elf_update(3E) elf_update(3E)
NAME
elf_update - update an ELF descriptor
SYNOPSIS
[flag... ] file... [library] ...
DESCRIPTION
causes the library to examine the information associated with an ELF descriptor, elf, and to recalculate the structural data needed to gen-
erate the file's image.
cmd may have the following values.
This value tells to recalculate various values, updating only the ELF descriptor's memory structures. Any modified structures are flagged
with the bit. A program thus can update the structural information and then reexamine them without changing the file
associated with the ELF descriptor. Because this does not change the file, the ELF descriptor may allow reading, writ-
ing, or both reading and writing (see elf_begin(3E)).
If cmd has this value, duplicates its actions and also writes any ``dirty'' information associated with the ELF descriptor
to the file. That is, when a program has used or the facilities to supply new (or update existing) information for an
ELF descriptor, those data will be examined, coordinated, translated if necessary (see elf_xlate(3E)), and written to the
file. When portions of the file are written, any bits are reset, indicating those items no longer need to be written to
the file (see elf_flag(3E)). The sections' data is written in the order of their section header entries, and the section
header table is written to the end of the file.
When the ELF descriptor was created with it must have allowed writing the file. That is, the command must have been
either or
If succeeds, it returns the total size of the file image (not the memory image), in bytes. Otherwise an error occurred, and the function
returns -1.
When updating the internal structures, sets some members itself. Members listed below are the application's responsibility and retain the
values given by the program.
Member Notes
-----------------------------------------------
e_ident[EI_DATA] Library controls other
e_ident values
e_type
e_machine
e_version
ELF Header e_entry
e_phoff Only when ELF_F_LAYOUT
asserted
e_shoff Only when ELF_F_LAYOUT
asserted
e_flags
e_shstrndx
-----------------------------------------------
Member Notes
-----------------------------------------------------------
p_type The application controls all
p_offset program header entries
p_vaddr
p_paddr
Program Header p_filesz
p_memsz
p_flags
p_align
-----------------------------------------------------------
Member Notes
--------------------------------------------------
sh_name
sh_type
sh_flags
sh_addr
sh_offset Only when ELF_F_LAYOUT
asserted
Section Header sh_size Only when ELF_F_LAYOUT
asserted
sh_link
sh_info
sh_addralign Only when ELF_F_LAYOUT
asserted
sh_entsize
--------------------------------------------------
Member Notes
-------------------------------------------------
d_buf
d_type
d_size
Data Descriptor d_off Only when ELF_F_LAYOUT
asserted
d_align
d_version
-------------------------------------------------
Note the program is responsible for two particularly important members (among others) in the ELF header. The member controls the version
of data structures written to the file. If the version is the library uses its own internal version. The entry controls the data encoding
used in the file. As a special case, the value may be to request the native data encoding for the host machine. An error occurs in this
case if the native encoding doesn't match a file encoding known by the library.
Further note that the program is responsible for the section header member. Although the library sets it for sections with known types, it
cannot reliably know the correct value for all sections. Consequently, the library relies on the program to provide the values for unknown
section type. If the entry size is unknown or not applicable, the value should be set to zero.
When deciding how to build the output file, obeys the alignments of individual data buffers to create output sections. A section's most
strictly aligned data buffer controls the section's alignment. The library also inserts padding between buffers, as necessary, to ensure
the proper alignment of each buffer.
Note
As mentioned above, the commands translate data as necessary, before writing them to the file. This translation is not always transparent
to the application program. If a program has obtained pointers to data associated with a file (for example, see elf_getehdr(3E) and
elf_getdata(3E)), the program should reestablish the pointers after calling
As elf_begin(3E) describes, a program may ``update'' a COFF file to make the image consistent for ELF. (COFF is an object file format that
preceded ELF on some computer architectures (Intel, for example). When a program calls elf_begin on a COFF file, the library translates
COFF structures to their ELF equivalents, allowing programs to read (but not to write) a COFF file as if it were ELF. This conversion hap-
pens only to the memory image and not to the file itself.) The command updates only the memory image; one can use the command to modify
the file as well. Absolute executable files files) require special alignment, which cannot normally be preserved between COFF and ELF.
Consequently, one may not update an executable COFF file with the command (though is allowed).
SEE ALSO
elf(3E), elf_begin(3E), elf_flag(3E), elf_fsize(3E), elf_getdata(3E), elf_getehdr(3E), elf_getshdr(3E), elf_xlate(3E).
elf_update(3E)