Linker scripts    
A linker script controls every link. Such a script is written in the linker command language. The main purpose of the linker script is to describe how the sections in the input files should be mapped into the output file, and to control the memory layout of the output file. However, when necessary, the linker script can also direct the linker to perform many other operations, using the linker commands. The following documentation discusses using the linker script and its commands.

Basic linker script concepts

The linker always uses a linker script. If you do not supply one yourself, the linker will use a default script that is compiled into the linker executable. You can use the  --verbose command line option to display the default linker script. Certain command line options, such as  -r or -N, will affect the default linker script. You may supply your own linker script by using the -T command line option. When you do this, your linker script will replace the default linker script.

You may also use linker scripts implicitly by naming them as input files to the linker, as though they were files to be linked. If the linker opens a file, which it can not recognize as an object file or as an archive file, it will try to read it as a linker script. If the file can not be parsed as a linker script, the linker will report an error. An implicit linker script will not replace the default linker script. Typically an implicit linker script would contain only the INPUT, GROUP, or VERSION commands.
We need to define some basic concepts and vocabulary in order to describe the linker script language. 
The linker combines input files into a single output file. The output file and each input file are in a special data format known as an object file format . Each file is called an object file . The output file is often called an executable , but for our purposes we will also call it an object file. Each object file has, among other things, a list of sections. We sometimes refer to a section in an input file as an input section ; similarly, a section in the output file is an output section .
Each section in an object file has a name and a size. Most sections also have an associated block of data, known as the section contents . A section may be marked as loadable , meaning that the contents should be loaded into memory when the output file is run. A section with no contents may be allocatable , which means that an area in memory should be set aside, but nothing in particular should be loaded there (in some cases this memory must be zeroed out). A section, which is neither loadable nor allocatable, typically contains some sort of debugging information.
Every loadable or allocatable output section has two addresses. The first is the VMA , or virtual memory address . This is the address the section will have when the output file is run. The second is the LMA , or load memory address . This is the address at which the section will be loaded. In most cases the two addresses will be the same. An example of when they might be different is when a data section is loaded into ROM, and then copied into RAM when the program starts up (this technique is often used to initialize global variables in a ROM based system). In this case the ROM address would be the LMA, and the RAM address would be the VMA.
You can see the sections in an object file by using the objdump program with the -h option.
Every object file also has a list of symbols , known as the symbol table . A symbol may be defined or undefined. Each symbol has a name, and each defined symbol has an address, among other information. If you compile a C or C++ program into an object file, you will get a defined symbol for every defined function and global or static variable. Every undefined function or global variable, which is referenced in the input file, will become an undefined symbol. You can see the symbols in an object file by using the nm program, or by using the objdump  program with the -t option.


Linker script format

 Linker scripts are text files. You write a linker script as a series of commands. Each command is either a keyword, possibly followed by arguments or an assignment to a symbol. You may separate commands using semicolons. Whitespace is generally ignored. 

Strings such as file or format names can normally be entered directly. If the file name contains a character such as a comma, which would otherwise serve to separate file names, you may put the file name in double quotes. There is no way to use a double quote character in a file name.
You may include comments in linker scripts just as in C, delimited by /* and */. As in C, comments are syntactically equivalent to whitespace. 


Simple linker script example

 Many linker scripts are fairly simple. The simplest possible linker script has just one command: SECTIONS. You use the SECTIONS command to describe the memory layout of the output file. The SECTIONS command is a powerful command. Here we will describe a simple use of it. Lets assume your program consists only of code, initialized data, and uninitialized data. These will be in the .text , .data, and .bss sections, respectively. Lets assume further that these are the only sections, which appear in your input files.

For this example, lets say that the code should be loaded at address 0x10000 , and that the data should start at address 0x8000000. The following linker script will do this function.
You write the SECTIONS command as the keyword SECTIONS, followed by a series of symbol assignments and output section descriptions enclosed in curly braces. The first line in the above example sets the special symbol ., which is the location counter. If you do not specify the address of an output section in some other way (other ways are described later), the address is set from the current value of the location counter. The location counter is then incremented by the size of the output section. The second line defines an output section, .text. The colon is required syntax, which may be ignored for now. Within the curly braces after the output section name, you list the names of the input sections, which should be placed into this output section. The * is a wildcard which matches any file name. The expression *(.text)  means all .text input sections in all input files.
Since the location counter is 0x10000 when the output section .text is defined, the linker will set the address of the .text  section in the output file to be 0x10000. The remaining lines define the .data and .bss sections in the output file. The .data output section will be at address 0x8000000 . When the .bss output section is defined, the value of the location counter will be 0x8000000 plus the size of the ` .data output section. The effect is that the .bss  output section will follow immediately after the .data  output section in memory.
Thats it! Thats a simple and complete linker script.


