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Full Discussion: Editing File Headers
Top Forums Shell Programming and Scripting Editing File Headers Post 302543585 by DGPickett on Monday 1st of August 2011 10:00:27 AM
Old 08-01-2011
Yes, the effect is the same, give or takes some renaming. Update in place destroys the original if anything goes wrong, and requires much more sophisticated tools. Most text tools truncate as they write. You need a very serious language, like PERL or C, to update the middle. Maybe it is a mindset hangover from tape, where, generally, the last thing you wrote is implicitly just before EOF.

A header could contain a file length and checksum as well as data type and format type magic strings, making updates more complex, but they are less popular in audio/video as you do not know these until the end, long after the header is written, and they do not want to go back and update, either.

Getting a patch to the software would be even nicer. I have patched stings in library file object code using a binary find and replace copier, and scary thing is, no checksum alarms went off! Maybe we need secure ELF? But these few bits are harder to locate and may break other things if you change them.
 

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GELF_CHECKSUM(3)					   BSD Library Functions Manual 					  GELF_CHECKSUM(3)

NAME
elf32_checksum, elf64_checksum, gelf_checksum -- return the checksum of an ELF object LIBRARY
ELF Access Library (libelf, -lelf) SYNOPSIS
#include <libelf.h> long elf32_checksum(Elf *elf); long elf64_checksum(Elf *elf); #include <gelf.h> long gelf_checksum(Elf *elf); DESCRIPTION
These functions return a simple checksum of the ELF object described by their argument elf. The checksum is computed in way that allows its value to remain unchanged in presence of modifications to the ELF object by utilities like strip(1). Function elf32_checksum() returns a checksum for an ELF descriptor elf of class ELFCLASS32. Function elf64_checksum() returns a checksum for an ELF descriptor elf of class ELFCLASS64. Function gelf_checksum() provides a class-independent way retrieving the checksum for ELF object elf. RETURN VALUES
These functions return the checksum of the ELF object, or zero in case an error was encountered. ERRORS
These functions may fail with the following errors: [ELF_E_ARGUMENT] Argument elf was NULL. [ELF_E_ARGUMENT] Argument elf was not a descriptor for an ELF file. [ELF_E_ARGUMENT] The ELF descriptor elf was not opened for reading or updating. [ELF_E_CLASS] For functions elf32_checksum() and elf64_checksum(), ELF descriptor elf did not match the class of the called function. [ELF_E_HEADER] The ELF object specified by argument elf had a malformed executable header. [ELF_E_RESOURCE] An out of memory condition was detected during processing. [ELF_E_SECTION] The ELF object specified by argument elf contained a section with a malformed section header. [ELF_E_VERSION] The ELF object was of an unsupported version. SEE ALSO
strip(1), elf(3), gelf(3) BSD
August 29, 2006 BSD
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