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Top Forums Programming A question regarding the gcc compiler ... Post 302475398 by jim mcnamara on Sunday 28th of November 2010 03:19:49 PM
Old 11-28-2010
The short answer is that gcc builds symbol tables on the fly by parsing your code into required symbols (a "myfile".o file plus the crt1.o file (where _start() lives) ), then invoking ld against those symbols.

Try nm on a.o file.

nm has to read an ELF file, then resolve the symbols. Every entry point and variable (except those optimized away) constitutes something nm has to re-engineer. If you are on linux or have support ELF image files, then you can get the readelf utility. Try it.

Or download readelf.c and check out what nm has to deal with.

rpm: file/src/readelf.c Source File


If you ever build large database programs, example: Oracle Pro*C, you will find it takes a while to build one.
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READELF(1)						       GNU Development Tools							READELF(1)

NAME
readelf - Displays information about ELF files. SYNOPSIS
readelf [-a|--all] [-h|--file-header] [-l|--program-headers|--segments] [-S|--section-headers|--sections] [-g|--section-groups] [-t|--section-details] [-e|--headers] [-s|--syms|--symbols] [-n|--notes] [-r|--relocs] [-u|--unwind] [-d|--dynamic] [-V|--version-info] [-A|--arch-specific] [-D|--use-dynamic] [-x <number or name>|--hex-dump=<number or name>] [-p <number or name>|--string-dump=<number or name>] [-R <number or name>|--relocated-dump=<number or name>] [-c|--archive-index] [-w[lLiaprmfFsoR]| --debug-dump[=rawline,=decodedline,=info,=abbrev,=pubnames,=aranges,=macro,=frames,=frames-interp,=str,=loc,=Ranges]] [-I|-histogram] [-v|--version] [-W|--wide] [-H|--help] elffile... DESCRIPTION
readelf displays information about one or more ELF format object files. The options control what particular information to display. elffile... are the object files to be examined. 32-bit and 64-bit ELF files are supported, as are archives containing ELF files. This program performs a similar function to objdump but it goes into more detail and it exists independently of the BFD library, so if there is a bug in BFD then readelf will not be affected. OPTIONS
The long and short forms of options, shown here as alternatives, are equivalent. At least one option besides -v or -H must be given. -a --all Equivalent to specifying --file-header, --program-headers, --sections, --symbols, --relocs, --dynamic, --notes and --version-info. -h --file-header Displays the information contained in the ELF header at the start of the file. -l --program-headers --segments Displays the information contained in the file's segment headers, if it has any. -S --sections --section-headers Displays the information contained in the file's section headers, if it has any. -g --section-groups Displays the information contained in the file's section groups, if it has any. -t --section-details Displays the detailed section information. Implies -S. -s --symbols --syms Displays the entries in symbol table section of the file, if it has one. -e --headers Display all the headers in the file. Equivalent to -h -l -S. -n --notes Displays the contents of the NOTE segments and/or sections, if any. -r --relocs Displays the contents of the file's relocation section, if it has one. -u --unwind Displays the contents of the file's unwind section, if it has one. Only the unwind sections for IA64 ELF files are currently supported. -d --dynamic Displays the contents of the file's dynamic section, if it has one. -V --version-info Displays the contents of the version sections in the file, it they exist. -A --arch-specific Displays architecture-specific information in the file, if there is any. -D --use-dynamic When displaying symbols, this option makes readelf use the symbol table in the file's dynamic section, rather than the one in the symbols section. -x <number or name> --hex-dump=<number or name> Displays the contents of the indicated section as a hexadecimal bytes. A number identifies a particular section by index in the section table; any other string identifies all sections with that name in the object file. -R <number or name> --relocated-dump=<number or name> Displays the contents of the indicated section as a hexadecimal bytes. A number identifies a particular section by index in the section table; any other string identifies all sections with that name in the object file. The contents of the section will be relocated before they are displayed. -p <number or name> --string-dump=<number or name> Displays the contents of the indicated section as printable strings. A number identifies a particular section by index in the section table; any other string identifies all sections with that name in the object file. -c --archive-index Displays the file symbol index infomation contained in the header part of binary archives. Performs the same function as the t command to ar, but without using the BFD library. -w[lLiaprmfFsoR] --debug-dump[=rawline,=decodedline,=info,=abbrev,=pubnames,=aranges,=macro,=frames,=frames-interp,=str,=loc,=Ranges] Displays the contents of the debug sections in the file, if any are present. If one of the optional letters or words follows the switch then only data found in those specific sections will be dumped. Note: the =decodedline option will display the interpreted contents of a .debug_line section whereas the =rawline option dumps the contents in a raw format. -I --histogram Display a histogram of bucket list lengths when displaying the contents of the symbol tables. -v --version Display the version number of readelf. -W --wide Don't break output lines to fit into 80 columns. By default readelf breaks section header and segment listing lines for 64-bit ELF files, so that they fit into 80 columns. This option causes readelf to print each section header resp. each segment one a single line, which is far more readable on terminals wider than 80 columns. -H --help Display the command line options understood by readelf. @file Read command-line options from file. The options read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed. Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional @file options; any such options will be processed recursively. SEE ALSO
objdump(1), and the Info entries for binutils. COPYRIGHT
Copyright (c) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". binutils-2.20 2009-10-16 READELF(1)
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