Executable and Linking Format
The type of object files created by the SPARC assembler version for SunOS 5.x are now Executable and Linking Format (ELF) files. These relocatable ELF files hold code and data suitable for linking with other object files to create an executable or a shared object file, and are the assembler normal output. The assembler can also write information to standard output (for example, under the -S option) and to standard error (for example, under the -V option). The SPARC assembler creates a default output file when standard input or multiple files are used.
This chapter is organized into the following sections:
The ELF object file format consists of:
Header
Sections
Locations
Addresses
Relocation tables
Symbol tables
String tables
For more information, see the System V Application Binary Interface: SPARC™ Processor Supplement.
3.1 ELF Header
The ELF header is always located at the beginning of the ELF file. It describes the ELF file organization and contains the actual sizes of the object file control structures. The initial bytes of an ELF header specify how the file is to be interpreted.
The ELF header contains the following information:
ehsize
ELF header size in bytes.
entry
Virtual address at which the process is to start. A value of 0 indicates no associated entry point.
flag
Processor-specific flags associated with the file.
ident
Marks the file as an object file and provides machine-independent data to decode and interpret the file contents.
machine
Specifies the required architecture for an individual file. A value of 2 specifies SPARC.
phentsize
Size in bytes of entries in the program header table. All entries are the same size.
phnum
Number of entries in program header table. A value of 0 indicates the file has no program header table.
phoff
Program header table file offset in bytes. The value of 0 indicates no program header.
shentsize
Size in bytes of the section header. A section header is one entry in the section header table; all entries are the same size.
shnum
Number of entries in section header table. A value of 0 indicates the file has no section header table.
shoff
Section header table file offset in bytes. The value of 0 indicates no section header.
shstrndx
Section header table index of the entry associated with the section name string table. A value of SHN_UNDEF indicates the file does not have a section name string table.
type
Identifies the object file type. Table 3-1 describes the reserved object file types.
version
Identifies the object file version.
Table 3-1 shows reserved object file types:
Table 3-1
Type | Value | Description |
|---|---|---|
none | 0 | No file type |
rel | 1 | Relocatable file |
exec | 2 | Executable file |
dyn | 3 | Shared object file |
core | 4 | Core file |
loproc | 0xff00 | Processor-specific |
hiproc | 0xffff | Processor-specific |
3.2 Sections
A section is the smallest unit of an object that can be relocated. The following sections are commonly present in an ELF file:
Section header
Executable text
Read-only data
Read-write data
Read-write uninitialized data (section header only)
Sections do not need to be specified in any particular order. The current section is the section to which code is generated.
These sections contain all other information in an object file and satisfy several conditions.
Every section must have one section header describing the section. However, a section header does not need to be followed by a section.
Each section occupies one contiguous sequence of bytes within a file. The section may be empty (that is, of zero-length).
A byte in a file can reside in only one section. Sections in a file cannot overlap.
An object file may have inactive space. The contents of the data in the inactive space are unspecified.