is performed. The keyword `ORIGIN' may be abbreviated to `org' or `o' (but not, for example, `ORG'). `LEN' is the size in bytes of the region (an expression). The keyword `LENGTH' may be abbreviated to `len' or `l'. For example, to specify that memory has two regions available for allocation--one starting at 0 for 256 kilobytes, and the other starting at `0x40000000' for four megabytes: MEMORY { rom : ORIGIN = 0, LENGTH = 256K ram : org = 0x40000000, l = 4M } Once you have defined a region of memory named MEM, you can direct specific output sections there by using a command ending in `>MEM' within the `SECTIONS' command (*note Section Options::.). If the combined output sections directed to a region are too big for the region, the linker will issue an error message.  File: ld.info, Node: SECTIONS, Next: Entry Point, Prev: MEMORY, Up: Commands Specifying Output Sections ========================== The `SECTIONS' command controls exactly where input sections are placed into output sections, their order in the output file, and to which output sections they are allocated. You may use at most one `SECTIONS' command in a script file, but you can have as many statements within it as you wish. Statements within the `SECTIONS' command can do one of three things: * define the entry point; * assign a value to a symbol; * describe the placement of a named output section, and which input sections go into it. You can also use the first two operations--defining the entry point and defining symbols--outside the `SECTIONS' command: *note Entry Point::., and *note Assignment::.. They are permitted here as well for your convenience in reading the script, so that symbols and the entry point can be defined at meaningful points in your output-file layout. If you do not use a `SECTIONS' command, the linker places each input section into an identically named output section in the order that the sections are first encountered in the input files. If all input sections are present in the first file, for example, the order of sections in the output file will match the order in the first input file. * Menu: * Section Definition:: Section Definitions * Section Placement:: Section Placement * Section Data Expressions:: Section Data Expressions * Section Options:: Optional Section Attributes  File: ld.info, Node: Section Definition, Next: Section Placement, Up: SECTIONS Section Definitions ------------------- The most frequently used statement in the `SECTIONS' command is the "section definition", which specifies the properties of an output section: its location, alignment, contents, fill pattern, and target memory region. Most of these specifications are optional; the simplest form of a section definition is SECTIONS { ... SECNAME : { CONTENTS } ... } SECNAME is the name of the output section, and CONTENTS a specification of what goes there--for example, a list of input files or sections of input files (*note Section Placement::.). As you might assume, the whitespace shown is optional. You do need the colon `:' and the braces `{}', however. SECNAME must meet the constraints of your output format. In formats which only support a limited number of sections, such as `a.out', the name must be one of the names supported by the format (`a.out', for example, allows only `.text', `.data' or `.bss'). If the output format supports any number of sections, but with numbers and not names (as is the case for Oasys), the name should be supplied as a quoted numeric string. A section name may consist of any sequence of characters, but any name which does not conform to the standard `ld' symbol name syntax must be quoted. *Note Symbol Names: Symbols.  File: ld.info, Node: Section Placement, Next: Section Data Expressions, Prev: Section Definition, Up: SECTIONS Section Placement ----------------- In a section definition, you can specify the contents of an output section by listing particular input files, by listing particular input-file sections, or by a combination of the two. You can also place arbitrary data in the section, and define symbols relative to the beginning of the section. The CONTENTS of a section definition may include any of the following kinds of statement. You can include as many of these as you like in a single section definition, separated from one another by whitespace. `FILENAME' You may simply name a particular input file to be placed in the current output section; *all* sections from that file are placed in the current section definition. If the file name has already been mentioned in another section definition, with an explicit section name list, then only those sections which have not yet been allocated are used. To specify a list of particular files by name: .data : { afile.o bfile.o cfile.o } The example also illustrates that multiple statements can be included in the contents of a section definition, since each file name is a separate statement. `FILENAME( SECTION )' `FILENAME( SECTION, SECTION, ... )' `FILENAME( SECTION SECTION ... )' You can name one or more sections from your input files, for insertion in the current output section. If you wish to specify a list of input-file sections inside the parentheses, you may separate the section names by either commas or whitespace. `* (SECTION)' `* (SECTION, SECTION, ...)' `* (SECTION SECTION ...)' Instead of explicitly naming particular input files in a link control script, you can refer to *all* files from