Simple linker script commands

 In the following documentation, the discussion describes the simple linker script commands. See also Command line options for ld and BFD.


Setting the entry point

The first instruction to execute in a program is called the entry point. You can use the ENTRY linker script command to set the entry point. The argument is a symbol name:
There are several ways to set the entry point. The linker will set the entry point by trying each of the following methods in order, and stopping when one of them succeeds:
Commands dealing with files

 Several linker script commands deal with files. See also Command line options for ld and BFD.

Commands dealing with object file formats

A couple of linker script commands deal with object file formats. See also Command line options for ld and BFD.

Other linker script commands

 The following options are a few other linker scripts commands. See also Command line options for ld and BFD.


Assigning values to symbols

 You may assign a value to a symbol in a linker script. This will define the symbol as a global symbol. 

Simple assignments

You may assign to a symbol using any of the C assignment operators: 

In the previous example, the floating_point symbol will be defined as zero. The _etext symbol will be defined as the address following the last .text input section. The _bdata symbol will be defined as the address following the .text output section aligned upward to a 4 byte boundary.


PROVIDE command

In some cases, it is desirable for a linker script to define a symbol only if it is referenced and is not defined by any object included in the link. For example, traditional linkers defined the symbol, etext. However, ANSI C requires that the user be able to use etext as a function name without encountering an error. The PROVIDE  keyword may be used to define a symbol, such as etext, only if it is referenced but not defined.

The syntax is PROVIDE( symbol = expression).

The following example shows how to use PROVIDE to define etext.
In the previous example, if the program defines _etext, the linker will give a multiple definition error. If, on the other hand, the program defines etext, the linker will silently use the definition in the program. If the program references etext but does not define it, the linker will use the definition in the linker script.

SECTIONS command

The SECTIONS  command tells the linker how to map input sections into output sections, and how to place the output sections in memory. The format of the SECTIONS command is:

Each sections - command may of be one of the following:
The ENTRY command and symbol assignments are permitted inside the SECTIONS command for convenience in using the location counter in those commands. This can also make the linker script easier to understand because you can use those commands at meaningful points in the layout of the output file. See Output section description and Overlay description.
If you do not use a SECTIONS command in your linker script, the linker will 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. The first section will be at address-zero.
Output section description
The full description of an output section looks like the following statement. 
Most output sections do not use most of the optional section attributes. The whitespace around SECTION is required, so that the section name is unambiguous. The colon and the curly braces are also required. The line breaks and other white space are optional. The expression, LMA, that follows the AT keyword specifies the load address of the section. This feature is designed to make it easy to build a ROM image; see also Output section LMA.
Each output-sections-command may be one of the following:
Output section name
The name of the output section is section. section 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 a name, which contains any unusual characters such as commas, must be quoted. The output section name /DISCARD/ is special. See Output section discarding.
Output section address

The address is an expression for the VMA (the virtual memory address) of the output section. If you do not provide address, the linker will set it based on REGION if present, or otherwise based on the current value of the location counter.

