ELF_BEGIN(3E)ELF_BEGIN(3E)NAMEelf_begin - make a file descriptor
SYNOPSIS
cc [flag ...] file ... -lelf [library ...]
#include <libelf.h>
Elf *elf_begin(int fildes, Elf_Cmd cmd, Elf *ref);
DESCRIPTION
elf_begin, elf_next, elf_rand, and elf_end work together to process ELF
object files, either individually or as members of archives. After
obtaining an ELF descriptor from elf_begin, the program may read an
existing file, update an existing file, or create a new file. fildes is
an open file descriptor that elf_begin uses for reading or writing. The
initial file offset [see lseek(2)] is unconstrained, and the resulting
file offset is undefined.
cmd may have the following values.
ELF_C_NULL When a program sets cmd to this value, elf_begin returns
a null pointer, without opening a new descriptor. ref is
ignored for this command. See elf_next(3E) and the
examples below for more information.
ELF_C_READ When a program wishes to examine the contents of an
existing file, it should set cmd to this value.
Depending on the value of ref, this command examines
archive members or entire files. Three cases can occur.
First, if ref is a null pointer, elf_begin allocates a
new ELF descriptor and prepares to process the entire
file. If the file being read is an archive, elf_begin
also prepares the resulting descriptor to examine the
initial archive member on the next call to elf_begin, as
if the program had used elf_next or elf_rand to ``move''
to the initial member.
Second, if ref is a non-null descriptor associated with
an archive file, elf_begin lets a program obtain a
separate ELF descriptor associated with an individual
member. The program should have used elf_next or
elf_rand to position ref appropriately (except for the
initial member, which elf_begin prepares; see the example
below). In this case, fildes should be the same file
descriptor used for the parent archive.
Finally, if ref is a non-null ELF descriptor that is not
an archive, elf_begin increments the number of
activations for the descriptor and returns ref, without
allocating a new descriptor and without changing the
descriptor's read/write permissions. To terminate the
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descriptor for ref, the program must call elf_end once
for each activation. See elf_next(3E) and the examples
below for more information.
ELF_C_READ_MMAP This command duplicates the actions of ELF_C_READ but
uses the mmap() system call to access the file data in a
more memory efficient manner. If, after libelf has
opened the file, another process truncates the file
libelf could get a Segmentation Violation attempting to
read data it believes is in the file (but which is no
longer there due to the truncation). Applications
calling libelf with ELF_C_READ_MMAP may therefore wish to
install a signal handler for SIGSEGV (Segmentation
Violation) and issue a message when it happens.
ELF_C_RDWR This command duplicates the actions of ELF_C_READ and
additionally allows the program to update the file image
[see elf_update(3E)]. That is, using ELF_C_READ gives a
read-only view of the file, while ELF_C_RDWR lets the
program read and write the file. ELF_C_RDWR is not valid
for archive members. If ref is non-null, it must have
been created with the ELF_C_RDWR command.
ELF_C_WRITE If the program wishes to ignore previous file contents,
presumably to create a new file, it should set cmd to
this value. ref is ignored for this command.
ELF_C_WRITE_FAST
is the same as ELF_C_WRITE except that instead of
malloc()ing memory for the output file (and then doing a
write(2) and free()) ELF_C_WRITE_FAST writes the output
directly to the file system (by doing writes as needed).
Avoiding the malloc() can help limit time-consuming
paging activity.
elf_begin ``works'' on all files (including files with zero bytes),
providing it can allocate memory for its internal structures and read any
necessary information from the file. Programs reading object files thus
may call elf_kind or elf_getehdr to determine the file type (only object
files have an ELF header). If the file is an archive with no more
members to process, or an error occurs, elf_begin returns a null pointer.
Otherwise, the return value is a non-null ELF descriptor.
Before the first call to elf_begin, a program must call elf_version to
coordinate versions.
SYSTEM SERVICES
When processing a file, the library decides when to read or write the
file, depending on the program's requests. Normally, the library assumes
the file descriptor remains usable for the life of the ELF descriptor.
If, however, a program must process many files simultaneously and the
underlying operating system limits the number of open files, the program
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can use elf_cntl to let it reuse file descriptors. After calling
elf_cntl with appropriate arguments, the program may close the file
descriptor without interfering with the library.
