Question

this, I hope, is a simple question. I am familiar with the idea that fork in unix/linux/etc does not actually copy an entire image, but maps shared memory as private with a copy-on-write flag. To illustrate this, I tried the following example below which I expected would show the large malloc’d region as private (copy on write). However, I get the output below. Can anyone help shed light on why this is? My assumption is that my OS works as expected (uname -a: Linux xxxxxx 2.6.32-279.19.1.el6.x86_64 #1 SMP Tue Dec 18 17:22:54 CST 2012 x86_64 x86_64 x86_64 GNU/Linux) but pmap is not working as I expect..

int main(int argc, char *argv[]) {

    pid_t pid;
    char syscmd[80];
    char *somebuffer=(char*)malloc(999999999l);
    pid = fork();
    if (0 == pid) { /* Child */
            sprintf(syscmd, "pmap -x %d", getpid());
            system(syscmd);

    } else { /* parent */
            wait(NULL);
    }

    return 0;
}

output:

9822:   ./a.out
Address           Kbytes     RSS   Dirty Mode   Mapping
0000000000400000       4       4       0 r-x--  a.out
0000000000600000       4       4       4 rw---  a.out
000000357e000000     128      24       0 r-x--  ld-2.12.so
000000357e21f000       4       4       4 r----  ld-2.12.so
000000357e220000       4       4       4 rw---  ld-2.12.so
000000357e221000       4       4       4 rw---    [ anon ]
000000357e400000    1572     120       0 r-x--  libc-2.12.so
000000357e589000    2048       0       0 -----  libc-2.12.so
000000357e789000      16      12       8 r----  libc-2.12.so
000000357e78d000       4       4       4 rw---  libc-2.12.so
000000357e78e000      20      16      16 rw---    [ anon ]
00007f64228ad000  976576      16      16 rw---    [ anon ]
00007f645e27b000       4       4       4 rw---    [ anon ]
00007f645e27c000       4       0       0 r-x--    [ anon ]
00007fff2a1eb000      84      12      12 rw---    [ stack ]
ffffffffff600000       4       0       0 r-x--    [ anon ]
----------------  ------  ------  ------
total kB          980480     228      76

:: why isn’t that large region (and probably others) marked as private?

Thanks!

Answer 1

I think the pmap program just fails to print this information. Why are you using an odd utility like that instead of just cat /proc/%d/maps?