man pages section 3: Extended Library Functions, Volume 1

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Updated: July 2014
 
 

elf_next(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.

  • 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.

  • 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.

  • 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

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe

See Also

creat(2), lseek(2), mmap(2), open(2), ar.h(3HEAD), elf(3ELF), elf32_getehdr(3ELF), elf_cntl(3ELF), elf_getarhdr(3ELF), elf_getarsym(3ELF), elf_getbase(3ELF), elf_getdata(3ELF), elf_getscn(3ELF), elf_kind(3ELF), elf_rawfile(3ELF), elf_update(3ELF), elf_version(3ELF), libelf(3LIB), attributes(5)