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
Short Answer: It doesn’t, it just happens to be zero in your case.(Also your test case doesn’t show that the data is zero. It only shows if one element is zero.) Long Answer: When you call malloc(), one of two things will happen: It recycles memory that was previous allocated and freed from the same … Read more
According to the specifications, malloc(0) will return either “a null pointer or a unique pointer that can be successfully passed to free()”. This basically lets you allocate nothing, but still pass the “artist” variable to a call to free() without worry. For practical purposes, it’s pretty much the same as doing: artist = NULL;
It is safer because you don’t have to mention the type name twice and don’t have to build the proper spelling for the “dereferenced” version of the type. For example, you don’t have to “count the stars” in int *****p = malloc(100 * sizeof *p); Compare that to the type-based sizeof in int *****p = … Read more
No, there is no way to get this information without depending strongly on the implementation details of malloc. In particular, malloc may allocate more bytes than you request (e.g. for efficiency in a particular memory architecture). It would be much better to redesign your code so that you keep track of n explicitly. The alternative … Read more
How should I pass a pointer to a function and allocate memory for the passed pointer from inside the called function? Ask yourself this: if you had to write a function that had to return an int, how would you do it? You’d either return it directly: int foo(void) { return 42; } or return … Read more
The short version: Always use calloc() instead of malloc()+memset(). In most cases, they will be the same. In some cases, calloc() will do less work because it can skip memset() entirely. In other cases, calloc() can even cheat and not allocate any memory! However, malloc()+memset() will always do the full amount of work. Understanding this … Read more
calloc() gives you a zero-initialized buffer, while malloc() leaves the memory uninitialized. For large allocations, most calloc implementations under mainstream OSes will get known-zeroed pages from the OS (e.g. via POSIX mmap(MAP_ANONYMOUS) or Windows VirtualAlloc) so it doesn’t need to write them in user-space. This is how normal malloc gets more pages from the OS … Read more
The answer is right there in the man page (at least on Linux): RETURN VALUE The alloca() function returns a pointer to the beginning of the allocated space. If the allocation causes stack overflow, program behaviour is undefined. Which isn’t to say it should never be used. One of the OSS projects I work on … Read more