Does the unlink function in C deallocate memory as well?

Question

So I'm going through a piece of code in one of the programs and it uses an unlink function , where

hist_name = malloc(128)

However, I did notice that the program did not use a "free" to free the memory , but it did have the unlink function shown below, at the end of the program:

unlink(hist_name);

My question is , does the unlink function free memory in addition to deleting the link to the file, or do I still need to insert a free(hist_name) statement to free the memory?

Answer 1

No. hist_name is leaked. unlink won't free the parameter.

Create a file called "a" and run this code (Compile with gcc test.c):

#include <unistd.h>
#include <stdlib.h>

int main(void){
    char* hist_name=malloc(128);
    //Fill hist_name
    hist_name[0]='a';
    hist_name[1]='\0';
    unlink(hist_name);
}

In the following, to show you, that unlink doesn't free your allocated memory, I will use valgrind. Valgrind is a tool that allows you to detect memory leaks and other memory issues (Like out-of-bounds accesses, use of uninitialized values,etc.)

So, if you run this program with valgrind (valgrind ./a.out), you get this output:

==2155== Memcheck, a memory error detector
==2155== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2155== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==2155== Command: ./a.out
==2155== 
==2155== 
==2155== HEAP SUMMARY:
==2155==     in use at exit: 128 bytes in 1 blocks
==2155==   total heap usage: 1 allocs, 0 frees, 128 bytes allocated
==2155== 
==2155== LEAK SUMMARY:
==2155==    definitely lost: 128 bytes in 1 blocks
==2155==    indirectly lost: 0 bytes in 0 blocks
==2155==      possibly lost: 0 bytes in 0 blocks
==2155==    still reachable: 0 bytes in 0 blocks
==2155==         suppressed: 0 bytes in 0 blocks
==2155== Rerun with --leak-check=full to see details of leaked memory
==2155== 
==2155== For lists of detected and suppressed errors, rerun with: -s
==2155== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

definitely lost: 128 bytes in 1 blocks

This means, you allocated memory once, 128 bytes, but didn't free it. ==> unlink doesn't free your memory for you.