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GNU Info File | 1993-09-06 | 49KB | 1,006 lines

This is Info file ld.info, produced by Makeinfo-1.52 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 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 * 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 a list of formats supported on various architectures. 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. 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 ] [ -i ] [ -lAR ] [ -LSEARCHDIR ] [ -M | -m ] [ -n | -N ] [ -noinhibit-exec ] [ -R FILENAME ] [ -relax ] [ -r | -Ur ] [ -S ] [ -s ] [ -T COMMANDFILE ] [ -Ttext TEXTORG ] [ -Tdata DATAORG ] [ -Tbss BSSORG ] [ -t ] [ -u SYM] [-v] [ -X ] [ -x ] [ -ySYMBOL ] [ { SCRIPT } ] 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 other forms of binary input files can also be specified with `-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'. Option arguments must either follow the option letter without intervening whitespace, or be given as separate arguments immediately following the option that requires them. `OBJFILE...' The object files to be linked. `-b INPUT-FORMAT' Specify the binary format for input object files that follow this option on the command line. You don't usually need to specify this, as `ld' is 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. `-format INPUT-FORMAT' has the same effect. *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'. You can also define the input format from a script, using the command `TARGET'; see *Note Other 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. `-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 Other 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. `-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, 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'. Ignored. Provided for compatibility with other tools. Perform an incremental link (same as option `-r'). `-lAR' Add archive file AR 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 AR specified. `-LSEARCHDIR' Add path SEARCHDIR to the list of paths that `ld' will search for archive libraries. You may use this option any number of times. The paths can also be specified in a link script with the `SEARCH_DIR' command. Print (to the standard output) a link map--diagnostic information about where symbols are mapped by `ld', and information on global common storage allocation. 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'. 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. `-R FILENAME' On some platforms, this 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. `-relax' An option with machine dependent effects. Currently this option is only supported on the H8/300. On some platforms, use 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. 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 as `-i'. Omit debugger symbol information (but not all symbols) from the output file. Omit all symbol information from the output file. `{ SCRIPT }' You can, if you wish, include a script of linker commands directly in the command line instead of referring to it via an input file. When the character `{' occurs on the command line, the linker switches to interpreting the command language until the end of the list of commands is reached; the end is indicated with a closing brace `}'. `ld' does not recognize other command-line options while parsing the script. *Note Commands::, for a description of the command language. `-Tbss BSSORG' `-Tdata DATAORG' `-Ttext TEXTORG' 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 completely override `ld''s default link format (rather than adding to it); COMMANDFILE must specify everything necessary to describe the target format. *Note Commands::. You may also include a script of link commands directly in the command line by bracketing it between `{' and `}'. Print the names of the input files as `ld' processes them. `-u SYM' Force SYM 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' *will* resolve references to constructors, unlike `-r'. Display the version number for `ld'. If `-s' or `-S' is also specified, delete only local symbols beginning with `L'. If `-s' or `-S' is also specified, delete all local symbols, not just those beginning with `L'. `-ySYMBOL' 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: Commands, Next: BFD, 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 format * 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 tries to interpret the file as a command file. You can also include a script directly on the `ld' command line, delimited by the characters `{' and `}'. * Menu: * Scripts:: Linker Scripts * Expressions:: Expressions * MEMORY:: MEMORY Command * SECTIONS:: SECTIONS Command * Entry Point:: The Entry Point * Other Commands:: Other 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 * Built-ins:: 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, point or hyphen and may include any letters, underscores, digits, points, and minus signs. 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; 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: Built-ins, 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: Built-ins, Prev: Assignment, Up: Expressions Built-In 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 Command ============== 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. The command files 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'. `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 SECTIONS Command ================ The `SECTIONS' command controls exactly where input sections are placed into output sections, their order and to which output sections they are allocated. You may use at most one `SECTIONS' command in a commands 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 what input sections make it up. The first two possibilities--defining the entry point, and defining symbols--can also be done outside the `SECTIONS' command: *note Entry Point::., *note Assignment::.. They are permitted here as well for your convenience in reading the script, so that symbols or the entry point can be defined at meaningful points in your output-file layout. When no `SECTIONS' command is specified, the default action of the linker is to place 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 Contents:: Section Contents * Section Options:: Optional Section Attributes File: ld.info, Node: Section Definition, Next: Section Contents, Up: SECTIONS Section Definitions ------------------- The most frequently used statement in the `SECTIONS' command is the "section definition", which you can use to specify 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. 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 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 Contents, Next: Section Options, Prev: Section Definition, Up: SECTIONS Section Contents ---------------- In a section definition, you can specify the contents of an output section by listing particular object 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. 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. 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. `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. 