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35f0a4f24e
For now, the elf writer is only capable of emitting an empty elf file, with a section table and a section table string table. This will be enhanced in the future :) git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22291 91177308-0d34-0410-b5e6-96231b3b80d8
231 lines
8.7 KiB
C++
231 lines
8.7 KiB
C++
//===-- ELFWriter.cpp - Target-independent ELF Writer code ----------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements the target-independent ELF writer. This file writes out
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// the ELF file in the following order:
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//
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// #1. ELF Header
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// #2. '.data' section
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// #3. '.bss' section
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// ...
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// #X. '.shstrtab' section
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// #Y. Section Table
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//
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// The entries in the section table are laid out as:
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// #0. Null entry [required]
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// #1. ".data" entry - global variables with initializers. [ if needed ]
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// #2. ".bss" entry - global variables without initializers. [ if needed ]
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// #3. ".text" entry - the program code
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// ...
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// #N. ".shstrtab" entry - String table for the section names.
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//
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// NOTE: This code should eventually be extended to support 64-bit ELF (this
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// won't be hard), but we haven't done so yet!
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/CodeGen/ELFWriter.h"
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#include "llvm/Module.h"
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#include "llvm/Target/TargetMachine.h"
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using namespace llvm;
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ELFWriter::ELFWriter(std::ostream &o, TargetMachine &tm) : O(o), TM(tm) {
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e_machine = 0; // e_machine defaults to 'No Machine'
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e_flags = 0; // e_flags defaults to 0, no flags.
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is64Bit = TM.getTargetData().getPointerSizeInBits() == 64;
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isLittleEndian = TM.getTargetData().isLittleEndian();
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}
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// doInitialization - Emit the file header and all of the global variables for
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// the module to the ELF file.
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bool ELFWriter::doInitialization(Module &M) {
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outbyte(0x7F); // EI_MAG0
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outbyte('E'); // EI_MAG1
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outbyte('L'); // EI_MAG2
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outbyte('F'); // EI_MAG3
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outbyte(is64Bit ? 2 : 1); // EI_CLASS
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outbyte(isLittleEndian ? 1 : 2); // EI_DATA
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outbyte(1); // EI_VERSION
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for (unsigned i = OutputBuffer.size(); i != 16; ++i)
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outbyte(0); // EI_PAD up to 16 bytes.
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// This should change for shared objects.
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outhalf(1); // e_type = ET_REL
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outhalf(e_machine); // e_machine = whatever the target wants
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outword(1); // e_version = 1
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outaddr(0); // e_entry = 0 -> no entry point in .o file
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outaddr(0); // e_phoff = 0 -> no program header for .o
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ELFHeader_e_shoff_Offset = OutputBuffer.size();
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outaddr(0); // e_shoff
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outword(e_flags); // e_flags = whatever the target wants
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assert(!is64Bit && "These sizes need to be adjusted for 64-bit!");
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outhalf(52); // e_ehsize = ELF header size
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outhalf(0); // e_phentsize = prog header entry size
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outhalf(0); // e_phnum = # prog header entries = 0
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outhalf(40); // e_shentsize = sect header entry size
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ELFHeader_e_shnum_Offset = OutputBuffer.size();
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outhalf(0); // e_shnum = # of section header ents
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ELFHeader_e_shstrndx_Offset = OutputBuffer.size();
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outhalf(0); // e_shstrndx = Section # of '.shstrtab'
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// Add the null section.
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SectionList.push_back(ELFSection());
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// Okay, the ELF header has been completed, emit the .data section next.
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ELFSection DataSection(".data", OutputBuffer.size());
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for (Module::global_iterator I = M.global_begin(), E = M.global_end();
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I != E; ++I)
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EmitDATASectionGlobal(I);
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// If the .data section is nonempty, add it to our list.
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if ((DataSection.Size = OutputBuffer.size()-DataSection.Offset)) {
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DataSection.Align = 4; // FIXME: Compute!
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SectionList.push_back(DataSection);
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}
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// Okay, emit the .bss section next.
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ELFSection BSSSection(".bss", OutputBuffer.size());
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for (Module::global_iterator I = M.global_begin(), E = M.global_end();
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I != E; ++I)
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EmitBSSSectionGlobal(I);
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// If the .bss section is nonempty, add it to our list.
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if ((BSSSection.Size = OutputBuffer.size()-BSSSection.Offset)) {
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BSSSection.Align = 4; // FIXME: Compute!
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SectionList.push_back(BSSSection);
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}
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return false;
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}
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// isCOMM - A global variable should be emitted to the common area if it is zero
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// initialized and has linkage that permits it to be merged with other globals.