If you provide address, the address of the output section will be set to precisely that specification. If you provide neither address nor region, then the address of the output section will be set to the current value of the location counter aligned to the alignment requirements of the output section. The alignment requirement of the output section is the strictest alignment of any input section contained within the output section. 
For example,  the following two declarations are subtly different.
The first will set the address of the .text output section to the current value of the location counter. The second will set it to the current value of the location counter aligned to the strictest alignment of a .text input section.
The address may be an arbitrary expression. See Expressions in linker scripts. For example, if you want to align the section on a 0x10 byte boundary, so that the lowest four bits of the section address are zero, you could use something like the following declaration. 
This works because ALIGN returns the current location counter aligned upward to the specified value.
Specifying address for a section will change the value of the location counter.
Input section description
The most common output section command is an input section description . The input section description is the most basic linker script operation. You use output sections to tell the linker how to lay out your program in memory. You use input section descriptions to tell the linker how to map the input files into your memory layout.
Input section basics
An input section description consists of a file name optionally followed by a list of section names in parentheses. The file name and the section name may be wildcard patterns, which we describe; see Input section wildcard patterns. The most common input section description is to include all input sections with a particular name in the output section. For example, to include all input .text sections, you would write:
Here the * is a wildcard which matches any file name.
There are two ways to include more than one section:
The difference between these is the order in which the .text and .rdata input sections will appear in the output section. In the first example, they will be intermingled. In the second example, all .text input sections will appear first, followed by all .rdata input sections.
You can specify a file name to include sections from a particular file. You would do this if one or more of your files contain special data that needs to be at a particular location in memory. For example, use the following input.
If you use a file name without a list of sections, then all sections in the input file will be included in the output section. This is not commonly done, but it may by useful on occasion. For example, use the following input.
When you use a file name, which does not contain any wild card characters, the linker will first see if you also specified the file name on the linker command line or in an INPUT command. If you did not, the linker will attempt to open the file as an input file, as though it appeared on the command line. This differs from an INPUT command, because the linker will not search for the file in the archive search path.
Input section wildcard patterns
In an input section description, either the file name or the section name or both may be wildcard patterns. The file name of * seen in many examples is a simple wildcard pattern for the file name. The wildcard patterns are like those used by the Unix shell.
When a file name is matched with a wildcard, the wildcard characters will not match a / character (used to separate directory names on Unix). A pattern consisting of a single * character is an exception; it will always match any file name, whether it contains a / or not. In a section name, the wildcard characters will match a / character.

File name wildcard patterns only match files which are explicitly specified on the command line or in an INPUT command. The linker does not search directories to expand wildcards.

If a file name matches more than one wildcard pattern, or if a file name appears explicitly and is also matched by a wildcard pattern, the linker will use the first match in the linker script. For example, the following input uses a sequence of input section descriptions that is probably in error, because the data.o rule will not be used.

If you ever get confused about where input sections are going, use the -M linker option to generate a map file. The map file shows precisely how input sections are mapped to output sections.
This example shows how wildcard patterns might be used to partition files. This linker script directs the linker to place all .text sections in .text and all .bss sections in .bss. The linker will place the .data section from all files beginning with an upper case character in .DATA; for all other files, the linker will place the .data section in .data.
Input section for common symbols

 A special notation is needed for common symbols because, in many object file formats, common symbols do not have a particular input section. The linker treats common symbols as though they are in an input section named COMMON.

You may use file names with the COMMON section just as with any other input sections. You can use this to place common symbols from a particular input file in one section while common symbols from other input files are placed in another section.
In most cases, common symbols in input files will be placed in the .bss section in the output file. For example, use the following input.
Some object file formats have more than one type of common symbol. For example, the MIPS ELF object file format distinguishes standard common symbols and small common symbols. In this case, the linker will use a different special section name for other types of common symbols. In the case of MIPS ELF, the linker uses COMMON for standard common symbols and .scommon for small common symbols. This permits you to map the different types of common symbols into memory at different locations.
You will sometimes see [COMMON] in old linker scripts. This notation is now considered obsolete. It is equivalent to *(COMMON).
Input section example

The following example is a complete linker script that tells the linker to read all of the sections from file, all.o, and place them at the start of output section, outputa, which starts at a 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 .