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") } } 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. `[ SECTION ]' `[ SECTION, SECTION, ... ]' `[ SECTION SECTION ... ]' This is an alternate notation to specify named sections from all unallocated input files; its effect is exactly the same as that of `* (SECTION...)' `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) } } There are still more kinds of statements permitted in the contents of output section definitions. The foregoing statements permitted you to 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 the 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 `objsym' 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 sample 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)' By including one of these three statements in a section definition, you can explicitly place one, two, or four bytes (respectively) at the current address of that section. Multiple-byte quantities are represented in whatever byte order is appropriate for the output file format (*note BFD::.). `FILL(EXPRESSION)' Specifies 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. File: ld.info, Node: Section Options, Prev: Section Contents, Up: SECTIONS Optional Section Attributes --------------------------- Here is the full syntax of a section definition, including all the optional portions: SECTIONS { ... SECNAME START BLOCK(ALIGN) (NOLOAD) : { CONTENTS } =FILL >REGION ... } SECNAME and CONTENTS are required. *Note Section Definition::, and *note Section Contents::. for details on CONTENTS. The remaining elements--START, `BLOCK(ALIGN)', `(NOLOAD)' `=FILL', and `>REGION'--are all optional. `START' You can force the output section to be loaded at a specified address by specifying START immediately following the section name. sTART can be represented as any expression. The following example generates section OUTPUT at location `0x40000000': SECTIONS { ... output 0x40000000: { ... } ... } `BLOCK(ALIGN)' You can include `BLOCK()' specification to advance the location counter `.' prior to the beginning of the section, so that the section will begin at the specified alignment. ALIGN is an expression. `(NOLOAD)' Use `(NOLOAD)' to prevent a section from being loaded into memory each time it is accessed. For example, in the script sample below, the `ROM' segment is addressed at memory location `0' and does not need to be loaded into each object file: SECTIONS { ROM 0 (NOLOAD) : { ... } ... } `=FILL' Including `=FILL' in a section definition specifies the initial fill value for that section. You may use any expression to specify FILL. Any unallocated holes in the current output section when written to the output file will be filled with the two least significant bytes of the value, repeated as necessary. You can also change the fill value with a `FILL' statement in the CONTENTS of a section definition. `>REGION' Assign this section to a previously defined region of memory. *Note MEMORY::. File: ld.info, Node: Entry Point, Next: Other Commands, Prev: SECTIONS, Up: Commands The Entry Point =============== The linker command language includes a command specifically for defining the first executable instruction in an output file (its "entry point"). Its argument is a symbol name: ENTRY(SYMBOL) Like symbol assignments, the `ENTRY' command may be placed either as an independent command in the command file, or among the section definitions within the `SECTIONS' command--whatever makes the most sense for your layout. `ENTRY' is only one of several ways of choosing the entry point. You may indicate it in any of the following ways (shown in descending order of priority: methods higher in the list override methods lower down). * the `-e' ENTRY command-line option; * the `ENTRY(SYMBOL' command in a linker control script; * the value of the symbol `start', if present; * the value of the symbol `_main', if present; * the address of the first byte of the `.text' section, if present; * The address `0'. For example, you can use these rules to generate an entry point with an assignment statement: if no symbol `start' is defined within your input files, you can simply define it, assigning it an appropriate value-- start = 0x2020; The example shows an absolute address, but you can use any expression. For example, if your input object files use some other symbol-name convention for the entry point, you can just assign the value of whatever symbol contains the start address to `start': start = other_symbol ; File: ld.info, Node: Other Commands, Prev: Entry Point, Up: Commands Other Commands ============== The command language includes a number of other commands that you can use for specialized purposes. They are similar in purpose to command-line options. `FLOAT' `NOFLOAT' These keywords were used in some older linkers to request a particular math subroutine library. `ld' doesn't use the keywords, assuming instead that any necessary subroutines are in libraries specified using the general mechanisms for linking to archives; but to permit the use of scripts that were written for the older linkers, the keywords `FLOAT' and `NOFLOAT' are accepted and ignored. `FORCE_COMMON_ALLOCATION' This command has the same effect as the `-d' command-line option: to make `ld' assign space to common symbols even if a relocatable output file is specified (`-r'). `INPUT ( FILE, FILE, ... )' `INPUT ( FILE FILE ... )' Use this command to include binary input files in the link, without including them in a particular section definition. Files specified this way are treated identically to object files listed on the command line. `OUTPUT ( FILENAME )' Use this command to name the link output file FILENAME. The effect of `OUTPUT(FILENAME)' is identical to the effect of `-o FILENAME', and whichever is encountered last will control the name actually used to name the output file. In particular, you can use this command to supply a default output-file name other than `a.out'. `OUTPUT_ARCH ( BFDNAME )' Specify a particular output machine architecture, with one of the names used by the BFD back-end routines (*note BFD::.). This command is often unnecessary; the architecture is most often set implicitly by either the system BFD configuration or as a side effect of the `OUTPUT_FORMAT' command. `OUTPUT_FORMAT ( BFDNAME )' Specify a particular output format, with one of the names used by the BFD back-end routines (*note BFD::.). This selection will only affect the output file; the related command `TARGET' affects primarily input files. `SEARCH_DIR ( PATH )' Add PATH to the list of paths where `ld' looks for archive libraries. `SEARCH_DIR(PATH)' has the same effect as `-LPATH' on the command line. `STARTUP ( FILENAME )' Ensure that FILENAME is the first input file used in the link process. `TARGET ( FORMAT )' Change the input-file object code format (like the command-line option `-b' or its synonym `-format'). The argument FORMAT is one of the strings used by BFD to name binary formats. In the current `ld' implementation, if `TARGET' is specified but `OUTPUT_FORMAT' is not, the last `TARGET' argument is also used as the default format for the `ld' output file. *Note BFD::. If you don't use the `TARGET' command, `ld' uses the value of the environment variable `GNUTARGET', if available, to select the output file format. If that variable is also absent, `ld' uses the default format configured for your machine in the BFD libraries.