Fix all the ambiguous or erroneous statements of the brk(2)
manpage by taking its text from NetBSD and editing it further. This also improves the page's mdoc(7) markup style. Reviewed by: ru Obtained from: NetBSD
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Yaroslav Tykhiy
parent
ef92e4471a
commit
b88d4f49fe
@ -32,7 +32,7 @@
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.\" @(#)brk.2 8.4 (Berkeley) 5/1/95
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.\" $FreeBSD$
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.\"
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.Dd May 1, 1995
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.Dd July 12, 1999
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.Dt BRK 2
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.Os
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.Sh NAME
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@ -49,96 +49,128 @@
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.Fn sbrk "int incr"
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.Sh DESCRIPTION
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.Bf -symbolic
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The brk and sbrk functions are historical curiosities
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left over from earlier days before the advent of virtual memory management.
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The
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.Fn brk
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and
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.Fn sbrk
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functions are legacy interfaces from before the
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advent of modern virtual memory management.
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.Ef
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.Pp
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The
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.Fn brk
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and
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.Fn sbrk
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functions are used to change the amount of memory allocated in a
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process's data segment.
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They do this by moving the location of the
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.Dq break .
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The break is the first address after the end of the process's
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uninitialized data segment (also known as the
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.Dq BSS ) .
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.Pp
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The
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.Fn brk
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function
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sets the break or lowest address
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of a process's data segment (uninitialized data) to
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.Fa addr
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(immediately above bss).
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Data addressing is restricted between
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.Fa addr
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and the lowest stack pointer to the stack segment.
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Memory is allocated by
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.Fa brk
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in page size pieces;
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if
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.Fa addr
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is not evenly divisible by the system page size, it is
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increased to the next page boundary.
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sets the break to
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.Fa addr .
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.Pp
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The
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.Fn sbrk
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function raises the break by
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.Fa incr
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bytes, thus allocating at least
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.Fa incr
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bytes of new memory in the data segment.
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If
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.Fa incr
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is negative,
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the break is lowered by
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.Fa incr
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bytes.
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.Sh NOTES
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While the actual process data segment size maintained by the kernel will only
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grow or shrink in page sizes, these functions allow setting the break
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to unaligned values (i.e., it may point to any address inside the last
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page of the data segment).
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.Pp
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The current value of the program break may be determined by calling
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.Fn sbrk 0 .
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See also
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.Xr end 3 .
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.Pp
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.\" The
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.\" .Nm sbrk
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.\" function
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.\" allocates chunks of
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.\" .Fa incr
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.\" bytes
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.\" to the process's data space
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.\" and returns an address pointer.
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.\" The
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.\" .Xr malloc 3
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.\" function utilizes
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.\" .Nm sbrk .
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.\" .Pp
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The current value of the program break is reliably returned by
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.Dq Li sbrk(0)
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(see also
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.Xr end 3 ) .
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The
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.Xr getrlimit 2
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system call may be used to determine
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the maximum permissible size of the
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.Em data
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segment;
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it will not be possible to set the break
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beyond the
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.Em rlim_max
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value returned from a call to
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.Xr getrlimit 2 ,
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e.g.\&
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.Dq Va etext No + Va rlp\->rlim_max .
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data segment.
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It will not be possible to set the break
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beyond
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.Dq Va etext No + Va rlim.rlim_max
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where the
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.Va rlim.rlim_max
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value is returned from a call to
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.Fn getrlimit RLIMIT_DATA &rlim .
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(See
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.Xr end 3
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for the definition of
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.Va etext ) .
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.Sh RETURN VALUES
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.Rv -std brk
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The
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.Fn brk
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function returns
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.Po Vt "char *" Pc Ns 0
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if successful;
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otherwise the value
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.Po Vt "char *" Pc Ns \-1
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is returned and the global variable
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.Va errno
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is set to indicate the error.
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.Pp
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The
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.Fn sbrk
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function returns a pointer to the base of the new storage if successful;
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otherwise the value \-1 is returned and the global variable
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function returns the prior break value if successful;
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otherwise the value
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.Po Vt "char *" Pc Ns \-1
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is returned and the global variable
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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.Fn Brk
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.Fn brk
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or
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.Fn sbrk
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will fail and no additional memory will be allocated if
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one of the following are true:
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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The requested break value was beyond the beginning of the data segment.
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.It Bq Er ENOMEM
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The limit, as set by
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The data segment size limit, as set by
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.Xr setrlimit 2 ,
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was exceeded.
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.It Bq Er ENOMEM
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The maximum possible size of a data segment (compiled into the
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system) was exceeded.
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.It Bq Er ENOMEM
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Insufficient space existed in the swap area
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to support the expansion.
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to support the expansion of the data segment.
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.El
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.Sh SEE ALSO
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.Xr execve 2 ,
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.Xr getrlimit 2 ,
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.Xr mmap 2 ,
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.Xr end 3 ,
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.Xr free 3 ,
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.Xr malloc 3
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.Sh BUGS
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Mixing
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.Fn brk
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or
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.Fn sbrk
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with
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.Xr malloc 3 ,
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.Xr free 3 ,
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or similar functions will result in non-portable program behavior.
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.Pp
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Setting the break may fail due to a temporary lack of
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swap space. It is not possible to distinguish this
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swap space.
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It is not possible to distinguish this
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from a failure caused by exceeding the maximum size of
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the data segment without consulting
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.Xr getrlimit 2 .
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