How to free memory allocated using mmap?

Question

I have allocated code using mmap, but unable to free it because of segmentation fault. I have done mprotect - PROT_WRITE to make it writable, but still, I am unable to free it. My code:

 1 #include <stdio.h> 2 #include <memory.h> 3 #include <stdlib.h> 4 #include <unistd.h> 5 #include <sys/mman.h> 6 #include <sys/types.h> 7 #include <fcntl.h> 8 9 int main() 10 { 11 void * allocation; 12 size_t size; 13 static int devZerofd = -1; 14 15 if ( devZerofd == -1 ) { 16 devZerofd = open("/dev/zero", O_RDWR); 17 if ( devZerofd < 0 ) 18 perror("open() on /dev/zero failed"); 19 } 20 21 allocation = (caddr_t) mmap(0, 5000, PROT_READ|PROT_NONE, MAP_PRIVATE, devZerofd, 0); 22 23 if ( allocation == (caddr_t)-1 ) 24 fprintf(stderr, "mmap() failed "); 25 26 if ( mprotect((caddr_t)allocation, 5000, PROT_WRITE) < 0 ) 27 fprintf(stderr, "mprotect failed"); 28 else 29 printf("mprotect done: memory allocated at address %u\n",allocation); 30 31 strcpy(allocation,"Hello, how are you"); 32 puts(allocation); 33 34 if ( mprotect((caddr_t)allocation, 5000, PROT_WRITE) < 0 ) 35 fprintf(stderr, "mprotect failed"); 36 37 free(allocation); 38 39 } 40 41

use munmap function its syntex is " int munmap(void *addr, size_t len);" — Jeegar Patel
– Jeegar Patel, Commented Aug 8, 2011 at 9:15

cnicutar · Accepted Answer · 2011-08-08 09:11:09Z

28

You need to use munmap for that. You don't need to do anything else (change protection bits etc). But you should check the return code of munmap.

munmap(allocation, 5000);

free(3) can only be used to free memory allocated via malloc(3).

answered Aug 8, 2011 at 9:11

cnicutar

184k26 gold badges378 silver badges398 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

RajSanpui Over a year ago

If i use munmap, the last 2 lines, mprotect - PROT_WRITE is also not needed, right? I mean can munmap be done on protected memory?

Bionix1441 Over a year ago

Can you please tell me if I can use free to free the memory allocated by mmap.

cnicutar Over a year ago

@Bionix1441 In general, no. It is only safe to call free for memory allocated using malloc or realloc.

Collectives™ on Stack Overflow

How to free memory allocated using mmap?

1 Answer 1

3 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Linked

Related