man3/end.3

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.TH END 3 2019-03-06 "GNU" "Linux Programmer's Manual"
.SH NAME
etext, edata, end \- end of program segments
.SH SYNOPSIS
.nf
.BI extern " etext" ;
.BI extern " edata" ;
.BI extern " end" ;
.fi
.SH DESCRIPTION
The addresses of these symbols indicate the end of various program
segments:
.TP
.I etext
This is the first address past the end of the text segment
(the program code).
.TP
.I edata
This is the first address past the end of the
initialized data segment.
.TP
.I end
This is the first address past the end of the
uninitialized data segment (also known as the BSS segment).
.SH CONFORMING TO
Although these symbols have long been provided on most UNIX systems,
they are not standardized; use with caution.
.SH NOTES
The program must explicitly declare these symbols;
they are not defined in any header file.
.PP
On some systems the names of these symbols are preceded by underscores,
thus:
.IR _etext ,
.IR _edata ,
and
.IR _end .
These symbols are also defined for programs compiled on Linux.
.PP
At the start of program execution,
the program break will be somewhere near
.IR &end
(perhaps at the start of the following page).
However, the break will change as memory is allocated via
.BR brk (2)
or
.BR malloc (3).
Use
.BR sbrk (2)
with an argument of zero to find the current value of the program break.
.SH EXAMPLE
When run, the program below produces output such as the following:
.PP
.in +4n
.EX
.RB "$" " ./a.out"
First address past:
    program text (etext)       0x8048568
    initialized data (edata)   0x804a01c
    uninitialized data (end)   0x804a024
.EE
.in
.SS Program source
\&
.EX
#include <stdio.h>
#include <stdlib.h>

extern char etext, edata, end; /* The symbols must have some type,
                                   or "gcc \-Wall" complains */

int
main(int argc, char *argv[])
{
    printf("First address past:\en");
    printf("    program text (etext)      %10p\en", &etext);
    printf("    initialized data (edata)  %10p\en", &edata);
    printf("    uninitialized data (end)  %10p\en", &end);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR objdump (1),
.BR readelf (1),
.BR sbrk (2),
.BR elf (5)