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






? [E] [N] [O] [R] [W] [X]











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

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.

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.