If we are talking about the standard free function then its prototype is void free(void *ptr); Therefore the cast is completely useless. Now some speculation. The author might have forgotten to include the stdlib.h header declaring this prototype, so the compiler is assuming the return type of it as int. Now during static analysis of … Read more
Several people have posted the right answer, but they keep deleting it for some reason. You need to cast it to a non-const pointer; free takes a void*, not a const void*: free((char*)str);
One-argument free(void *) (introduced in Unix V7) has another major advantage over the earlier two-argument mfree(void *, size_t) which I haven’t seen mentioned here: one argument free dramatically simplifies every other API that works with heap memory. For example, if free needed the size of the memory block, then strdup would somehow have to return … 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
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
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
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
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