man3/end.3

   1 .\" Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
   2 .\"     <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH END 3 2020-06-09 "GNU" "Linux Programmer's Manual"
   7 .SH NAME
   8 etext, edata, end \- end of program segments
   9 .SH SYNOPSIS
  10 .nf
  11 .BI extern " etext" ;
  12 .BI extern " edata" ;
  13 .BI extern " end" ;
  14 .fi
  15 .SH DESCRIPTION
  16 The addresses of these symbols indicate the end of various program
  17 segments:
  18 .TP
  19 .I etext
  20 This is the first address past the end of the text segment
  21 (the program code).
  22 .TP
  23 .I edata
  24 This is the first address past the end of the
  25 initialized data segment.
  26 .TP
  27 .I end
  28 This is the first address past the end of the
  29 uninitialized data segment (also known as the BSS segment).
  30 .SH CONFORMING TO
  31 Although these symbols have long been provided on most UNIX systems,
  32 they are not standardized; use with caution.
  33 .SH NOTES
  34 The program must explicitly declare these symbols;
  35 they are not defined in any header file.
  36 .PP
  37 On some systems the names of these symbols are preceded by underscores,
  38 thus:
  39 .IR _etext ,
  40 .IR _edata ,
  41 and
  42 .IR _end .
  43 These symbols are also defined for programs compiled on Linux.
  44 .PP
  45 At the start of program execution,
  46 the program break will be somewhere near
  47 .I &end
  48 (perhaps at the start of the following page).
  49 However, the break will change as memory is allocated via
  50 .BR brk (2)
  51 or
  52 .BR malloc (3).
  53 Use
  54 .BR sbrk (2)
  55 with an argument of zero to find the current value of the program break.
  56 .SH EXAMPLES
  57 When run, the program below produces output such as the following:
  58 .PP
  59 .in +4n
  60 .EX
  61 .RB "$" " ./a.out"
  62 First address past:
  63     program text (etext)       0x8048568
  64     initialized data (edata)   0x804a01c
  65     uninitialized data (end)   0x804a024
  66 .EE
  67 .in
  68 .SS Program source
  69 \&
  70 .EX
  71 #include <stdio.h>
  72 #include <stdlib.h>
  73
  74 extern char etext, edata, end; /* The symbols must have some type,
  75                                    or "gcc \-Wall" complains */
  76
  77 int
  78 main(int argc, char *argv[])
  79 {
  80     printf("First address past:\en");
  81     printf("    program text (etext)      %10p\en", &etext);
  82     printf("    initialized data (edata)  %10p\en", &edata);
  83     printf("    uninitialized data (end)  %10p\en", &end);
  84
  85     exit(EXIT_SUCCESS);
  86 }
  87 .EE
  88 .SH SEE ALSO
  89 .BR objdump (1),
  90 .BR readelf (1),
  91 .BR sbrk (2),
  92 .BR elf (5)