I am trying to understand memory mapping in Linux, and wrote below program for the same.
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#define MEM_SIZE 1*1024*1024*1024 // 1GB
#define PAGE_SIZE 4*1024
#define NO_OF_PAGES (MEM_SIZE)/(PAGE_SIZE)
// Global array of size 1 GB
int mem[MEM_SIZE/sizeof(int)] = {
0,1,2,
};
void *func(void *args)
{
while (1) {
for (int i=0; i<NO_OF_PAGES; i++) {
mem[(i*PAGE_SIZE)/sizeof(int)] = 1; // Update first 4-bytes of each page.
}
// All pages are updated.
fflush(stdout);
printf("."); // Each dot(.) represents update of all pages in global data.
}
return NULL;
}
int main() {
pthread_t t;
pthread_create(&t, NULL, func, NULL);
printf("A thread is continually updating all pages of global 1GB data. (pid : %d)\n", getpid());
pthread_join(t, NULL);
return 0;
}
It executes like this,
[ ] gcc test.c
[ ] ./a.out
A thread is continually updating all pages of global 1GB data. (pid : 4086)
.......................
Here, a thread is continually updating all pages of the global data used in this program.
Hence, this process should consume ~1GB (MEM_SIZE) in memory.
But the process memory looks like below, (I can not see 1GB of data memory being used here).
[ ]# cat /proc/4086/maps
55eac3b7c000-55eac3b7d000 r--p 00000000 08:01 1197055 /root/testing/a.out
55eac3b7d000-55eac3b7e000 r-xp 00001000 08:01 1197055 /root/testing/a.out
55eac3b7e000-55eac3b7f000 r--p 00002000 08:01 1197055 /root/testing/a.out
55eac3b7f000-55eac3b80000 r--p 00002000 08:01 1197055 /root/testing/a.out
55eac3b80000-55eb03b81000 rw-p 00003000 08:01 1197055 /root/testing/a.out
55eb047b5000-55eb047d6000 rw-p 00000000 00:00 0 [heap]
7f19c791b000-7f19c791c000 ---p 00000000 00:00 0
7f19c791c000-7f19c811e000 rw-p 00000000 00:00 0
7f19c811e000-7f19c8140000 r--p 00000000 08:01 1969450 /usr/lib/libc.so.6
7f19c8140000-7f19c829b000 r-xp 00022000 08:01 1969450 /usr/lib/libc.so.6
7f19c829b000-7f19c82f2000 r--p 0017d000 08:01 1969450 /usr/lib/libc.so.6
7f19c82f2000-7f19c82f6000 r--p 001d4000 08:01 1969450 /usr/lib/libc.so.6
7f19c82f6000-7f19c82f8000 rw-p 001d8000 08:01 1969450 /usr/lib/libc.so.6
7f19c82f8000-7f19c8307000 rw-p 00000000 00:00 0
7f19c8311000-7f19c8312000 r--p 00000000 08:01 1969441 /usr/lib/ld-linux-x86-64.so.2
7f19c8312000-7f19c8338000 r-xp 00001000 08:01 1969441 /usr/lib/ld-linux-x86-64.so.2
7f19c8338000-7f19c8342000 r--p 00027000 08:01 1969441 /usr/lib/ld-linux-x86-64.so.2
7f19c8342000-7f19c8344000 r--p 00031000 08:01 1969441 /usr/lib/ld-linux-x86-64.so.2
7f19c8344000-7f19c8346000 rw-p 00033000 08:01 1969441 /usr/lib/ld-linux-x86-64.so.2
7ffe8b517000-7ffe8b538000 rw-p 00000000 00:00 0 [stack]
7ffe8b599000-7ffe8b59d000 r--p 00000000 00:00 0 [vvar]
7ffe8b59d000-7ffe8b59f000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 --xp 00000000 00:00 0 [vsyscall]
[ ]#
Though, meminfo reflects the consumption of ~1GB.
