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           #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:
                  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 behav-
                  ior, as follows:
                  Bit 0  If this bit is set, then  print  a  one-line  message  on
                         stderr  that  provides details about the error.  The mes-
                         sage 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  mes-
                         sage  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 func-
                         tion  where the error was detected and the brief descrip-
                         tion 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 unde-
                          fined results).
                       1  Print a detailed error message and continue execution.
                  Using a nonzero M_CHECK_ACTION value can be useful because  oth-
                  erwise  a crash may happen much later, and the true cause of the
                  problem is then very hard to track down.
                  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  sig-
                  nificance  and  which servers only as a safeguard.  Setting this
                  parameter to 0 disables the use of mmap(2) for  servicing  large
                  allocation requests.
                  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):  deal-
                  located  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 sat-
                  ized  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  pro-
                  grams  incorrectly rely on allocated memory being initialized to
                  zero, or reuse values in memory that has already been freed.
                  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
                  The default value for this parameter is 128*1024.
                  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 sig-
                  nificant 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 wast-
                  ing  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
                  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 sin-
                  gle 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.
                  Controls the same parameter as mallopt() M_MMAP_MAX.
                  Controls the same parameter as mallopt() M_MMAP_THRESHOLD.
                  Controls the same parameter as mallopt() M_PERTURB.
                  Controls the same parameter as mallopt() M_TRIM_THRESHOLD.
                  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  imple-
           mented 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


           The program below demonstrates the use of M_CHECK_ACTION.  If the  pro-
           gram  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: =========
               ======= Memory map: ========
               001e4000-001fe000 r-xp 00000000 08:06 1083555    /lib/
               001fe000-001ff000 r--p 00019000 08:06 1083555    /lib/
               [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
               $ ./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>
               p = malloc(1000);
               if (p == NULL) {
                   fprintf(stderr, "malloc() failed");
               printf("main(): returned from first free() call\n");
               printf("main(): returned from second free() call\n");


           mmap(2), sbrk(2), mallinfo(3), malloc(3), malloc_hook(3),
           malloc_info(3), malloc_stats(3), malloc_trim(3), mcheck(3), mtrace(3),

    Linux 2012-04-30 MALLOPT(3)


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