elf_begin(3E)
elf_begin - Make a file descriptor
Showing IRIX 6.5.30 (default release). Last changed in IRIX 6.5.19.
NAME elf_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 is unconstrained, and the resulting file offset is undefined. cmd may have the following values: ELF_C_NULL elf_begin returns a null pointer, without opening a new descriptor. ref is ignored for this command. See elf_next(3E) and the EXAMPLES section for more information. ELF_C_READ Specifies that the program examine the contents of an existing file. Depending on the value of ref, this command examines archive members or entire files. The following three cases may 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 descriptor for ref, the program must call elf_end once for each activation. See elf_next(3E) and the EXAMPLES section 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 The program ignores previous file contents, presumably to create a new file. ref is ignored for this command. ELF_C_WRITE_FAST This command is the same as ELF_C_WRITE except that instead of performing malloc() on 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 a write as needed). Avoiding running 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. Usually, 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 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 Example 1: The following is a prototype for reading a file. 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); Example 2: 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 */ } Example 3: The following outline shows how you 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); Example 4: The following outline shows how you might update an existing ELF file. 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 previous example, 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 call to creat protects against this. Note 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 want to provide both write and read permissions. 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 that are generally available. SEE ALSO creat(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)