Using mmap() instead of malloc()

Question

I am trying to complete an exercise that is done with system calls and need to allocate memory for a struct *. My code is:

myStruct * entry = (myStruct *)mmap(0, SIZEOF(myStruct), PROT_READ|PROT_WRITE, MAP_ANONYMOUS, -1, 0);

To clarify, I cannot use malloc() but can use mmap(). I was having no issues with this on Windows in Netbeans, now however I'm compiling and running from command line on Ubuntu I am getting "Segmentation Fault" each time I try to access it.

Is there a reason why it will work on one and not the other, and is mmap() a valid way of allocating memory in this fashion? My worry was I was going to be allocating big chunks of memory for each mmap() call initially, now I just cannot get it to run.

Additionally, the error returned my mmap is 22 - Invalid Argument (I did some troubleshooting while writing the question so the error check isn't in the above code). Address is 0, the custom SIZEOF() function works in other mmap arguments, I am using MAP_ANONYMOUS so the fd and offsetparameters must -1 and 0 respectively.

Is there something wrong with the PROT_READ|PROT_WRITE sections?

Just a wild guess, if you use 4096 or 8192 or some other power of two instead of SIZEOF(myStruct), does it work ? — Jabberwocky
– Jabberwocky, Commented Oct 19, 2016 at 8:52
I think the error is from your SIZEOF(). and you should add MAP PRIVATE — albttx
– albttx, Commented Oct 19, 2016 at 9:01
the SIZEOF() is not correct the actual item is spelled sizeof() I.E. all lower case. — user3629249
– user3629249, Commented Oct 19, 2016 at 9:03
the first parameter to mmap() is an address pointer, not an integer, Strongly suggest using NULL rather than 0 — user3629249
– user3629249, Commented Oct 19, 2016 at 9:05
Please, always make a complete, compilable example - an minimal reproducible example. Always. — davmac
– davmac, Commented Oct 19, 2016 at 10:10

Community · Accepted Answer · 2017-05-23 12:00:45Z

You need to specify MAP_PRIVATE in your flags.

myStruct * entry = (myStruct *)mmap(0, SIZEOF(myStruct), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

From the manual page:

The flags argument determines whether updates to the mapping are visible to other processes mapping the same region, and whether updates are carried through to the underlying file. This behavior is determined by including exactly one of the following values in flags:

You need exactly one of the flags MAP_PRIVATE or MAP_SHARED - but you didn't give either of them.

A complete example:

#include <sys/mman.h> #include <stdio.h> typedef struct { int a; int b; } myStruct; int main() { myStruct * entry = (myStruct *)mmap(0, sizeof(myStruct), PROT_READ|PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (entry == MAP_FAILED) { printf("Map failed.\n"); } else { entry->a = 4; printf("Success: entry=%p, entry->a = %d\n", entry, entry->a); } return 0; }

(The above, without MAP_PRIVATE of course, is a good example of what you might have provided as a an MCVE. This makes it much easier for others to help you, since they can see exactly what you've done, and test their proposed solutions. You should always provide an MCVE).

This was it, I misread the manual! I started with MAP_PRIVATE, moved to MAP_ANONYMOUS, needed both... Thanks a lot.

Toby Speight · Accepted Answer · 2016-10-19 10:30:13Z

The man page for mmap() says that you must specify exactly one of MAP_SHARED and MAP_PRIVATE in the flags argument. In your case, to act like malloc(), you'll want MAP_PRIVATE:

myStruct *entry = mmap(0, sizeof *entry, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

(I've also made this more idiomatic C by omitting the harmful cast and matching the sizeof to the actual variable rather than its type).

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