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NAME

       mallopt - set memory allocation parameters

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <malloc.h>

       int mallopt(int param, int value);

DESCRIPTION

       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.

RETURN VALUE

       On success, mallopt() returns 1.  On error, it returns 0.

ERRORS

       On error, errno is not set.

VERSIONS

       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.

STANDARDS

       None.

HISTORY

       glibc 2.0.

BUGS

       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.

EXAMPLES

       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);
       }

SEE ALSO

       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)