mirror of https://github.com/mkerrisk/man-pages
252 lines
6.6 KiB
Groff
252 lines
6.6 KiB
Groff
.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\"
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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 this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" 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.\" manual page may be incorrect or out-of-date.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 no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" 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.
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.\" Modified Sat Jul 24 19:00:59 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Clarification concerning realloc, iwj10@cus.cam.ac.uk (Ian Jackson), 950701
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.\" Documented MALLOC_CHECK_, Wolfram Gloger (wmglo@dent.med.uni-muenchen.de)
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.\" 2007-09-15 mtk: added notes on malloc()'s use of sbrk() and mmap().
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.\"
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.TH MALLOC 3 2007-09-15 "GNU" "Linux Programmer's Manual"
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.SH NAME
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calloc, malloc, free, realloc \- Allocate and free dynamic memory
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.SH SYNOPSIS
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.nf
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.B #include <stdlib.h>
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.sp
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.BI "void *calloc(size_t " "nmemb" ", size_t " "size" );
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.br
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.BI "void *malloc(size_t " "size" );
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.br
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.BI "void free(void " "*ptr" );
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.br
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.BI "void *realloc(void " "*ptr" ", size_t " "size" );
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.fi
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.SH DESCRIPTION
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.BR calloc ()
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allocates memory for an array of
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.I nmemb
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elements of
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.I size
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bytes each and returns a pointer to the allocated memory.
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The memory is set to zero.
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If
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.I nmemb
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or
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.I size
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is 0, then
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.BR calloc ()
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returns either NULL,
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.\" glibc does this:
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or a unique pointer value that can later be successfully passed to
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.BR free ().
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.PP
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.BR malloc ()
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allocates
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.I size
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bytes and returns a pointer to the allocated memory.
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The memory is not cleared.
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If
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.I size
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is 0, then
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.BR malloc ()
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returns either NULL,
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.\" glibc does this:
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or a unique pointer value that can later be successfully passed to
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.BR free ().
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.PP
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.BR free ()
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frees the memory space pointed to by
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.IR ptr ,
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which must have been returned by a previous call to
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.BR malloc (),
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.BR calloc ()
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or
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.BR realloc ().
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Otherwise, or if
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.I free(ptr)
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has already been called before, undefined behavior occurs.
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If
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.I ptr
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is NULL, no operation is performed.
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.PP
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.BR realloc ()
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changes the size of the memory block pointed to by
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.I ptr
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to
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.I size
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bytes.
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The contents will be unchanged to the minimum of the old and new sizes;
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newly allocated memory will be uninitialized.
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If
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.I ptr
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is NULL, the call is equivalent to
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.IR malloc(size) ;
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if
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.I size
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is equal to zero,
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the call is equivalent to
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.IR free(ptr) .
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Unless
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.I ptr
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is NULL, it must have been returned by an earlier call to
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.BR malloc (),
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.BR calloc ()
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or
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.BR realloc ().
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If the area pointed to was moved, a
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.I free(ptr)
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is done.
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.SH "RETURN VALUE"
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For
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.BR calloc ()
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and
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.BR malloc (),
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the value returned is a pointer to the allocated memory, which is suitably
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aligned for any kind of variable, or NULL if the request fails.
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.PP
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.BR free ()
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returns no value.
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.PP
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.BR realloc ()
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returns a pointer to the newly allocated memory, which is suitably
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aligned for any kind of variable and may be different from
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.IR ptr ,
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or NULL if the request fails.
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If
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.I size
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was equal to 0, either NULL or a pointer suitable to be passed to
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.BR free ()
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is returned.
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If
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.BR realloc ()
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fails the original block is left untouched; it is not freed or moved.
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.SH "CONFORMING TO"
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C89, C99.
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.SH NOTES
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Normally,
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.BR malloc ()
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allocates memory from the heap, and adjusts the size of the heap
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as required, using
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.BR sbrk (2).
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When allocating blocks of memory larger than
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.B MMAP_THRESHOLD
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bytes, the glibc
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.BR malloc ()
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implementation allocates the memory as a private anonymous mapping using
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.BR mmap (2).
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.B MMAP_THRESHOLD
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is 128 kB by default, but is adjustable using
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.BR mallopt (3).
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.\" FIXME . there is no mallopt(3) man page yet.
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Allocations performed using
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.BR mmap (2)
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are unaffected by the
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.B RLIMIT_DATA
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resource limit (see
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.BR getrlimit (2)).
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The Unix98 standard requires
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.BR malloc (),
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.BR calloc (),
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and
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.BR realloc ()
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to set
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.I errno
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to
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.B ENOMEM
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upon failure.
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Glibc assumes that this is done
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(and the glibc versions of these routines do this); if you
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use a private malloc implementation that does not set
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.IR errno ,
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then certain library routines may fail without having
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a reason in
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.IR errno .
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.LP
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Crashes in
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.BR malloc (),
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.BR calloc (),
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.BR realloc (),
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or
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.BR free ()
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are almost always related to heap corruption, such as overflowing
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an allocated chunk or freeing the same pointer twice.
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.PP
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Recent versions of Linux libc (later than 5.4.23) and glibc (2.x)
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include a
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.BR malloc ()
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implementation which is tunable via environment variables.
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When
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.B MALLOC_CHECK_
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is set, a special (less efficient) implementation is used which
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is designed to be tolerant against simple errors, such as double
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calls of
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.BR free ()
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with the same argument, or overruns of a single byte (off-by-one
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bugs).
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Not all such errors can be protected against, however, and
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memory leaks can result.
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If
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.B MALLOC_CHECK_
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is set to 0, any detected heap corruption is silently ignored;
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if set to 1, a diagnostic message is printed on \fIstderr\fP;
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if set to 2,
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.BR abort (3)
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is called immediately;
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if set to 3, a diagnostic message is printed on \fIstderr\fP
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and the program is aborted.
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Using a nonzero
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.B MALLOC_CHECK_
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value can be useful because otherwise
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a crash may happen much later, and the true cause for the problem
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is then very hard to track down.
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.SH BUGS
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By default, Linux follows an optimistic memory allocation strategy.
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This means that when
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.BR malloc ()
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returns non-NULL there is no guarantee that the memory really
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is available.
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This is a really bad bug.
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In case it turns out that the system is out of memory,
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one or more processes will be killed by the infamous OOM killer.
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In case Linux is employed under circumstances where it would be
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less desirable to suddenly lose some randomly picked processes,
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and moreover the kernel version is sufficiently recent,
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one can switch off this overcommitting behavior using a command like:
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.in +4n
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.nf
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# echo 2 > /proc/sys/vm/overcommit_memory
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.fi
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.in
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See also the kernel Documentation directory, files
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.I vm/overcommit-accounting
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and
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.IR sysctl/vm.txt .
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.SH "SEE ALSO"
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.BR brk (2),
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.\" .BR mallopt (3),
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.BR mmap (2),
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.BR alloca (3),
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.BR posix_memalign (3)
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