Others have answered how malloc(0) works. I will answer one of the questions that you asked that hasn’t been answered yet (I think). The question is about realloc(malloc(0), 0): What does malloc(0) return? Would the answer be same for realloc(malloc(0),0)? The standard says this about realloc(ptr, size): if ptr is NULL, it behaves like malloc(size), … Read more
malloc is used for dynamic memory allocation. As said, it is dynamic allocation which means you allocate the memory at run time. For example when you don’t know the amount of memory during compile time. One example should clear this. Say you know there will be maximum 20 students. So you can create an array … Read more
I read that the maximum memory malloc can allocate is limited to physical memory (on heap). Wrong: most computers/OSs support virtual memory, backed by disk space. Some questions: does malloc allocate memory from HDD also? malloc asks the OS, which in turn may well use some disk space. What was the reason for above behavior? … Read more
The sbrksystem call moves the “border” of the data segment. This means it moves a border of an area in which a program may read/write data (letting it grow or shrink, although AFAIK no malloc really gives memory segments back to the kernel with that method). Aside from that, there’s also mmap which is used … Read more
It might appear to work fine, but it isn’t very safe at all. By writing data outside the allocated block of memory you are overwriting some data you shouldn’t. This is one of the greatest causes of segfaults and other memory errors, and what you’re observing with it appearing to work in this short program … Read more
You can’t malloc a class with non-trivial constructor in C++. What you get from malloc is a block of raw memory, which does not contain a properly constructed object. Any attempts to use that memory as a “real” object will fail. Instead of malloc-ing object, use new example *ex = new example; Your original code … Read more
You have a few options: GLIBC-specific solution (mostly Linux). If your compilation environment is glibc with gcc, the preferred way is to use malloc hooks. Not only it lets you specify custom malloc and free, but will also identify the caller by the return address on the stack. POSIX-specific solution. Define malloc and free as … Read more
Read one character at a time (using getc(stdin)) and grow the string (realloc) as you go. Here’s a function I wrote some time ago. Note it’s intended only for text input. char *getln() { char *line = NULL, *tmp = NULL; size_t size = 0, index = 0; int ch = EOF; while (ch) { … Read more
Case 1: In char Buffer[MAX_BUF]; Buffer is an array of size MAX_BUF. The allocation technique is called VLA. Case 2: In const int MAX_BUF = 1000; char* Buffer = malloc(MAX_BUF); Buffer is a pointer which is allocated a memory of size MAX_BUF which is 1000. and, an array is not the same as a pointer, … Read more
Question: “is malloc reentrant”? Answer: no, it is not. Here is one definition of what makes a routine reentrant. None of the common versions of malloc allow you to re-enter it (e.g. from a signal handler). Note that a reentrant routine may not use locks, and almost all malloc versions in existence do use locks … Read more