I have a program which mallocs and later frees a lot of memory ...

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I have a program which mallocs and later frees a lot of memory, but I can see from the operating system that memory usage doesn't actually go back down.

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Most implementations of malloc/free do not return freed memory to the operating system, but merely make it available for future malloc calls within the same program.

2016-03-28, 1502👍, 0💬

How does free know how many bytes to free?
How does free know how many bytes to free? The malloc/free implementation remembers the size of each block as it is allocated, so it is not necessary to remind it of the size when freeing. (Typically, the size is stored adjacent to the allocated block, which is why things usually break badly if the ... 2016-03-24, 1512👍, 0💬

I am allocating structures which contain pointers to other dynamically ....
I'm allocating structures which contain pointers to other dynamically-allocated objects. When I free a structure, do I also have to free each subsidiary pointer? Yes. malloc knows nothing about structure declarations or about the contents of allocated memory; it especially does not know whether allo... 2016-03-31, 1510👍, 0💬

I have a program which mallocs and later frees a lot of memory ...
I have a program which mallocs and later frees a lot of memory, but I can see from the operating system that memory usage doesn't actually go back down. Most implementations of malloc/free do not return freed memory to the operating system, but merely make it available for future malloc calls within... 2016-03-28, 1503👍, 0💬

Why isn't a pointer null after calling free? ...
Why isn't a pointer null after calling free? How unsafe is it to use (assign, compare) a pointer value after it's been freed? When you call free, the memory pointed to by the passed pointer is freed, but the value of the pointer in the caller probably remains unchanged, because C's pass-by-value sem... 2016-04-04, 1496👍, 0💬

When I call malloc to allocate memory for a pointer which is local to a function ....
When I call malloc to allocate memory for a pointer which is local to a function, do I have to explicitly free it? Yes. Remember that a pointer is different from what it points to. Local variablesare deallocated when the function returns, but in the case of a pointer variable, this means that the po... 2016-03-31, 1471👍, 0💬

Must I free allocated memory before the program exits?
Must I free allocated memory before the program exits? You shouldn't have to. A real operating system definitively reclaims all memory and other resources when a program exits; the system cannot afford to have memory integrity depend on the whims of random programs. (Strictly speaking, it is not eve... 2016-03-28, 1421👍, 0💬

So can I query the malloc package to find out how big an allocated block is?
So can I query the malloc package to find out how big an allocated block is? Unfortunately, there is no standard or portable way. (Some compilers provide nonstandard extensions.) If you need to know, you'll have to keep track of it yourself. 2016-03-24, 1377👍, 0💬

Is it legal to pass a null pointer as the first argument to realloc? Why would you want to?
Is it legal to pass a null pointer as the first argument to realloc? Why would you want to? ANSI C sanctions this usage (and the related realloc(..., 0), which frees), although several earlier implementations do not support it, so it may not be fully portable. Passing an initially-null pointer to re... 2016-03-16, 1344👍, 0💬

Having dynamically allocated an array , can I change its size?
Having dynamically allocated an array , can I change its size? Yes. This is exactly what realloc is for. Given a region of malloced memory its size can be changed using code like: dynarray = realloc(dynarray, 20 * sizeof(int)); Note that realloc may not always be able to enlarge memory regions in-pl... 2016-03-21, 1325👍, 0💬

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