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Full Discussion: How raw disks are read?
Top Forums UNIX for Advanced & Expert Users How raw disks are read? Post 302802743 by jlliagre on Saturday 4th of May 2013 04:59:01 PM
Old 05-04-2013
A raw disk is accessed through a file descriptor so is technically still a (single and big) file, as far as POSIX is concerned. There are operations that won't work like truncating or enlarging the file but otherwise, it can be read by seeking to the expected location and reading the data there. Same for writing.
 

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elf_cntl(3E)															      elf_cntl(3E)

NAME
elf_cntl - control a file descriptor SYNOPSIS
[flag... ] file... [library] ... DESCRIPTION
instructs the library to modify its behavior with respect to an ELF descriptor, elf. As elf_begin(3E) describes, an ELF descriptor can have multiple activations, and multiple ELF descriptors may share a single file descriptor. Generally, commands apply to all activations of elf. Moreover, if the ELF descriptor is associated with an archive file, descriptors for members within the archive will also be affected as described below. Unless stated otherwise, operations on archive members do not affect the descriptor for the containing ar- chive. The cmd argument tells what actions to take and may have the following values. This value tells the library not to use the file descriptor associated with elf. A program should use this command when it has requested all the information it cares to use and wishes to avoid the overhead of reading the rest of the file. The memory for all completed operations remains valid, but later file operations, such as the initial for a section, will fail if the data is not in memory already. This command is similar to except it forces the library to read the rest of the file. A program should use this command when it must close the file descriptor but has not yet read everything it needs from the file. After completes the command, future operations, such as will use the memory version of the file without needing to use the file descriptor. If succeeds, it returns zero. Otherwise, elf was null or an error occurred, and the function returns -1. Note If the program wishes to use the ``raw'' operations (see described in elf_getdata(3E), and elf_rawfile(3E)) after disabling the file descriptor with or it must execute the raw operations explicitly beforehand. Otherwise, the raw file operations will fail. Calling makes the entire image available, thus supporting subsequent calls. SEE ALSO
elf(3E), elf_begin(3E), elf_getdata(3E), elf_rawfile(3E). elf_cntl(3E)
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