All data associated with an ELF descriptor remain allocated until elf_end
terminates the descriptor's last activation. After the descriptors have
been terminated, the storage is released; attempting to reference such
data gives undefined behavior. Consequently, a program that deals with
multiple input (or output) files must keep the ELF descriptors active
until it finishes with them.
EXAMPLES
A prototype for reading a file appears below. If the file is a simple
object file, the program executes the loop one time, receiving a null
descriptor in the second iteration. In this case, both elf and arf will
have the same value, the activation count will be two, and the program
calls elf_end twice to terminate the descriptor. If the file is an
archive, the loop processes each archive member in turn, ignoring those
that are not object files.
if (elf_version(EV_CURRENT) == EV_NONE)
{
/* library out of date */
/* recover from error */
}
cmd = ELF_C_READ;
arf = elf_begin(fildes, cmd, (Elf *)0);
while ((elf = elf_begin(fildes, cmd, arf)) != 0)
{
if ((ehdr = elf32_getehdr(elf)) != 0)
{
/* process the file ... */
}
cmd = elf_next(elf);
elf_end(elf);
}
elf_end(arf);
Alternatively, the next example illustrates random archive processing.
After identifying the file as an archive, the program repeatedly
processes archive members of interest. For clarity, this example omits
error checking and ignores simple object files. Additionally, this
fragment preserves the ELF descriptors for all archive members, because
it does not call elf_end to terminate them.
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ELF_BEGIN(3E)ELF_BEGIN(3E)elf_version(EV_CURRENT);
arf = elf_begin(fildes, ELF_C_READ, (Elf *)0);
if (elf_kind(arf) != ELF_K_AR)
{
/* not an archive */
}
/* initial processing */
/* set offset = ... for desired member header */
while (elf_rand(arf, offset) == offset)
{
if ((elf = elf_begin(fildes, ELF_C_READ, arf)) == 0)
break;
if ((ehdr = elf32_getehdr(elf)) != 0)
{
/* process archive member ... */
}
/* set offset = ... for desired member header */
}
The following outline shows how one might create a new ELF file. This
example is simplified to show the overall flow.
elf_version(EV_CURRENT);
fildes = open("path/name", O_RDWR|O_TRUNC|O_CREAT, 0666);
if ((elf = elf_begin(fildes, ELF_C_WRITE, (Elf *)0)) == 0)
return;
ehdr = elf32_newehdr(elf);
phdr = elf32_newphdr(elf, count);
scn = elf_newscn(elf);
shdr = elf32_getshdr(scn);
data = elf_newdata(scn);
elf_update(elf, ELF_C_WRITE);
elf_end(elf);
Finally, the following outline shows how one might update an existing ELF
file. Again, this example is simplified to show the overall flow.
elf_version(EV_CURRENT);
fildes = open("path/name", O_RDWR);
elf = elf_begin(fildes, ELF_C_RDWR, (Elf *)0);
/* add new or delete old information ... */
close(creat("path/name", 0666));
elf_update(elf, ELF_C_WRITE);
elf_end(elf);
In the example above, the call to creat truncates the file, thus ensuring
the resulting file will have the ``right'' size. Without truncation, the
updated file might be as big as the original, even if information were
deleted. The library truncates the file, if it can, with ftruncate [see
truncate(2)]. Some systems, however, do not support ftruncate, and the
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call to creat protects against this.
Notice that both file creation examples open the file with write and read
permissions. The library is not compiled to use mmap so there is no need
for write permission. But for maximum portability with implementations
that do use mmap for file creation you may wish to provide both write and
read permissions.
SEE ALSOcreat(2), lseek(2), mmap(2), open(2), truncate(2), elf(3E), elf_cntl(3E),
elf_end(3E), elf_getarhdr(3E), elf_getbase(3E), elf_getdata(3E),
elf_getehdr(3E), elf_getphdr(3E), elf_getscn(3E), elf_kind(3E),
elf_next(3E), elf_rand(3E), elf_rawfile(3E), elf_update(3E),
elf_version(3E), ar(4).
NOTES
COFF is an object file format that preceded ELF . COFF object files
cannot be used with this library.
ELF_C_READ_MMAP and ELF_C_WRITE_FAST are options specific to this
implementation, not options which are generally available.
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