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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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mallopt(3) Library Functions Manual mallopt(3)
mallopt - set memory allocation parameters
Standard C library (libc, -lc)
#include <malloc.h>
int mallopt(int param, int value);
The mallopt() function adjusts parameters that control the
behavior of the memory-allocation functions (see malloc(3)). The
param argument specifies the parameter to be modified, and value
specifies the new value for that parameter.
The following values can be specified for param:
M_ARENA_MAX
If this parameter has a nonzero value, it defines a hard
limit on the maximum number of arenas that can be created.
An arena represents a pool of memory that can be used by
malloc(3) (and similar) calls to service allocation
requests. Arenas are thread safe and therefore may have
multiple concurrent memory requests. The trade-off is
between the number of threads and the number of arenas.
The more arenas you have, the lower the per-thread
contention, but the higher the memory usage.
The default value of this parameter is 0, meaning that the
limit on the number of arenas is determined according to
the setting of M_ARENA_TEST.
This parameter has been available since glibc 2.10 via
--enable-experimental-malloc, and since glibc 2.15 by
default. In some versions of the allocator there was no
limit on the number of created arenas (e.g., CentOS 5, RHEL
5).
When employing newer glibc versions, applications may in
some cases exhibit high contention when accessing arenas.
In these cases, it may be beneficial to increase
M_ARENA_MAX to match the number of threads. This is
similar in behavior to strategies taken by tcmalloc and
jemalloc (e.g., per-thread allocation pools).
M_ARENA_TEST
This parameter specifies a value, in number of arenas
created, at which point the system configuration will be
examined to determine a hard limit on the number of created
arenas. (See M_ARENA_MAX for the definition of an arena.)
The computation of the arena hard limit is implementation-
defined and is usually calculated as a multiple of the
number of available CPUs. Once the hard limit is computed,
the result is final and constrains the total number of
arenas.
The default value for the M_ARENA_TEST parameter is 2 on
systems where sizeof(long) is 4; otherwise the default
value is 8.
This parameter has been available since glibc 2.10 via
--enable-experimental-malloc, and since glibc 2.15 by
default.
The value of M_ARENA_TEST is not used when M_ARENA_MAX has
a nonzero value.
M_CHECK_ACTION
Setting this parameter controls how glibc responds when
various kinds of programming errors are detected (e.g.,
freeing the same pointer twice). The 3 least significant
bits (2, 1, and 0) of the value assigned to this parameter
determine the glibc behavior, as follows:
Bit 0 If this bit is set, then print a one-line message on
stderr that provides details about the error. The
message starts with the string "*** glibc
detected ***", followed by the program name, the
name of the memory-allocation function in which the
error was detected, a brief description of the
error, and the memory address where the error was
detected.
Bit 1 If this bit is set, then, after printing any error
message specified by bit 0, the program is
terminated by calling abort(3). Since glibc 2.4, if
bit 0 is also set, then, between printing the error
message and aborting, the program also prints a
stack trace in the manner of backtrace(3), and
prints the process's memory mapping in the style of
/proc/pid/maps (see proc(5)).
Bit 2 (since glibc 2.4)
This bit has an effect only if bit 0 is also set.
If this bit is set, then the one-line message
describing the error is simplified to contain just
the name of the function where the error was
detected and the brief description of the error.
The remaining bits in value are ignored.
Combining the above details, the following numeric values
are meaningful for M_CHECK_ACTION:
0 Ignore error conditions; continue execution
(with undefined results).
1 Print a detailed error message and continue
execution.
2 Abort the program.
3 Print detailed error message, stack trace, and
memory mappings, and abort the program.
5 Print a simple error message and continue
execution.
7 Print simple error message, stack trace, and
memory mappings, and abort the program.
Since glibc 2.3.4, the default value for the M_CHECK_ACTION
parameter is 3. In glibc 2.3.3 and earlier, the default
value is 1.
Using a nonzero M_CHECK_ACTION value can be useful because
otherwise a crash may happen much later, and the true cause
of the problem is then very hard to track down.
M_MMAP_MAX
This parameter specifies the maximum number of allocation
requests that may be simultaneously serviced using mmap(2).
This parameter exists because some systems have a limited
number of internal tables for use by mmap(2), and using
more than a few of them may degrade performance.
The default value is 65,536, a value which has no special
significance and which serves only as a safeguard. Setting
this parameter to 0 disables the use of mmap(2) for
servicing large allocation requests.
M_MMAP_THRESHOLD
For allocations greater than or equal to the limit
specified (in bytes) by M_MMAP_THRESHOLD that can't be
satisfied from the free list, the memory-allocation
functions employ mmap(2) instead of increasing the program
break using sbrk(2).