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static bool isCOMM(GlobalVariable *GV) {
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return GV->getInitializer()->isNullValue() &&
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(GV->hasLinkOnceLinkage() || GV->hasInternalLinkage() ||
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GV->hasWeakLinkage());
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}
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// EmitDATASectionGlobal - Emit a global variable to the .data section if it
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// belongs there.
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void ELFWriter::EmitDATASectionGlobal(GlobalVariable *GV) {
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if (!GV->hasInitializer()) return;
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// Do not emit a symbol here if it should be emitted to the common area.
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if (isCOMM(GV)) return;
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EmitGlobal(GV);
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}
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void ELFWriter::EmitBSSSectionGlobal(GlobalVariable *GV) {
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if (!GV->hasInitializer()) return;
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// FIXME: We don't support BSS yet!
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return;
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EmitGlobal(GV);
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}
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void ELFWriter::EmitGlobal(GlobalVariable *GV) {
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}
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bool ELFWriter::runOnMachineFunction(MachineFunction &MF) {
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return false;
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}
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/// doFinalization - Now that the module has been completely processed, emit
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/// the ELF file to 'O'.
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bool ELFWriter::doFinalization(Module &M) {
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// Emit the string table for the sections in the ELF file we have.
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EmitSectionTableStringTable();
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// Emit the .o file section table.
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EmitSectionTable();
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// Emit the .o file to the specified stream.
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O.write((char*)&OutputBuffer[0], OutputBuffer.size());
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// Free the output buffer.
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std::vector<unsigned char>().swap(OutputBuffer);
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return false;
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}
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/// EmitSectionTableStringTable - This method adds and emits a section for the
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/// ELF Section Table string table: the string table that holds all of the
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/// section names.
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void ELFWriter::EmitSectionTableStringTable() {
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// First step: add the section for the string table to the list of sections:
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SectionList.push_back(ELFSection(".shstrtab", OutputBuffer.size()));
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SectionList.back().Type = 3; // SHT_STRTAB
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// Now that we know which section number is the .shstrtab section, update the
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// e_shstrndx entry in the ELF header.
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fixhalf(SectionList.size()-1, ELFHeader_e_shstrndx_Offset);
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// Set the NameIdx of each section in the string table and emit the bytes for
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// the string table.
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unsigned Index = 0;
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for (unsigned i = 0, e = SectionList.size(); i != e; ++i) {
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// Set the index into the table. Note if we have lots of entries with
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// common suffixes, we could memoize them here if we cared.
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SectionList[i].NameIdx = Index;
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// Add the name to the output buffer, including the null terminator.
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OutputBuffer.insert(OutputBuffer.end(), SectionList[i].Name.begin(),
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SectionList[i].Name.end());
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// Add a null terminator.
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OutputBuffer.push_back(0);
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// Keep track of the number of bytes emitted to this section.
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Index += SectionList[i].Name.size()+1;
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}
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// Set the size of .shstrtab now that we know what it is.
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SectionList.back().Size = Index;
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}
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/// EmitSectionTable - Now that we have emitted the entire contents of the file
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/// (all of the sections), emit the section table which informs the reader where
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/// the boundaries are.
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void ELFWriter::EmitSectionTable() {
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// Now that all of the sections have been emitted, set the e_shnum entry in
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// the ELF header.
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fixhalf(SectionList.size(), ELFHeader_e_shnum_Offset);
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// Now that we know the offset in the file of the section table (which we emit
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// next), update the e_shoff address in the ELF header.
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fixaddr(OutputBuffer.size(), ELFHeader_e_shoff_Offset);
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// Emit all of the section table entries.
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for (unsigned i = 0, e = SectionList.size(); i != e; ++i) {
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const ELFSection &S = SectionList[i];
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outword(S.NameIdx); // sh_name - Symbol table name idx
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outword(S.Type); // sh_type - Section contents & semantics
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outword(S.Flags); // sh_flags - Section flags.
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outaddr(S.Addr); // sh_addr - The mem address this section appears in.
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outaddr(S.Offset); // sh_offset - The offset from the start of the file.
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outword(S.Size); // sh_size - The section size.
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outword(S.Link); // sh_link - Section header table index link.
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outword(S.Info); // sh_info - Auxillary information.
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outword(S.Align); // sh_addralign - Alignment of section.
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outword(S.EntSize); // sh_entsize - Size of each entry in the section.
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}
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// Release the memory allocated for the section list.
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std::vector<ELFSection>().swap(SectionList);
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}
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