问题
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char* f(void) {
char *x;
x = malloc(sizeof(char) * 4);
strcpy(x, "abc");
return(x);
}
int main(void) {
char *a;
a = f();
printf("%s", a);
free(a);
return(0);
}
Does the variable x in the function have to be freed? If so, how is that possible when I need to return it?
回答1:
Does the variable x in the function have to be freed?
Yes (kinda, see my later comment). Every call to malloc requires a later call to free. Otherwise, you have a leak. Remember though; you are not "free[ing] x", you are freeing the memory that x refers to.
When you return x a copy of the value (an address) of x is made and returned to the caller. x was declared with automatic storage duration. It is the memory it refers to that must be freed.
If so how is that possible when I need to return it.
Your design has placed the onus on the caller to free the memory. You have already done this in main. Of course, using this method requires that you document the function so that users of your code know that they are receiving an address to memory which was dynamically allocated.
A better approach (IMO) is to take a buffer as an input parameter. Now it is very clear who is responsible for managing this memory (i.e., the caller). Perhaps I don't even want to dynamically allocate it. With this design it is my choice.
void f(char *buf, size_t buf_size) {
strncpy(buf, "abc", buf_size - 1);
buf[buf_size-1] = '\0';
}
On a side note, you should always be checking the return value of malloc. It can fail, in which case a null pointer will be returned. Also, sizeof(char) is guaranteed to be 1 by the standard, so you can remove that bit and just say malloc(n).
回答2:
Yes it should be freed by the caller. Such as the free in your main.
When you return x in the function f, a copy of the address is passed back to the caller. The caller can call free on that address.
回答3:
Yes you need to free, but it's already being freed when you do free(a); in main (since a is assigned to equal x in the line a = f();).
回答4:
x is destroyed after you exit the function. The value of x is an address that points to a dynamically allocated object. The object still exists after you exit the function.
To free the dynamically object you have to pass the value of x that is returned by the f function to the free function.
回答5:
When you call malloc it allocates memory and returns the initial address of that memory block.
In your case you return this address back to the callee, now the callee, main, is responsible for deallocating it.
The only thing required to deallocate a memory block is the initial address, AFIK the OS is in charge of allocating/freeing memory it uses fairly complex algorithms, but the programs only need to keep track the initial memory address. This address can be stored or moved around just like any other integer value because in 32 bit systems thats all it is just an int value.
回答6:
You code work well, you freed a which is the string malloced by f.
来源：https://stackoverflow.com/questions/11892962/how-do-i-free-a-pointer-returned-from-a-function