Allocating memory using mmap(2) has the significant
advantage that the allocated memory blocks can always be
independently released back to the system. (By contrast,
the heap can be trimmed only if memory is freed at the top
end.) On the other hand, there are some disadvantages to
the use of mmap(2): deallocated space is not placed on the
free list for reuse by later allocations; memory may be
wasted because mmap(2) allocations must be page-aligned;
and the kernel must perform the expensive task of zeroing
out memory allocated via mmap(2). Balancing these factors
leads to a default setting of 128*1024 for the
M_MMAP_THRESHOLD parameter.
The lower limit for this parameter is 0. The upper limit
is DEFAULT_MMAP_THRESHOLD_MAX: 512*1024 on 32-bit systems
or 4*1024*1024*sizeof(long) on 64-bit systems.
Note: Nowadays, glibc uses a dynamic mmap threshold by
default. The initial value of the threshold is 128*1024,
but when blocks larger than the current threshold and less
than or equal to DEFAULT_MMAP_THRESHOLD_MAX are freed, the
threshold is adjusted upward to the size of the freed
block. When dynamic mmap thresholding is in effect, the
threshold for trimming the heap is also dynamically
adjusted to be twice the dynamic mmap threshold. Dynamic
adjustment of the mmap threshold is disabled if any of the
M_TRIM_THRESHOLD, M_TOP_PAD, M_MMAP_THRESHOLD, or
M_MMAP_MAX parameters is set.
M_MXFAST (since glibc 2.3)
Set the upper limit for memory allocation requests that are
satisfied using "fastbins". (The measurement unit for this
parameter is bytes.) Fastbins are storage areas that hold
deallocated blocks of memory of the same size without
merging adjacent free blocks. Subsequent reallocation of
blocks of the same size can be handled very quickly by
allocating from the fastbin, although memory fragmentation
and the overall memory footprint of the program can
increase.
The default value for this parameter is 64*sizeof(size_t)/4
(i.e., 64 on 32-bit architectures). The range for this
parameter is 0 to 80*sizeof(size_t)/4. Setting M_MXFAST to
0 disables the use of fastbins.
M_PERTURB (since glibc 2.4)
If this parameter is set to a nonzero value, then bytes of
allocated memory (other than allocations via calloc(3)) are
initialized to the complement of the value in the least
significant byte of value, and when allocated memory is
released using free(3), the freed bytes are set to the
least significant byte of value. This can be useful for
detecting errors where programs incorrectly rely on
allocated memory being initialized to zero, or reuse values
in memory that has already been freed.
The default value for this parameter is 0.
M_TOP_PAD
This parameter defines the amount of padding to employ when
calling sbrk(2) to modify the program break. (The
measurement unit for this parameter is bytes.) This
parameter has an effect in the following circumstances:
• When the program break is increased, then M_TOP_PAD
bytes are added to the sbrk(2) request.
• When the heap is trimmed as a consequence of calling
free(3) (see the discussion of M_TRIM_THRESHOLD) this
much free space is preserved at the top of the heap.
In either case, the amount of padding is always rounded to
a system page boundary.
Modifying M_TOP_PAD is a trade-off between increasing the
number of system calls (when the parameter is set low) and
wasting unused memory at the top of the heap (when the
parameter is set high).
The default value for this parameter is 128*1024.
M_TRIM_THRESHOLD
When the amount of contiguous free memory at the top of the
heap grows sufficiently large, free(3) employs sbrk(2) to
release this memory back to the system. (This can be
useful in programs that continue to execute for a long
period after freeing a significant amount of memory.) The
M_TRIM_THRESHOLD parameter specifies the minimum size (in
bytes) that this block of memory must reach before sbrk(2)
is used to trim the heap.
The default value for this parameter is 128*1024. Setting
M_TRIM_THRESHOLD to -1 disables trimming completely.
Modifying M_TRIM_THRESHOLD is a trade-off between
increasing the number of system calls (when the parameter
is set low) and wasting unused memory at the top of the
heap (when the parameter is set high).
Environment variables
A number of environment variables can be defined to modify some of
the same parameters as are controlled by mallopt(). Using these
variables has the advantage that the source code of the program
need not be changed. To be effective, these variables must be
defined before the first call to a memory-allocation function.
(If the same parameters are adjusted via mallopt(), then the
mallopt() settings take precedence.) For security reasons, these
variables are ignored in set-user-ID and set-group-ID programs.
The environment variables are as follows (note the trailing
underscore at the end of the name of some variables):
MALLOC_ARENA_MAX
Controls the same parameter as mallopt() M_ARENA_MAX.
MALLOC_ARENA_TEST
Controls the same parameter as mallopt() M_ARENA_TEST.
MALLOC_CHECK_
This environment variable controls the same parameter as
mallopt() M_CHECK_ACTION. If this variable is set to a
nonzero value, then a special implementation of the memory-
allocation functions is used. (This is accomplished using
the malloc_hook(3) feature.) This implementation performs
additional error checking, but is slower than the standard
set of memory-allocation functions. (This implementation
does not detect all possible errors; memory leaks can still
occur.)
The value assigned to this environment variable should be a
single digit, whose meaning is as described for
M_CHECK_ACTION. Any characters beyond the initial digit
are ignored.