Output section data
You can include explicit bytes of data in an output section by using BYTE, SHORT, LONG, QUAD, or SQUAD as an output section command. Each keyword is followed by an expression in parentheses providing the value to store; see Expressions in linker scripts. The value of the expression is stored at the current value of the location counter.
The BYTE , SHORT, LONG, and QUAD commands store one, two, four, and eight bytes (respectively). After storing the bytes, the location counter is incremented by the number of bytes stored. For example, this will store the byte 1 followed by the four byte value of the symbol, addr:
When using a 64-bit host or target, QUAD and SQUAD are the same; they both store an 8-byte, or 64-bit, value. When both host and target are 32 bits, an expression is computed as 32 bits. In this case QUAD stores a 32-bit value zero extended to 64 bits, and SQUAD stores a 32-bit value sign extended to 64 bits.
If the object file format of the output file has an explicit endianness, which is the normal case, the value will be stored in that endianness. When the object file format does not have an explicit endianness, as is true of, for example, S-records, the value will be stored in the endianness of the first input object file. 
You may use the FILL command to set the fill pattern for the current section. It is followed by an expression in parentheses. Any otherwise unspecified regions of memory within the section (for example, gaps left due to the required alignment of input sections) are filled with the two least significant bytes of the expression, repeated as necessary. A FILL statement covers memory locations after the point at which 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.
This example shows how to fill unspecified regions of memory with the value, 0x9090 :
The FILL command is similar to the =fillexp output section attribute (see Output section fill); but it only affects the part of the section following the FILL command, rather than the entire section. If both are used, the FILL command takes precedence.
Output section keywords
There are a couple of keywords, which can appear as output section commands. 
Output section discarding

The linker will not create output section which do not have any contents. This is for convenience when referring to input sections that may or may not be present in any of the input files. For example, the following input will only create a .foo section in the output file if there is a .foo section in at least one input file.

If you use anything other than an input section description as an output section command, such as a symbol assignment, then the output section will always be created, even if there are no matching input sections.
The special output section name, /DISCARD/ may be used to discard input sections. Any input sections which are assigned to an output section named /DISCARD/ are not included in the output file.
Output section attributes

We showed with Output section description that the full description of an output section looked like the following input.

We’ve already described section, address, and output-sections-command. In the following discussion, we will describe the remaining section attributes. 
Output section type
Each output section may have a type. The type is a keyword in parentheses. The following types are defined:
The linker normally sets the attributes of an output section, based on the input sections, which map into it. You can override this by using the section type. For example, in the script sample below, the ROM section is addressed at memory location 0 and does not need to be loaded when the program is run. The contents of the ROM section will appear in the linker output file as usual.
Output section LMA

Every section has a virtual address (VMA) and a load address (LMA); see Basic linker script concepts. The address expression that, may appear in an output section description sets the VMA. The linker will normally set the LMA equal to the VMA. You can change that by using the AT keyword. The expression, LMA, that follows the AT keyword specifies the load address of the section. This feature is designed to make it easy to build a ROM image. For example, the following linker script creates three output sections: one called .text, which starts at 0x1000, one called .mdata, which is loaded at the end of the .textsection even though its VMA is 0x2000, and one called .bssto hold uninitialized data at address, 0x3000. The symbol _data is defined with the value, 0x2000, which shows that the location counter holds the VMA value, not the LMA value.

The run-time initialization code for use with a program generated with this linker script would include something like the following, to copy the initialized data from the ROM image to its runtime address. Notice how this code takes advantage of the symbols defined by the linker script.
Output section region 
You can assign a section to a previously defined region of memory by using >region, where region is the defined region for the section. The following example shows usage.
Output section phdr 
You can assign a section to a previously defined program segment by using :phdr. If a section is assigned to one or more segments, then all subsequent allocated sections will be assigned to those segments as well, unless they use an explicitly :phdr modifier. To prevent a section from being assigned to a segment when it would normally default to one, use :NONE. See PHDRS command.
The following example shows usage.
Output section fill 
You can set the fill pattern for an entire section by using =fillexp. fillexp is an expression; see Expressions in linker scripts. Any otherwise unspecified regions of memory within the output section (for example, gaps left due to the required alignment of input sections) 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 command in the output section commands. See Output section data.
Here is a simple example:
Overlay description

An overlay description provides an easy way to describe sections, which are to be loaded as part of a single memory image but are to be run at the same memory address. At run time, some sort of overlay manager will copy the overlaid sections in and out of the runtime memory address as required, perhaps by simply manipulating addressing bits. This approach can be useful, for example, when a certain region of memory is faster than another region of memory. 

