Linux error from munmap

Question

I have a simple question regarding mmap and munmap in Linux : is it possible that mmap succeeds but munmap fails?

Assuming all the parameters are correctly given, for example, see the following code snippet. In what circumstances munmap failed! will be printed??

char *addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

... exit if mmap was not successful ...
... do some stuff using mmaped area ...

if( munmap(addr, 4096) == -1 ){
    printf("munmap failed!\n");
}

score 1 · Answer 1 · answered Apr 01 '14 at 07:43

Yes, it can fail. From mmunmap man pages :

Upon successful completion, munmap() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

The error codes indicates that people do pass invalid parameters. But if you pass pointer you got from mmap(), and correct size, then it will not fail.

Assuming all the parameters are correctly given,

Then it will not fail. That is why most implementations (99%) just don't check the return value of unmmap(). In such case, even if it fails, you can't do anything (other then informing the user).

Unfortunately on Linux munmap() _can_ fail even when all parameters are valid and you are not splitting mapping in halfs. See http://stackoverflow.com/questions/43743555/munmap-failure-with-enomem-with-private-anonymous-mapping for example. — , May 03 '17 at 09:29

score 0 · Answer 2 · answered Jan 25 '17 at 23:25

munmap can fail with EINVAL if it receives an invalid parameter.

It can also fail with ENOMEM if the process's maximum number of mappings would have been exceeded. That can happen if you try to unmap a region in the middle of an existing map, and that results in two smaller maps.

You can see the maximum number of mappings per process with:

sysctl vm.max_map_count

You can increase it with, for example:

sysctl -w vm.max_map_count=131060

Be warned that if you don't check munmap's return, you will not have any clue when that causes a crash or another kind of failure, specially in long-running processes where the error can linger.

An munmap EINVAL, for instance, can point to a memory leak.

Also notice that mmap fails with MAP_FAILED, which is (void*)-1 and not NULL.

Linux error from munmap

2 Answers2