mirror of https://github.com/mkerrisk/man-pages
123 lines
3.2 KiB
Groff
123 lines
3.2 KiB
Groff
.\" Copyright (C) 2017, Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of
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.\" this manual under the conditions for verbatim copying, provided that
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.\" the entire resulting derived work is distributed under the terms of
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.\" a permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume.
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.\" no responsibility for errors or omissions, or for damages resulting.
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.\" from the use of the information contained herein. The author(s) may.
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.\" not have taken the same level of care in the production of this.
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.\" manual, which is licensed free of charge, as they might when working.
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH GETENTROPY 3 2017-09-15 "Linux" "Linux Programmer's Manual"
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.SH NAME
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getentropy \- fill a buffer with random bytes
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.PP
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.BI "int getentropy(void *" buffer ", size_t " length );
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.fi
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.PP
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR getentropy ():
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.nf
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_DEFAULT_SOURCE
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.fi
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.SH DESCRIPTION
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The
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.BR getentropy ()
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function writes
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.I length
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bytes of high-quality random data to the buffer starting
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at the location pointed to by
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.IR buffer .
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The maximum permitted value for the
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.I length
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argument is 256.
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.PP
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A successful call to
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.BR getentropy ()
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always provides the requested number of bytes of entropy.
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.SH RETURN VALUE
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On success, this function returns zero.
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On error, \-1 is returned, and
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.I errno
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EFAULT
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Part or all of the buffer specified by
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.I buffer
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and
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.I length
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is not in valid addressable memory.
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.TP
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.B EIO
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.I length
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is greater than 256.
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.TP
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.B EIO
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An unspecified error occurred while trying to overwrite
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.I buffer
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with random data.
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.TP
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.B ENOSYS
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This kernel version does not implement the
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.BR getrandom (2)
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system call required to implement this function.
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.SH VERSIONS
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The
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.BR getentropy ()
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function first appeared in glibc 2.25.
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.SH CONFORMING TO
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This function is nonstandard.
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It is also present on OpenBSD.
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.SH NOTES
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The
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.BR getentropy ()
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function is implemented using
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.BR getrandom (2).
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.PP
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Whereas the glibc wrapper makes
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.BR getrandom (2)
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a cancellation point,
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.BR getentropy ()
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is not a cancellation point.
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.PP
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.BR getentropy ()
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is also declared in
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.BR <sys/random.h> .
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(No feature test macro need be defined to obtain the declaration from
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that header file.)
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.PP
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A call to
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.BR getentropy ()
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may block if the system has just booted and the kernel has
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not yet collected enough randomness to initialize the entropy pool.
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In this case,
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.BR getentropy ()
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will keep blocking even if a signal is handled,
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and will return only once the entropy pool has been initialized.
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.SH SEE ALSO
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.BR getrandom (2),
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.BR urandom (4),
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.BR random (7)
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