Overlays are described using the OVERLAY command. The OVERLAY command is used within a SECTIONS command, like an output section description. The full syntax of the OVERLAY command is as follows:
Everything is optional except OVERLAY  (a keyword), and each section must have a name (secname1 and secname2 in the previous examples input). The section definitions within the OVERLAY construct are identical to those within the general SECTIONS  construct, except that no addresses and no memory regions may be defined for sections within an OVERLAY. See SECTIONS command.
The sections are all defined with the same starting address. The load addresses of the sections are arranged, so that they are consecutive in memory, starting at the load address used for the OVERLAY as a whole (as with normal section definitions. The load address is optional, and defaults to the start address. The start address is also optional, and defaults to the current value of the location counter).
If the NOCROSSREFS keyword is used, and there any references among the sections, the linker will report an error. Since the sections all run at the same address, it normally does not make sense for one section to refer directly to another.
For each section within the OVERLAY, the linker automatically defines two symbols. The symbol, __load_start_ secname, is defined as the starting load address of the section. The symbol, __load_stop_ secname, is defined as the final load address of the section. Any characters within secname that are not legal within C identifiers are removed. C (or assembler) code may use these symbols to move the overlaid sections around as necessary.
At the end of the overlay, the value of the location counter is set to the start address of the overlay plus the size of the largest section.
Here is an example. Remember that this would appear inside a SECTIONS construct.
This will define both .text0 and .text1 to start at address 0x1000. .text0 will be loaded at address 0x4000, and .text1will be loaded immediately after .text0. The following symbols will be defined: __load_start_text0, __load_stop_text0 , __load_start_text1, and __load_stop_text1.
C code to copy overlay .text1 into the overlay area might look like the following example input.
Note that the OVERLAY command is just syntactic sugar, since everything it does can be done using the more basic commands. The previous example could have been input identically as follows.
MEMORY command

The linkers default configuration permits allocation of all available memory. You can override this by using the MEMORY  command.

The MEMORY command describes the location and size of blocks of memory in the target. You can use it to describe which memory regions may be used by the linker, and which memory regions it must avoid. You can then assign sections to particular memory regions. The linker will set section addresses based on the memory regions, and will warn about regions that become too full. The linker will not shuffle sections around to fit into the available regions.
A linker script 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:
The name is a name used in the linker script to refer to the region. The region name has no meaning outside of the linker script. Region names are stored in a separate name space, and will not conflict with symbol names, file names, or section names. Each memory region must have a distinct name.
The attr string is an optional list of attributes that specify whether to use a particular memory region for an input section, which is not explicitly mapped in the linker script. If you do not specify an output section for some input section, the linker will create an output section with the same name as the input section. If you define region attributes, the linker will use them to select the memory region for the output section that it creates. See SECTIONS command.
The attr string must consist only of the following characters:
If an unmapped section matches any of the listed attributes other than !, it will be placed in the memory region. The ! attribute reverses this test, so that an unmapped section will be placed in the memory region only if it does not match any of the listed attributes.
The ORIGIN is an expression for the start address of the memory region. The expression must evaluate to a constant before memory allocation is performed, which means that you may not use any section relative symbols. The ORIGIN keyword may be abbreviated to org or o (but not, for example, to ORG).
The len is an expression for the size in bytes of the memory region. As with the origin expression, the expression must evaluate to a constant before memory allocation is performed. The LENGTH keyword may be abbreviated to len or l.
In the following example, we specify that there are two memory regions available for allocation: one starting at 0 for 256 kilobytes, and the other starting at 0x40000000 for four megabytes. The linker will place into the rom memory region every section, which is not explicitly mapped into a memory region, and is either read-only or executable. The linker will place other sections, which are not explicitly mapped into a memory region into the ram memory region.
If you have defined a memory region named mem, you can direct the linker to place specific output sections into that memory region by using the >region output section attribute. If no address was specified for the output section, the linker will set the address to the next available address within the memory region. If the combined output sections directed to a memory region are too large for the region, the linker will issue an error message. See Output section region.


PHDRS command

The ELF object file format uses program headers, also knows as segments . The program headers describe how the program should be loaded into memory. You can print them out by using the objdump program with the -p option.

