illumos: manual page: elf

ELF_BEGIN(3ELF) ELF Library Functions ELF_BEGIN(3ELF)

NAME

elf_begin, elf_end, elf_memory, elf_next, elf_rand - process ELF object
files

SYNOPSIS

cc [ flag... ] file ... -lelf [ library ... ]
#include <libelf.h>

Elf *elf_begin(int fildes, Elf_Cmd cmd, Elf *ref);

int elf_end(Elf *elf);

Elf *elf_memory(char *image, size_t sz);

Elf_Cmd elf_next(Elf *elf);

size_t elf_rand(Elf *elf, size_t offset);

DESCRIPTION

The elf_begin(), elf_end(), elf_memory(), elf_next(), and elf_rand()
functions work together to process Executable and Linking Format (ELF)
object files, either individually or as members of archives. After
obtaining an ELF descriptor from elf_begin() or elf_memory(), the program
can read an existing file, update an existing file, or create a new file.
The fildes argument is an open file descriptor that elf_begin() uses for
reading or writing. The elf argument is an ELF descriptor previously
returned from elf_begin(). The initial file offset (see lseek(2)) is
unconstrained, and the resulting file offset is undefined.

The cmd argument can take 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 the examples below for more
information.

ELF_C_READ
When a program wants 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.

o 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.

o 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.

o 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 descriptor for
ref, the program must call elf_end() once for
each activation. See the examples below for
more information.

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(3ELF)). 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 wants to ignore previous file contents,
presumably to create a new file, it should set cmd to this
value. ref is ignored for this command.

The elf_begin() function operates 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 can call elf_kind(3ELF) or elf32_getehdr(3ELF) 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.

The elf_end() function is used to terminate an ELF descriptor, elf, and
to deallocate data associated with the descriptor. Until the program
terminates a descriptor, the data remain allocated. A null pointer is
allowed as an argument, to simplify error handling. If the program wants
to write data associated with the ELF descriptor to the file, it must use
elf_update() before calling elf_end().

Calling elf_end() removes one activation and returns the remaining
activation count. The library does not terminate the descriptor until the
activation count reaches 0. Consequently, a 0 return value indicates the
ELF descriptor is no longer valid.

The elf_memory() function returns a pointer to an ELF descriptor. The
ELF image has read operations enabled ( ELF_C_READ). The image argument
is a pointer to an image of the Elf file mapped into memory. The sz
argument is the size of the ELF image. An ELF image that is mapped in
with elf_memory() can be read and modified, but the ELF image size cannot
be changed.

The elf_next() function provides sequential access to the next archive
member. Having an ELF descriptor, elf, associated with an archive member,
elf_next() prepares the containing archive to access the following member
when the program calls elf_begin(). After successfully positioning an
archive for the next member, elf_next() returns the value ELF_C_READ.
Otherwise, the open file was not an archive, elf was NULL, or an error
occurred, and the return value is ELF_C_NULL. In either case, the return
value can be passed as an argument to elf_begin(), specifying the
appropriate action.

The elf_rand() function provides random archive processing, preparing elf
to access an arbitrary archive member. The elf argument must be a
descriptor for the archive itself, not a member within the archive. The
offset argument specifies the byte offset from the beginning of the
archive to the archive header of the desired member. See
elf_getarsym(3ELF) for more information about archive member offsets.
When elf_rand() works, it returns offset. Otherwise, it returns 0,
because an error occurred, elf was NULL, or the file was not an archive
(no archive member can have a zero offset). A program can mix random and
sequential archive processing.

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
can use elf_cntl() to let it reuse file descriptors. After calling
elf_cntl() with appropriate arguments, the program can 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

Example 1: A sample program of calling the elf_begin() function.

A prototype for reading a file appears on the next page. 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 2, 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.

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 */
}

An archive starts with a ``magic string'' that has SARMAG bytes; the
initial archive member follows immediately. An application could thus
provide the following function to rewind an archive (the function returns
-1 for errors and 0 otherwise).

#include <ar.h>
#include <libelf.h>
int
rewindelf(Elf *elf)
{
if (elf_rand(elf, (size_t)SARMAG) == SARMAG)
return 0;
return -1;
}

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 */
...
/* ensure that the memory image of the file is complete */
elf_update(elf, ELF_C_NULL);
elf_update(elf, ELF_C_WRITE); /* update file */
elf_end(elf);

Notice that both file creation examples open the file with write and read
permissions. On systems that support mmap(2), the library uses it to
enhance performance, and mmap(2) requires a readable file descriptor.
Although the library can use a write-only file descriptor, the
application will not obtain the performance advantages of mmap(2).

ATTRIBUTES