Before starting this program:
[ ]# cat /proc/meminfo | grep 'MemFree\|SwapFree'
MemFree: 6740324 kB
SwapFree: 0 kB
and after starting the program:
[ ]# cat /proc/meminfo | grep 'MemFree\|SwapFree'
MemFree: 5699312 kB
SwapFree: 0 kB
In the map, we can see the following zones:
7f19c791b000-7f19c791c000 ---p 00000000 00:00 0 : Red zone to protect the stack of the thread = non RW 4KB long page (stack overflow detection)
7f19c791c000-7f19c811e000 rw-p 00000000 00:00 0 : Stack of the thread (it grows from high to low memory addresses)
55eac3b80000-55eb03b81000 rw-p 00003000 08:01 1197055 : the 1 GB memory space (mem[] table) + 4KB
You could add to your program the display of the mem[] table address:
printf("A thread is continually updating all pages of global 1GB data (mem[]#%p). (pid : %d)\n", mem, getpid());
The size command displays the size of the data segment which is ~1GB:
$ gcc test.c -l pthread
$ size a.out
text data bss dec hex filename
2571 1073742504 16 1073745091 40000cc3 a.out
Execution which shows mem[]'s address at: 0x562c48575020
$ ./a.out
A thread is continually updating all pages of global 1GB data (mem[]#0x562c48575020). (pid : 10029)
.............................................
Display of the map from another terminal where we can see the address of mem[] located in the zone 562c48575000-562c88576000:
$ cat /proc/`pidof a.out`/maps
562c48571000-562c48572000 r--p 00000000 08:05 9044419 /home/xxx/a.out
562c48572000-562c48573000 r-xp 00001000 08:05 9044419 /home/xxx/a.out
562c48573000-562c48574000 r--p 00002000 08:05 9044419 /home/xxx/a.out
562c48574000-562c48575000 r--p 00002000 08:05 9044419 /home/xxx/a.out
562c48575000-562c88576000 rw-p 00003000 08:05 9044419 /home/xxx/a.out
562c89668000-562c89689000 rw-p 00000000 00:00 0 [heap]
7f02946be000-7f02946bf000 ---p 00000000 00:00 0
7f02946bf000-7f0294ebf000 rw-p 00000000 00:00 0
7f0294ebf000-7f0294ee1000 r--p 00000000 08:05 6031519 /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f0294ee1000-7f0295059000 r-xp 00022000 08:05 6031519 /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f0295059000-7f02950a7000 r--p 0019a000 08:05 6031519 /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f02950a7000-7f02950ab000 r--p 001e7000 08:05 6031519 /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f02950ab000-7f02950ad000 rw-p 001eb000 08:05 6031519 /usr/lib/x86_64-linux-gnu/libc-2.31.so
7f02950ad000-7f02950b1000 rw-p 00000000 00:00 0
7f02950b7000-7f02950bd000 r--p 00000000 08:05 6031549 /usr/lib/x86_64-linux-gnu/libpthread-2.31.so
7f02950bd000-7f02950ce000 r-xp 00006000 08:05 6031549 /usr/lib/x86_64-linux-gnu/libpthread-2.31.so
7f02950ce000-7f02950d4000 r--p 00017000 08:05 6031549 /usr/lib/x86_64-linux-gnu/libpthread-2.31.so
7f02950d4000-7f02950d5000 r--p 0001c000 08:05 6031549 /usr/lib/x86_64-linux-gnu/libpthread-2.31.so
7f02950d5000-7f02950d6000 rw-p 0001d000 08:05 6031549 /usr/lib/x86_64-linux-gnu/libpthread-2.31.so
7f02950d6000-7f02950da000 rw-p 00000000 00:00 0
7f02950fc000-7f02950ff000 rw-p 00000000 00:00 0
7f02950ff000-7f0295100000 r--p 00000000 08:05 6031511 /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f0295100000-7f0295123000 r-xp 00001000 08:05 6031511 /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f0295123000-7f029512b000 r--p 00024000 08:05 6031511 /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f029512c000-7f029512d000 r--p 0002c000 08:05 6031511 /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f029512d000-7f029512e000 rw-p 0002d000 08:05 6031511 /usr/lib/x86_64-linux-gnu/ld-2.31.so
7f029512e000-7f029512f000 rw-p 00000000 00:00 0
7f0295131000-7f0295133000 rw-p 00000000 00:00 0
7ffcb340f000-7ffcb3430000 rw-p 00000000 00:00 0 [stack]
7ffcb35ff000-7ffcb3603000 r--p 00000000 00:00 0 [vvar]
7ffcb3603000-7ffcb3605000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 --xp 00000000 00:00 0 [vsyscall]
I am trying to understand the basic of linux process memory layout and i got this program:
#include <stdio.h> // standard io
#include <stdlib.h> // C standard library
#include <pthread.h> // threading
#include <unistd.h> // unix standard library
#include <sys/types.h> // system types for linux
// getchar basically is like "read"
// it prompts the user for input
// in this case, the input is thrown away
// which makes similar to a "pause" continuation primitive
// but a pause that is resolved through user input, which we promptly throw away!