When you run an ELF program on a native ELF system, the system loader reads the program headers in order to figure out how to load the program. This will only work if the program headers are set correctly. This documentation does not describe the details of how the system loader interprets program headers; for more information, see the ELF ABI.
The linker will create reasonable program headers by default. However, in some cases, you may need to specify the program headers more precisely. You may use the PHDRS command for this purpose. When the linker sees the PHDRS command in the linker script, it will not create any program headers other than the ones specified.
The linker only pays attention to the PHDRS command when generating an ELF output file. In other cases, the linker will simply ignore PHDRS.
This is the syntax of the PHDRS command. The words, PHDRS, FILEHDR, AT, and FLAGS, are keywords.
The name is used only for reference in the SECTIONS command of the linker script. It is not put into the output file. Program header names are stored in a separate name space, and will not conflict with symbol names, file names, or section names. Each program header must have a distinct name.
Certain program header types describe segments of memory, which the system loader will load from the file. In the linker script, you specify the contents of these segments by placing allocatable output sections in the segments. You use the : phdr output section attribute to place a section in a particular segment. See Output section phdr.
It is normal to put certain sections in more than one segment. This merely implies that one segment of memory contains another. You may repeat : phdr, using it once for each segment which should contain the section.
If you place a section in one or more segments using : phdr, then the linker will place all subsequent allocatable sections which do not specify : phdr in the same segments. This is for convenience, since generally a whole set of contiguous sections will be placed in a single segment. To prevent a section from being assigned to a segment when it would normally default to one, use :NONE.
You may use the FILEHDR and PHDRS keywords appear after the program header type to further describe the contents of the segment. The FILEHDR keyword means that the segment should include the ELF file header. The PHDRS keyword means that the segment should include the ELF program headers themselves.
The type may be one of the following. The numbers indicate the value of the keyword.
You can specify that a segment should be loaded at a particular address in memory by using an AT expression. This is identical to the AT command used as an output section attribute. The AT command for a program header, overrides the output section attribute. See Output section, LMA.
The linker will normally set the segment flags based on the sections, which comprise the segment. You may use the FLAGS keyword to specify the segment flags, explicitly. The value of flags must be an integer. It is used to set the p_flags field of the program header.
The following example shows the script for PHDRS. This shows a typical set of program headers used on a native ELF system. 
VERSION command

The linker supports symbol versions when using ELF. Symbol versions are only useful when using shared libraries. The dynamic linker can use symbol versions to select a specific version of a function when it runs a program that may have been linked against an earlier version of the shared library.

You can include a version script directly in the main linker script, or you can supply the version script as an implicit linker script. You can also use the --version-script  linker option.
The syntax of the VERSION command is a simple declaration like the following example.
The format of the version script commands is identical to that used by Suns linker in Solaris 2.5. The version script defines a tree of version nodes. You specify the node names and interdependencies in the version script. You can specify which symbols are bound to which version nodes, and you can reduce a specified set of symbols to local scope so that they are not globally visible outside of the shared library.
The easiest way to demonstrate the version script language is with a few examples.
This example version script defines three version nodes. The first version node defined is VERS_1.1; it has no other dependencies. The script binds the symbol, foo1, to VERS_1.1. It reduces a number of symbols to local scope so that they are not visible outside of the shared library.
Next, the version script defines node, VERS_1.2. This node depends upon VERS_1.1 . The script binds the symbol, foo2, to the version node, VERS_1.2.
Finally, the version script defines node, VERS_2.0. This node depends upon VERS_1.2. The script binds the symbols, bar1 and bar2, to the version node, VERS_2.0.
When the linker finds a symbol defined in a library, which is not specifically bound to a version node, it will effectively bind it to an unspecified base version of the library. You can bind all otherwise unspecified symbols to a given version node by using global: * somewhere in the version script.
The names of the version nodes have no specific meaning other than what they might suggest to the person reading them. The 2.0 version could just as well have appeared in between 1.1 and 1.2. However, this would be a confusing way to write a version script.
When you link an application against a shared library that has versioned symbols, the application itself knows which version of each symbol it requires, and it also knows which version nodes it needs from each shared library it is linked against. Thus at runtime, the dynamic loader can make a quick check to make sure that the libraries you have linked against do in fact supply all of the version nodes that the application will need to resolve all of the dynamic symbols. In this way it is possible for the dynamic linker to know with certainty that all external symbols that it needs will be resolvable without having to search for each symbol reference.
The symbol versioning is in effect a much more sophisticated way of doing minor version checking that SunOS does. The fundamental problem that is being addressed here is that typically references to external functions are bound on an as-needed basis, and are not all bound when the application starts up. If a shared library is out of date, a required interface may be missing; when the application tries to use that interface, it may suddenly and unexpectedly fail. With symbol versioning, the user will get a warning when they start their program if the libraries being used with the application are too old.
There are several GNU extensions to Suns versioning approach. The first of these is the ability to bind a symbol to a version node in the source file where the symbol is defined instead of in the versioning script. This was done mainly to reduce the burden on the library maintainer. You can do this by putting something like this in the C source file:
This renames the function, original_foo, to be an alias for foo bound to the version node VERS_1.1. The local: directive can be used to prevent the symbol, original_foo, from being exported.
The second GNU extension is to allow multiple versions of the same function to appear in a given, shared library. In this way you can make an incompatible change to an interface without increasing the major version number of the shared library, while still allowing applications linked against the old interface to continue to function.
To do this, you must use multiple .symver directives in the source file. The following example shows the usage.
In this example, foo@ represents the symbol, foo, bound to the unspecified base version of the symbol. The source file that contains this example would define four C functions: original_foo, old_foo , old_foo1, and new_foo.
When you have multiple definitions of a given symbol, there needs to be some way to specify a default version to which external references to this symbol will be bound. You can do this with the foo@@VERS_2.0 type of .symver directive. You can only declare one version of a symbol as the default in this manner; otherwise you would effectively have multiple definitions of the same symbol.
If you wish to bind a reference to a specific version of the symbol within the shared library, you can use the aliases of convenience (i.e., old_foo ), or you can use the .symver directive to bind specifically to an external version of the function in question.