the `ld' command line: use `*' instead of a particular file name before the parenthesized input-file section list. If you have already explicitly included some files by name, `*' refers to all *remaining* files--those whose places in the output file have not yet been defined. For example, to copy sections `1' through `4' from an Oasys file into the `.text' section of an `a.out' file, and sections `13' and `14' into the `.data' section: SECTIONS { .text :{ *("1" "2" "3" "4") } .data :{ *("13" "14") } } `[ SECTION ... ]' used to be accepted as an alternate way to specify named sections from all unallocated input files. Because some operating systems (VMS) allow brackets in file names, that notation is no longer supported. `FILENAME`( COMMON )'' `*( COMMON )' Specify where in your output file to place uninitialized data with this notation. `*(COMMON)' by itself refers to all uninitialized data from all input files (so far as it is not yet allocated); FILENAME`(COMMON)' refers to uninitialized data from a particular file. Both are special cases of the general mechanisms for specifying where to place input-file sections: `ld' permits you to refer to uninitialized data as if it were in an input-file section named `COMMON', regardless of the input file's format. For example, the following command script arranges the output file into three consecutive sections, named `.text', `.data', and `.bss', taking the input for each from the correspondingly named sections of all the input files: SECTIONS { .text : { *(.text) } .data : { *(.data) } .bss : { *(.bss) *(COMMON) } } The following example reads all of the sections from file `all.o' and places them at the start of output section `outputa' which starts at location `0x10000'. All of section `.input1' from file `foo.o' follows immediately, in the same output section. All of section `.input2' from `foo.o' goes into output section `outputb', followed by section `.input1' from `foo1.o'. All of the remaining `.input1' and `.input2' sections from any files are written to output section `outputc'. SECTIONS { outputa 0x10000 : { all.o foo.o (.input1) } outputb : { foo.o (.input2) foo1.o (.input1) } outputc : { *(.input1) *(.input2) } }  File: ld.info, Node: Section Data Expressions, Next: Section Options, Prev: Section Placement, Up: SECTIONS Section Data Expressions ------------------------ The foregoing statements arrange, in your output file, data originating from your input files. You can also place data directly in an output section from the link command script. Most of these additional statements involve expressions; *note Expressions::.. Although these statements are shown separately here for ease of presentation, no such segregation is needed within a section definition in the `SECTIONS' command; you can intermix them freely with any of the statements we've just described. `CREATE_OBJECT_SYMBOLS' Create a symbol for each input file in the current section, set to the address of the first byte of data written from that input file. For instance, with `a.out' files it is conventional to have a symbol for each input file. You can accomplish this by defining the output `.text' section as follows: SECTIONS { .text 0x2020 : { CREATE_OBJECT_SYMBOLS *(.text) _etext = ALIGN(0x2000); } ... } If `sample.ld' is a file containing this script, and `a.o', `b.o', `c.o', and `d.o' are four input files with contents like the following-- /* a.c */ afunction() { } int adata=1; int abss; `ld -M -T sample.ld a.o b.o c.o d.o' would create a map like this, containing symbols matching the object file names: 00000000 A __DYNAMIC 00004020 B _abss 00004000 D _adata 00002020 T _afunction 00004024 B _bbss 00004008 D _bdata 00002038 T _bfunction 00004028 B _cbss 00004010 D _cdata 00002050 T _cfunction 0000402c B _dbss 00004018 D _ddata 00002068 T _dfunction 00004020 D _edata 00004030 B _end 00004000 T _etext 00002020 t a.o 00002038 t b.o 00002050 t c.o 00002068 t d.o `SYMBOL = EXPRESSION ;' `SYMBOL F= EXPRESSION ;' SYMBOL is any symbol name (*note Symbols::.). "F=" refers to any of the operators `&= += -= *= /=' which combine arithmetic and assignment. When you assign a value to a symbol within a particular section definition, the value is relative to the beginning of the section (*note Assignment::.). If you write SECTIONS { abs = 14 ; ... .data : { ... rel = 14 ; ... } abs2 = 14 + ADDR(.data); ... } `abs' and `rel' do not have the same value; `rel' has the same value as `abs2'. `BYTE(EXPRESSION)' `SHORT(EXPRESSION)' `LONG(EXPRESSION)' `QUAD(EXPRESSION)' By including one of these four statements in a section definition, you can explicitly place one, two, four, or eight bytes (respectively) at the current address of that section. `QUAD' is only supported when using a 64 bit host or target. Multiple-byte quantities are represented in whatever byte order is appropriate for the output file format (*note BFD::.). `FILL(EXPRESSION)' Specify the "fill pattern" for the current section. Any otherwise unspecified regions of memory within the section (for example, regions you skip over by assigning a new value to the location counter `.') are filled with the two least significant bytes from the EXPRESSION argument. A `FILL' statement covers memory locations *after* the point it occurs in the section definition; by including more than one `FILL' statement, you can have different fill patterns in different parts of an output section. .