void * thread_func (void * arg) {
printf("Before malloc in thread 1\n");
getchar();
char * addr = (char *) malloc(1000);
printf("After malloc and before free in thread 1\n");
getchar();
free(addr);
printf("After free in thread 1\n");
getchar();
}
int main () {
char * addr;
printf("Welcome to per thread arena example::%d\n", getpid());
printf("Before malloc in the main thread\n");
getchar();
addr = (char *) malloc(1000);
printf("After malloc and before free in main thread\n");
getchar();
free(addr);
printf("After free in main thread\n");
getchar();
// pointer to the thread 1
pthread_t thread_1;
// pthread_* functions return 0 upon succeeding, and other numbers upon failing
int pthread_status;
pthread_status = pthread_create(&thread_1, NULL, thread_func, NULL);
if (pthread_status != 0) {
printf("Thread creation error\n");
return -1;
}
// returned status code from thread_1
void * thread_1_status;
pthread_status = pthread_join(thread_1, &thread_1_status);
if (pthread_status != 0) {
printf("Thread join error\n");
return -1;
}
return 0;
}
When I started the program, the content in /proc/<pid>/maps is:
00400000-00401000 r-xp 00000000 08:01 1323314 /home/oscp/xg/c/memory_layout/a.out
00600000-00601000 r--p 00000000 08:01 1323314 /home/oscp/xg/c/memory_layout/a.out
00601000-00602000 rw-p 00001000 08:01 1323314 /home/oscp/xg/c/memory_layout/a.out
7fcc372d7000-7fcc37491000 r-xp 00000000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37491000-7fcc37691000 ---p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37691000-7fcc37695000 r--p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37695000-7fcc37697000 rw-p 001be000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37697000-7fcc3769c000 rw-p 00000000 00:00 0
7fcc3769c000-7fcc376b5000 r-xp 00000000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc376b5000-7fcc378b4000 ---p 00019000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc378b4000-7fcc378b5000 r--p 00018000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc378b5000-7fcc378b6000 rw-p 00019000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc378b6000-7fcc378ba000 rw-p 00000000 00:00 0
7fcc378ba000-7fcc378dd000 r-xp 00000000 08:01 1053733 /lib/x86_64-linux-gnu/ld-2.19.so
7fcc37abe000-7fcc37ac1000 rw-p 00000000 00:00 0
7fcc37ad8000-7fcc37adc000 rw-p 00000000 00:00 0
7fcc37adc000-7fcc37add000 r--p 00022000 08:01 1053733 /lib/x86_64-linux-gnu/ld-2.19.so
7fcc37add000-7fcc37ade000 rw-p 00023000 08:01 1053733 /lib/x86_64-linux-gnu/ld-2.19.so
7fcc37ade000-7fcc37adf000 rw-p 00000000 00:00 0
7ffdc1cff000-7ffdc1d20000 rw-p 00000000 00:00 0 [stack]
7ffdc1dd8000-7ffdc1ddb000 r--p 00000000 00:00 0 [vvar]
7ffdc1ddb000-7ffdc1ddd000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
What are the purposes of these memory regions?
7fcc37491000-7fcc37691000 ---p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
...
7fcc37abe000-7fcc37ac1000 rw-p 00000000 00:00 0
7fcc37ad8000-7fcc37adc000 rw-p 00000000 00:00 0
Then I press enter a few times after running the program. After it prints "Before malloc in thread 1". The memory layout looks like below:
00400000-00401000 r-xp 00000000 08:01 1323314 /home/oscp/xg/c/memory_layout/a.out
00600000-00601000 r--p 00000000 08:01 1323314 /home/oscp/xg/c/memory_layout/a.out
00601000-00602000 rw-p 00001000 08:01 1323314 /home/oscp/xg/c/memory_layout/a.out
00632000-00653000 rw-p 00000000 00:00 0 [heap]
7fcc36ad6000-7fcc36ad7000 ---p 00000000 00:00 0
7fcc36ad7000-7fcc372d7000 rw-p 00000000 00:00 0
7fcc372d7000-7fcc37491000 r-xp 00000000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37491000-7fcc37691000 ---p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37691000-7fcc37695000 r--p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37695000-7fcc37697000 rw-p 001be000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37697000-7fcc3769c000 rw-p 00000000 00:00 0
7fcc3769c000-7fcc376b5000 r-xp 00000000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc376b5000-7fcc378b4000 ---p 00019000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc378b4000-7fcc378b5000 r--p 00018000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc378b5000-7fcc378b6000 rw-p 00019000 08:01 1053877 /lib/x86_64-linux-gnu/libpthread-2.19.so
7fcc378b6000-7fcc378ba000 rw-p 00000000 00:00 0
7fcc378ba000-7fcc378dd000 r-xp 00000000 08:01 1053733 /lib/x86_64-linux-gnu/ld-2.19.so
7fcc37abe000-7fcc37ac1000 rw-p 00000000 00:00 0
7fcc37ad8000-7fcc37adc000 rw-p 00000000 00:00 0
7fcc37adc000-7fcc37add000 r--p 00022000 08:01 1053733 /lib/x86_64-linux-gnu/ld-2.19.so
7fcc37add000-7fcc37ade000 rw-p 00023000 08:01 1053733 /lib/x86_64-linux-gnu/ld-2.19.so
7fcc37ade000-7fcc37adf000 rw-p 00000000 00:00 0
7ffdc1cff000-7ffdc1d20000 rw-p 00000000 00:00 0 [stack]
7ffdc1dd8000-7ffdc1ddb000 r--p 00000000 00:00 0 [vvar]
7ffdc1ddb000-7ffdc1ddd000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
What are the purposes of these two regions?
