This is actually a really hard, imponderable question. Pro (in favor of freeing everything before exit): no bugs or memory leaks later if code is rearranged no false positives from valgrind or memory leak checker no memory leaks if you’re running under a buggy OS, or no OS at all Con (just exit, don’t worry … Read more
Un-deterministic. Cannot say. You need to supply the exact pointer which was returned by malloc(). Passing a pointer to free() which is not returned by malloc() [and family] is undefined behaviour. As per your question, if the malloc()-ed pointer is stored in p, and you do p++ and call free(p);, then there p is not … Read more
7.20.3.2 The free function Synopsis #include <stdlib.h> void free(void *ptr); Description The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. See ISO-IEC 9899. That being said, when looking at different codebases in the wild, you’ll … Read more
If you’re using glibc, you can set the MALLOC_CHECK_ environment variable to 2, this will cause glibc to use an error tolerant version of malloc, which will cause your program to abort at the point where the double free is done. You can set this from gdb by using the set environment MALLOC_CHECK_ 2 command … Read more
Setting unused pointers to NULL is a defensive style, protecting against dangling pointer bugs. If a dangling pointer is accessed after it is freed, you may read or overwrite random memory. If a null pointer is accessed, you get an immediate crash on most systems, telling you right away what the error is. For local … Read more
The following analysis applies only to glibc (based on the ptmalloc2 algorithm). There are certain options that seem helpful to return the freed memory back to the system: mallopt() (defined in malloc.h) does provide an option to set the trim threshold value using one of the parameter option M_TRIM_THRESHOLD, this indicates the minimum amount of … Read more
When you malloc a block, it actually allocates a bit more memory than you asked for. This extra memory is used to store information such as the size of the allocated block, and a link to the next free/used block in a chain of blocks, and sometimes some “guard data” that helps the system to … Read more
When you call malloc(), you specify the amount of memory to allocate. The amount of memory actually used is slightly more than this, and includes extra information that records (at least) how big the block is. You can’t (reliably) access that other information – and nor should you :-). When you call free(), it simply … Read more
Just about every modern operating system will recover all the allocated memory space after a program exits. The only exception I can think of might be something like Palm OS where the program’s static storage and runtime memory are pretty much the same thing, so not freeing might cause the program to take up more … Read more
OK some answers about malloc were already posted. The more interesting part is how free works (and in this direction, malloc too can be understood better). In many malloc/free implementations, free does normally not return the memory to the operating system (or at least only in rare cases). The reason is that you will get … Read more