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:

The remaining bits in value are ignored.

Combining the above details, the following numeric values are meaningful for M_CHECK_ACTION:

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.

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 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): 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 upwards 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.

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.

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.

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:

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.

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).