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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
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MALLOPT(3) Linux Programmer's Manual MALLOPT(3)
mallopt - set memory allocation parameters
#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). In glibc versions since 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 version 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.
This function is not specified by POSIX or the C standards. 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.
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 ini‐
tializing 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 MAL‐
LOC_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("main(): returned from first free() call\n");
free(p);
printf("main(): returned from second free() call\n");
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 release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 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)
Copyright and license for this manual page
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