ELF Library Functions elfupdate(3ELF)
NAME
elfupdate - update an ELF descriptor
SYNOPSIS
cc [ flag ... ] file ... -lelf [ library ... ]
#include
offt elfupdate(Elf *elf, ElfCmd cmd);
DESCRIPTION
The elfupdate() function causes the library to examine the
information associated with an ELF descriptor, elf, and to
recalculate the structural data needed to generate the
file's image.
The cmd argument can have the following values:
ELFCNUL This value tells elfupdate() to recalculate
various values, updating only the ELF
descriptor's memory structures. Any modified
structures are flagged with the ELFDIRTY
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,
writing, or both reading and writing (see
elfbegin(3ELF)).
ELFCWRITE If cmd has this value, elfupdate() dupli-
cates its ELFCNUL actions and also writes
any ``dirty'' information associated with the
ELF descriptor to the file. That is, when a
program has used elfgetdata(3ELF) or the
elfflagdata(3ELF) facilities to supply new
(or update existing) information for an ELF
descriptor, those data will be examined,
coordinated, translated if necessary (see
elf32xlatetof(3ELF)), and written to the
file. When portions of the file are written,
any ELFDIRTY bits are reset, indicating
those items no longer need to be written to
the file (see elfflagdata(3ELF)). The sec-
tions' data are 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 elfbegin(), it must have allowed writ-
ing the file. That is, the elfbegin()
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ELF Library Functions elfupdate(3ELF)
command must have been either ELFCRDWR or
ELFCWRITE.
If elfupdate() 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, elfupdate() sets
some members itself. Members listed below are the
application's responsibility and retain the values given by
the program.
The following table shows ELF Header members:
Member Notes
eident[EIDATA] Library controls other eident values
etype
emachine
eversion
eentry
ephoff Only when ELFLAYOUT asserted
eshoff Only when ELFLAYOUT asserted
eflags
eshstrndx
The following table shows the Program Header members:
Member Notes
ptype The application controls all
poffset program header entries
pvaddr
ppaddr
pfilesz
pmemsz
pflags
palign
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ELF Library Functions elfupdate(3ELF)
The following table shows the Section Header members:
Member Notes
shname
shtype
shflags
shaddr
shoffset Only when ELFLAYOUT asserted
shsize Only when ELFLAYOUT asserted
shlink
shinfo
shaddralign Only when ELFLAYOUT asserted
shentsize
The following table shows the Data Descriptor members:
Member Notes
dbuf
dtype
dsize
doff Only when ELFLAYOUT asserted
dalign
dversion
Note that the program is responsible for two particularly
important members (among others) in the ELF header. The
eversion member controls the version of data structures
written to the file. If the version is EVNONE, the library
uses its own internal version. The eident[EIDATA] entry
controls the data encoding used in the file. As a special
case, the value may be ELFDATANONE 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
shentsize 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 types. If the entry size is unknown or not
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ELF Library Functions elfupdate(3ELF)
applicable, the value should be set to 0.
When deciding how to build the output file, elfupdate()
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.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability Stable
MT-Level MT-Safe
SEE ALSO
elf(3ELF), elf32fsize(3ELF), elf32getehdr(3ELF),
elf32getshdr(3ELF), elf32xlatetof(3ELF), elfbegin(3ELF),
elfflagdata(3ELF), elfgetdata(3ELF), libelf(3LIB), attri-
butes(5)
NOTES
As mentioned above, the ELFCWRITE command translates data
as necessary, before writing them to the file. This transla-
tion is not always transparent to the application program.
If a program has obtained pointers to data associated with a
file (for example, see elf32getehdr(3ELF) and
elfgetdata(3ELF)), the program should reestablish the
pointers after calling elfupdate().
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