For security reasons, the effect of MALLOC_CHECK_ is
disabled by default for set-user-ID and set-group-ID
programs. However, if the file /etc/suid-debug exists (the
content of the file is irrelevant), then MALLOC_CHECK_ also
has an effect for set-user-ID and set-group-ID programs.
MALLOC_MMAP_MAX_
Controls the same parameter as mallopt() M_MMAP_MAX.
MALLOC_MMAP_THRESHOLD_
Controls the same parameter as mallopt() M_MMAP_THRESHOLD.
MALLOC_PERTURB_
Controls the same parameter as mallopt() M_PERTURB.
MALLOC_TRIM_THRESHOLD_
Controls the same parameter as mallopt() M_TRIM_THRESHOLD.
MALLOC_TOP_PAD_
Controls the same parameter as mallopt() M_TOP_PAD.
On success, mallopt() returns 1. On error, it returns 0.
On error, errno is not set.
A similar function exists on many System V derivatives, but the
range of values for param varies across systems. The SVID defined
options M_MXFAST, M_NLBLKS, M_GRAIN, and M_KEEP, but only the
first of these is implemented in glibc.
None.
glibc 2.0.
Specifying an invalid value for param does not generate an error.
A calculation error within the glibc implementation means that a
call of the form:
mallopt(M_MXFAST, n)
does not result in fastbins being employed for all allocations of
size up to n. To ensure desired results, n should be rounded up
to the next multiple greater than or equal to
(2k+1)*sizeof(size_t), where k is an integer.
If mallopt() is used to set M_PERTURB, then, as expected, the
bytes of allocated memory are initialized to the complement of the
byte in value, and when that memory is freed, the bytes of the
region are initialized to the byte specified in value. However,
there is an off-by-sizeof(size_t) error in the implementation:
instead of initializing precisely the block of memory being freed
by the call free(p), the block starting at p+sizeof(size_t) is
initialized.
The program below demonstrates the use of M_CHECK_ACTION. If the
program is supplied with an (integer) command-line argument, then
that argument is used to set the M_CHECK_ACTION parameter. The
program then allocates a block of memory, and frees it twice (an
error).
The following shell session shows what happens when we run this
program under glibc, with the default value for M_CHECK_ACTION:
$ ./a.out;
main(): returned from first free() call
*** glibc detected *** ./a.out: double free or corruption (top): 0x09d30008 ***
======= Backtrace: =========
/lib/libc.so.6(+0x6c501)[0x523501]
/lib/libc.so.6(+0x6dd70)[0x524d70]
/lib/libc.so.6(cfree+0x6d)[0x527e5d]
./a.out[0x80485db]
/lib/libc.so.6(__libc_start_main+0xe7)[0x4cdce7]
./a.out[0x8048471]
======= Memory map: ========
001e4000-001fe000 r-xp 00000000 08:06 1083555 /lib/libgcc_s.so.1
001fe000-001ff000 r--p 00019000 08:06 1083555 /lib/libgcc_s.so.1
[some lines omitted]
b7814000-b7817000 rw-p 00000000 00:00 0
bff53000-bff74000 rw-p 00000000 00:00 0 [stack]
Aborted (core dumped)
The following runs show the results when employing other values
for M_CHECK_ACTION:
$ ./a.out 1; # Diagnose error and continue
main(): returned from first free() call
*** glibc detected *** ./a.out: double free or corruption (top): 0x09cbe008 ***
main(): returned from second free() call
$ ./a.out 2; # Abort without error message
main(): returned from first free() call
Aborted (core dumped)
$ ./a.out 0; # Ignore error and continue
main(): returned from first free() call
main(): returned from second free() call
The next run shows how to set the same parameter using the
MALLOC_CHECK_ environment variable:
$ MALLOC_CHECK_=1 ./a.out;
main(): returned from first free() call
*** glibc detected *** ./a.out: free(): invalid pointer: 0x092c2008 ***
main(): returned from second free() call
Program source
#include <malloc.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
char *p;
if (argc > 1) {
if (mallopt(M_CHECK_ACTION, atoi(argv[1])) != 1) {
fprintf(stderr, "mallopt() failed");
exit(EXIT_FAILURE);
}
}
p = malloc(1000);
if (p == NULL) {
fprintf(stderr, "malloc() failed");
exit(EXIT_FAILURE);
}
free(p);
printf("%s(): returned from first free() call\n", __func__);
free(p);
printf("%s(): returned from second free() call\n", __func__);
exit(EXIT_SUCCESS);
}
mmap(2), sbrk(2), mallinfo(3), malloc(3), malloc_hook(3),
malloc_info(3), malloc_stats(3), malloc_trim(3), mcheck(3),
mtrace(3), posix_memalign(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 mallopt(3)
Pages that refer to this page: mallinfo(3), malloc(3), malloc_get_state(3), malloc_info(3), malloc_stats(3), malloc_trim(3), mcheck(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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