Expressions in linker scripts

The syntax for expressions in the linker script language is identical to that of C expressions. All expressions are evaluated as integers. All expressions are evaluated in the same size, which is 32 bits if both the host and target are 32 bits, and is otherwise 64 bits. You can use and set symbol values in expressions. The linker defines several special purpose builtin functions for use in expressions.



All constants are integers. As in C, the linker considers an integer beginning with 0 to be octal, and an integer beginning with 0x or 0X to be hexadecimal.

The linker considers other integers to be decimal.
In addition, you can use the suffixes K and M to scale a constant by 1024, or 1024*1024, respectively. For example, the following example declarations all refer to the same quantity.
Symbol names

Unless quoted, symbol names start with a letter, underscore, or period and may include letters, digits, underscores, periods, 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. 

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.


The location counter

The special linker dot (.) variable always contains the current output location counter. Since the . symbol always refers to a location in an output section, it may only appear in an expression within a SECTIONS command. The . symbol may appear anywhere that an ordinary symbol is allowed in an expression.

Assigning a value to . 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.
In the previous example, the .text section from file1 is located at the beginning of the output section, output. It is followed by a 1000 byte gap. Then the .text section from file2 appears, also with a 1000 byte gap following before the .text section from file3. The notation, = 0x1234, specifies data to write in the gaps.



The linker recognizes the standard C set of arithmetic operators, with the standard bindings and precedence levels; see Table 1: Arithmetic operators with precedence levels and bindings associations

Table 1: Arithmetic operators with precedence levels and bindings associations
! - ~ 
* / % 
+ - 
>> << 
== != > < <= >= 
? : 
&= += -= *= /= 

1. Prefix operators 

The linker evaluates expressions lazily. It only computes the value of an expression when absolutely necessary. 

The linker needs some information, such as the value of the start address of the first section, and the origins and 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 linker script. 
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. 
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 ., must be evaluated during section 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, will cause the error message, non constant expression for initial address:
The section of an expression

When the linker evaluates an expression, the result is either absolute or relative to some section. A relative expression is expressed as a fixed offset from the base of a section. 

The position of the expression within the linker script determines whether it is absolute or relative. An expression, which appears within an output section definition, is relative to the base of the output section. An expression, which appears elsewhere, will be absolute.
A symbol set to a relative expression will be relocatable if you request relocatable output using the -r option. That means that a further link operation may change the value of the symbol. The symbols section will be the section of the relative expression.
A symbol set to an absolute expression will retain the same value through any further link operation. The symbol will be absolute, and will not have any particular associated section.
You can use the builtin function ABSOLUTE to force an expression to be absolute when it would otherwise be relative. For example, to create an absolute symbol set to the address of the end of the output section .data :
If ABSOLUTE were not used, _edata would be relative to the .data section.


Builtin functions

The linker script language includes a number of builtin functions for use in linker script expressions. 

ADDR( section )
MAX(exp1, exp2)
MIN(exp1, exp2)
SIZEOF( section )