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https://github.com/freebsd/freebsd-src
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8269e7673c
Remove core system call implementations and documentation to lib/libsys and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>. Update paths to allow libc to find them in their new home. Reviewed by: kib, emaste, imp Pull Request: https://github.com/freebsd/freebsd-src/pull/908
187 lines
4.7 KiB
Groff
187 lines
4.7 KiB
Groff
.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.Dd June 2, 2018
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.Dt BRK 2
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.Os
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.Sh NAME
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.Nm brk ,
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.Nm sbrk
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.Nd change data segment size
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft int
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.Fn brk "const void *addr"
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.Ft void *
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.Fn sbrk "intptr_t incr"
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.Sh DESCRIPTION
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.Bf -symbolic
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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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They are deprecated and not present on the arm64 or riscv architectures.
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The
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.Xr mmap 2
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interface should be used to allocate pages instead.
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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 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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.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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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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.Pp
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The
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.Fn sbrk
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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 "void *" 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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The
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.Fn brk
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and
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.Fn sbrk
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functions
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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 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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Insufficient space existed in the swap area
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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 HISTORY
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The
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.Fn brk
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function appeared in
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.At v7 .
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.Fx 11.0
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introduced the arm64 and riscv architectures which do not support
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.Fn brk
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or
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.Fn sbrk .
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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.
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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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.Pp
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.Fn sbrk
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is sometimes used to monitor heap use by calling with an argument of 0.
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The result is unlikely to reflect actual utilization in combination with an
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.Xr mmap 2
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based malloc.
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.Pp
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.Fn brk
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and
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.Fn sbrk
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are not thread-safe.
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