7fcc36ad6000-7fcc36ad7000 ---p 00000000 00:00 0
7fcc36ad7000-7fcc372d7000 rw-p 00000000 00:00 0
After it prints "After malloc and before free in thread 1", it creates another two regions below:
7fcc30000000-7fcc30021000 rw-p 00000000 00:00 0
7fcc30021000-7fcc34000000 ---p 00000000 00:00 0
What are the purposes of these two regions?
Your question covers many completely different things, so the answer will be long.
The first question is the meaning of
7fcc37491000-7fcc37691000 ---p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
in
7fcc372d7000-7fcc37491000 r-xp 00000000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37491000-7fcc37691000 ---p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37691000-7fcc37695000 r--p 001ba000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
7fcc37695000-7fcc37697000 rw-p 001be000 08:01 1053757 /lib/x86_64-linux-gnu/libc-2.19.so
This inaccessible memory region is a gap between adjacent ELF segments of the library (which is supposed to occupy the contiguous chunk of memory). The ---p protection mode forbids using this gap for occasional memory allocation. If you strace(1) the process when it loads the library, you may see something like this:
mmap(NULL, 1848896, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3</usr/lib/libc-2.28.so>, 0) = 0x7f9673d8f000
mprotect(0x7f9673db1000, 1671168, PROT_NONE) = 0
mmap(0x7f9673db1000, 1355776, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3</usr/lib/libc-2.28.so>, 0x22000) = 0x7f9673db1000
mmap(0x7f9673efc000, 311296, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3</usr/lib/libc-2.28.so>, 0x16d000) = 0x7f9673efc000
mmap(0x7f9673f49000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3</usr/lib/libc-2.28.so>, 0x1b9000) = 0x7f9673f49000
The first mmap() maps the first ELF segment into the memory but reserves the space for whole library. This is done to allow the kernel to choose the location for the library on its discretion. To protect any possible gaps between segments mprotect(..., PROT_NONE) is called; then all remaining segments are mapped into the memory using mmap() - this also changes the protection mode of the appropriate memory pages from ---p to whatever mode the segment requires. You may have some fun by taking a look at how it actually works. If you want to verify how this ---p gap is formed during the loading, you may also use readelf(1) with library's binary and do some hexadecimal math with segments' locations and alignments, collating the results with the output of strace.
The second question are the following anonymous mappings:
7fcc36ad6000-7fcc36ad7000 ---p 00000000 00:00 0
7fcc36ad7000-7fcc372d7000 rw-p 00000000 00:00 0
This looks like a thread stack for thread 1. The second mapping is the stack itself (372d7000 - 36ad7000 = 800000 = 8 MiB, which is a default stack size limit in many distros, which, in turn, is a default stack size for pthread), and the first one is a stack guard page. This page with mode ---p protects the stack from overflowing, and triggers the segfault when the overflow happens (because of a write to this write-protected page).
Note: in older Linux kernels, the thread stacks were annotated with [stack:TID] names in maps file, but this feature was removed, so I cannot guarantee that this mapping is really a thread stack (though it looks like). However, you may use strace to find the exact thread's stack location from child_stack argument of clone() syscall and compare with this mapping.
Going on. The third question is
7fcc30000000-7fcc30021000 rw-p 00000000 00:00 0
7fcc30021000-7fcc34000000 ---p 00000000 00:00 0
Well, this is what malloc() in thread 1 did to allocate the memory you have requested. In short, the whole region 7fcc30000000-7fcc34000000 is a heap, from which allocations are done. The rw-p interval 7fcc30000000-7fcc30021000, allocated from this heap, will grow as you will request more and more memory with malloc(). When this heap will deplete, new one will be requested using mmap().
