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MALLOC_TRIM(3) Linux Programmer's Manual MALLOC_TRIM(3)


malloc_trim - release free memory from the heap


#include <malloc.h>

int malloc_trim(size_t pad);


The malloc_trim() function attempts to release free memory from the heap (by calling sbrk(2) or madvise(2) with suitable arguments).

The pad argument specifies the amount of free space to leave untrimmed at the top of the heap. If this argument is 0, only the minimum amount of memory is maintained at the top of the heap (i.e., one page or less). A nonzero argument can be used to maintain some trailing space at the top of the heap in order to allow future allocations to be made without having to extend the heap with sbrk(2).


The malloc_trim() function returns 1 if memory was actually released back to the system, or 0 if it was not possible to release any memory.


No errors are defined.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
malloc_trim () Thread safety MT-Safe


This function is a GNU extension.


This function is automatically called by free(3) in certain circumstances; see the discussion of M_TOP_PAD and M_TRIM_THRESHOLD in mallopt(3).

Only the main heap (using sbrk(2)) honors the pad argument; thread heaps do not.

Since glibc 2.8 this function frees memory in all arenas and in all chunks with whole free pages.

Before glibc 2.8 this function only freed memory at the top of the heap in the main arena.


sbrk(2), malloc(3), mallopt(3)


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2020-08-13 Linux