man3/end.3

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  26 .TH END 3 2017-09-15 "GNU" "Linux Programmer's Manual"
  27 .SH NAME
  28 etext, edata, end \- end of program segments
  29 .SH SYNOPSIS
  30 .nf
  31 .BI extern " etext" ;
  32 .BI extern " edata" ;
  33 .BI extern " end" ;
  34 .fi
  35 .SH DESCRIPTION
  36 The addresses of these symbols indicate the end of various program
  37 segments:
  38 .TP
  39 .I etext
  40 This is the first address past the end of the text segment
  41 (the program code).
  42 .TP
  43 .I edata
  44 This is the first address past the end of the
  45 initialized data segment.
  46 .TP
  47 .I end
  48 This is the first address past the end of the
  49 uninitialized data segment (also known as the BSS segment).
  50 .SH CONFORMING TO
  51 Although these symbols have long been provided on most UNIX systems,
  52 they are not standardized; use with caution.
  53 .SH NOTES
  54 The program must explicitly declare these symbols;
  55 they are not defined in any header file.
  56 .PP
  57 On some systems the names of these symbols are preceded by underscores,
  58 thus:
  59 .IR _etext ,
  60 .IR _edata ,
  61 and
  62 .IR _end .
  63 These symbols are also defined for programs compiled on Linux.
  64 .PP
  65 At the start of program execution,
  66 the program break will be somewhere near
  67 .IR &end
  68 (perhaps at the start of the following page).
  69 However, the break will change as memory is allocated via
  70 .BR brk (2)
  71 or
  72 .BR malloc (3).
  73 Use
  74 .BR sbrk (2)
  75 with an argument of zero to find the current value of the program break.
  76 .SH EXAMPLE
  77 When run, the program below produces output such as the following:
  78 .PP
  79 .in +4n
  80 .EX
  81 .RB "$" " ./a.out"
  82 First address past:
  83     program text (etext)       0x8048568
  84     initialized data (edata)   0x804a01c
  85     uninitialized data (end)   0x804a024
  86 .EE
  87 .in
  88 .SS Program source
  89 \&
  90 .EX
  91 #include <stdio.h>
  92 #include <stdlib.h>
  93
  94 extern char etext, edata, end; /* The symbols must have some type,
  95                                    or "gcc \-Wall" complains */
  96
  97 int
  98 main(int argc, char *argv[])
  99 {
 100     printf("First address past:\\n");
 101     printf("    program text (etext)      %10p\\n", &etext);
 102     printf("    initialized data (edata)  %10p\\n", &edata);
 103     printf("    uninitialized data (end)  %10p\\n", &end);
 104
 105     exit(EXIT_SUCCESS);
 106 }
 107 .EE
 108 .SH SEE ALSO
 109 .BR objdump (1),
 110 .BR readelf (1),
 111 .BR sbrk (2),
 112 .BR elf (5)