.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
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.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
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.\" have taken the same level of care in the production of this manual,
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.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" License.
.\" Modified Sat Jul 24 19:00:59 1993 by Rik Faith (faith@cs.unc.edu)
.\" Clarification concerning realloc, iwj10@cus.cam.ac.uk (Ian Jackson), 950701
.\" Documented MALLOC_CHECK_, Wolfram Gloger (wmglo@dent.med.uni-muenchen.de)
.\"
.TH MALLOC 3  "April 4, 1993" "GNU" "Linux Programmer's Manual"
.SH NAME
calloc, malloc, free, realloc \- Allocate and free dynamic memory
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "void *calloc(size_t " "nmemb" ", size_t " "size" ");"
.nl
.BI "void *malloc(size_t " "size" ");"
.nl
.BI "void free(void " "*ptr" ");"
.nl
.BI "void *realloc(void " "*ptr" ", size_t "  "size" ");"
.fi
.SH DESCRIPTION
.B calloc()
allocates memory for an array of 
.I nmemb
elements of 
.I size
bytes each and returns a pointer to the allocated memory. 
The memory is set to zero.
.PP
.B malloc()
allocates
.I size
bytes and returns a pointer to the allocated memory. 
The memory is not cleared.
.PP
.B free()
frees the memory space pointed to by
.IR ptr ,
which must have been returned by a previous call to
.BR malloc() ,
.B calloc()
or
.BR realloc() .
Otherwise, or if
.BI "free(" "ptr" )
has already been called before, undefined behaviour occurs.
If
.I ptr
is
.BR NULL ,
no operation is performed.
.PP
.B realloc()
changes the size of the memory block pointed to by
.I ptr
to
.I size
bytes.
The contents will be unchanged to the minimum of the old and new sizes;
newly allocated memory will be uninitialized.
If
.I ptr
is
.BR NULL ,
the call is equivalent to
.BR malloc(size) ;
if size is equal to zero,
the call is equivalent to
.BI "free(" "ptr" ) .
Unless
.I ptr
is
.BR NULL ,
it must have been returned by an earlier call to
.BR malloc() ,
.BR calloc()
or
.BR realloc() .
.SH "RETURN VALUES"
For
.BR calloc() " and " malloc() ,
the value returned is a pointer to the allocated memory, which is suitably
aligned for any kind of variable, or
.B NULL
if the request fails.
.PP
.B free()
returns no value.
.PP
.B realloc()
returns a pointer to the newly allocated memory, which is suitably
aligned for any kind of variable and may be different from
.IR ptr ,
or
.B NULL
if the request fails or if size was equal to 0.  If
.B realloc()
fails the original block is left untouched - it is not freed or moved.
.SH "CONFORMING TO"
ANSI-C
.SH "SEE ALSO"
.BR brk (2)
.SH NOTES
The Unix98 standard requires
.BR malloc() ,
.BR calloc() ,
and
.BR realloc ()
to set
.I errno
to ENOMEM upon failure. Glibc assumes that this is done
(and the glibc versions of these routines do this); if you
use a private malloc implementation that does not set
.IR errno ,
then certain library routines may fail without having
a reason in
.IR errno .
.LP
Crashes in
.BR malloc() ,
.BR free()
or
.BR realloc()
are almost always related to heap corruption, such as overflowing
an allocated chunk or freeing the same pointer twice.
.PP
Recent versions of Linux libc (later than 5.4.23) and GNU libc (2.x)
include a malloc implementation which is tunable via environment
variables.  When
.BR MALLOC_CHECK_
is set, a special (less efficient) implementation is used which
is designed to be tolerant against simple errors, such as double
calls of
.BR free()
with the same argument, or overruns of a single byte (off-by-one
bugs).  Not all such errors can be proteced against, however, and
memory leaks can result.
If
.BR MALLOC_CHECK_
is set to 0, any detected heap corruption is silently ignored;
if set to 1, a diagnostic is printed on stderr;
if set to 2,
.BR abort()
is called immediately.  This can be useful because otherwise
a crash may happen much later, and the true cause for the problem
is then very hard to track down.