Segment Declarations (Linker and Libraries Guide)

Linker and Libraries Guide

Segment Declarations

A segment declaration creates a new segment in the output file, or changes the attribute values of an existing segment. An existing segment is one that you previously defined or one of the four built-in segments described immediately following.

A segment declaration has the following syntax.

        segment_name = {segment_attribute_value}*;

For each segment_name, you can specify any number of segment_attribute_values in any order, each separated by a space. Only one attribute value is allowed for each segment attribute. The segment attributes and their valid values are as shown in the following table.

Table E–1 Mapfile Segment Attributes


Attribute	Value
`segment_type`	`LOAD \| NOTE \| NULL \| STACK`
`segment_flags`	`? [E] [N] [O] [R] [W] [X]`
`virtual_address`	`Vnumber`
`physical_address`	`Pnumber`
`length`	`Lnumber`
`rounding`	`Rnumber`
`alignment`	`Anumber`

Four built-in segments exist with the following default attribute values.

text – LOAD, ?RX, no virtual_address, physical_address, or length specified. alignment values are set to defaults per CPU type.
data – LOAD, ?RWX, no virtual_address, physical_address, or length specified. alignment values are set to defaults per CPU type.
bss – disabled, LOAD, ?RWX, no virtual_address, physical_address, or length specified. alignment values are set to defaults per CPU type.
note – NOTE.

By default, the bss segment is disabled. Any sections of type SHT_NOBITS, which are its sole input, are captured in the data segment. See Table 7–5 for a full description of SHT_NOBITS sections. The simplest bss declaration is sufficient to enable the creation of a bss segment.

        bss =;

Any SHT_NOBITS sections is captured by this segment, rather than captured in the data segment. In its simplest form, this segment is aligned using the same defaults as applied to any other segment. The declaration can also provide additional segment attributes that both enable the segment creation, and assign the specified attributes.

The link-editor behaves as if these segments are declared before your mapfile is read in. See Mapfile Option Defaults.

Note the following when entering segment declarations.

A number can be hexadecimal, decimal, or octal, following the same rules as in the C language.
No space is allowed between the V, P, L, R, or A and the number.
The segment_type value can be either LOAD, NOTE, NULL or STACK. If unspecified, the segment type defaults to LOAD.
The segment_flags values are R for readable, W for writable, X for executable, and O for order. No spaces are allowed between the question mark (?) and the individual flags that make up the segment_flags value.
The segment_flags value for a LOAD segment defaults to RWX.
NOTE segments cannot be assigned any segment attribute value other than a segment_type.
One segment_type of value STACK is permitted. Only the access requirements of the segment, selected from the segment_flags, can be specified.
Implicitly declared segments default to segment_type value LOAD, segment_flags value RWX, a default virtual_address, physical_address, and alignment value, and have no length limit.

Note –
The link-editor calculates the addresses and length of the current segment based on the previous segment's attribute values.
LOAD segments can have an explicitly specified virtual_address value or physical_address value, as well as a maximum segment length value.
If a segment has a segment_flags value of ? with nothing following, the value defaults to not readable, not writable, and not executable.
The alignment value is used in calculating the virtual address of the beginning of the segment. This alignment only affects the segment for which the alignment is specified. Other segments still have the default alignment unless their alignment values are also changed.
If any of the virtual_address, physical_address, or length attribute values are not set, the link-editor calculates these values as the output file is created.
If an alignment value is not specified for a segment, the alignment is set to the built-in default. This default differs from one CPU to another and might even differ between software revisions.
If both a virtual_address and an alignment value are specified for a segment, the virtual_address value takes priority.
If a virtual_address value is specified for a segment, the alignment field in the program header contains the default alignment value.
If the rounding value is set for a segment, that segment's virtual address is rounded to the next address that conforms to the value that is given. This value only effects the segments that the value is specified for. If no value is given, no rounding is performed.

Note –

If a virtual_address value is specified, the segment is placed at that virtual address. For the system kernel, this method creates a correct result. For files that start through exec(2), this method creates an incorrect output file because the segments do not have correct offsets relative to their page boundaries.

The ?E flag allows the creation of an empty segment. This empty segment has no sections associated with the segment. This segment can be a LOAD segment or a NULL segment. Empty LOAD segments can only be specified for executables. These segments must have a specified size and alignment. These segments result in the creation of memory reservations at process startup. Empty NULL segments provide for adding program header entries that can be used by post-processing utilities. These segments should have no additional attributes specified. Multiple definitions for LOAD segments and NULL segments are permitted.

The ?N flag enables you to control whether the ELF header, and any program headers are included as part of the first loadable segment. By default, the ELF header and program headers are included with the first segment. The information in these headers is used within the mapped image, typically by the runtime linker. The use of the ?N option causes the virtual address calculations for the image to start at the first section of the first segment.

The ?O flag enables you control the order of sections in the output file. This flag is intended for use in conjunction with the -xF option to the compilers. When a file is compiled with the -xF option, each function in that file is placed in a separate section with the same attributes as the .text section. These sections are called .text%function_name.

For example, a file containing three functions, main(), foo() and bar(), when compiled with the -xF option, yields a relocatable object file with text for the three functions being placed in sections called .text%main, .text%foo, and .text%bar. Because the -xF option forces one function per section, the use of the ?O flag to control the order of sections in effect controls the order of functions.

Consider the following user-defined mapfile.

        text = LOAD ?RXO;
        text: .text%foo;
        text: .text%bar;
        text: .text%main;

The first declaration associates the ?O flag with the default text segment.

If the order of function definitions in the source file is main, foo, and bar, then the final executable contains functions in the order foo, bar, and main.

For static functions with the same name, the file names must also be used. The ?O flag forces the ordering of sections as requested in the mapfile. For example, if a static function bar() exists in files a.o and b.o, and function bar() from file a.o is to be placed before function bar() from file b.o, then the mapfile entries should read as follows.

        text: .text%bar: a.o;
        text: .text%bar: b.o;

The syntax allows for the following entry.

        text: .text%bar: a.o b.o;

However, this entry does not guarantee that function bar() from file a.o is placed before function bar() from file b.o. The second format is not recommended as the results are not reliable.