As you probably noticed, I don't have an explanation for the following mappings in your question:
7fcc37abe000-7fcc37ac1000 rw-p 00000000 00:00 0
7fcc37ad8000-7fcc37adc000 rw-p 00000000 00:00 0
I can't recognize those guys quickly and not sure that these are ordinary allocations. Probably this needs separate investigation, as this topic is already too long.
an example for /proc/pid/maps
0022a000-00245000 r-xp 00000000 ca:01 11633540 /lib/ld-2.5.so
00245000-00246000 r--p 0001a000 ca:01 11633540 /lib/ld-2.5.so
00246000-00247000 rw-p 0001b000 ca:01 11633540 /lib/ld-2.5.so
00249000-003a3000 r-xp 00000000 ca:01 11633640 /lib/i686/nosegneg/libc-2.5.so
003a3000-003a5000 r--p 0015a000 ca:01 11633640 /lib/i686/nosegneg/libc-2.5.so
003a5000-003a6000 rw-p 0015c000 ca:01 11633640 /lib/i686/nosegneg/libc-2.5.so
003a6000-003a9000 rw-p 003a6000 00:00 0
00ada000-00adb000 r-xp 00ada000 00:00 0 [vdso]
08048000-08049000 r-xp 00000000 00:16 4735574 /home/yimingwa/test/Ctest/link_test/SectionMapping.elf
08049000-0804a000 rw-p 00000000 00:16 4735574 /home/yimingwa/test/Ctest/link_test/SectionMapping.elf
b7fcf000-b7fd0000 rw-p b7fcf000 00:00 0
b7fe1000-b7fe2000 rw-p b7fe1000 00:00 0
bfe82000-bfe98000 rw-p bffe8000 00:00 0 [stack]
the 4th column means “If the region was mapped from a file, this is the major and minor device number (in hex) where the file lives”
In the above, ca:01 I can find through /proc/devices /dev
Question is that what does "00：16" 00 means which major device?
When I look at a process's memory map using
cat /proc/pid/maps
There are entries like this:
40321000-40336000 r-xp 00000000 b3:15 875 /system/lib/libm.so
40336000-40337000 r--p 00014000 b3:15 875 /system/lib/libm.so
40337000-40338000 rw-p 00015000 b3:15 875 /system/lib/libm.so
40338000-40345000 r-xp 00000000 b3:15 789 /system/lib/libcutils.so
40345000-40346000 r--p 0000c000 b3:15 789 /system/lib/libcutils.so
40346000-40347000 rw-p 0000d000 b3:15 789 /system/lib/libcutils.so
40347000-40355000 rw-p 00000000 00:00 0
40355000-403bc000 r-xp 00000000 b3:15 877 /system/lib/libmedia.so
403bc000-403bd000 ---p 00000000 00:00 0
403bd000-403d0000 r--p 00067000 b3:15 877 /system/lib/libmedia.so
403d0000-403d1000 rw-p 0007a000 b3:15 877 /system/lib/libmedia.so
403d1000-403d5000 rw-p 00000000 00:00 0
403d5000-403d8000 rw-p 00000000 00:00 0
I understand the .so represents the shared libraries the process maps. It seems each .so has 3 entries and their permissions are
r-xp
r--p
rw-p
So how do I interpret this? Can I assume the r-xp is the code section of the library, since it has the x (execute) permission? How about the r--p and rw-p, are they the data sections?
What about the empty entries? For example, the last 6 entries about libmedia have three empty entires (00:00 0). What are these?
403bc000-403bd000 ---p 00000000 00:00 0
403bd000-403d0000 r--p 00067000 b3:15 877 /system/lib/libmedia.so
403d0000-403d1000 rw-p 0007a000 b3:15 877 /system/lib/libmedia.so
403d1000-403d5000 rw-p 00000000 00:00 0
403d5000-403d8000 rw-p 00000000 00:00 0
Can I assume the r-xp is the code section of the library, since it has
the x (execute) permission?
Yes, but this is known as text segment(which stores the instruction). You should also note that it does not have write permission as it should not have.
How about the r--p and rw-p, are they the data sections?
Yes,These segments store the static/global variable. However constant global variable would be stored into r--p segment as it should not be modifiable by any program.
What about the empty entries? For example, the last 6 entries about
libmedia have three empty entires (00:00 0). What are these?
These might be the guard segment(kernel inserts these segments to protect the overflow scenario). The "p" indicates that its private.
EDIT
For complete information, you may want to refer the following link:
http://linux.die.net/man/5/proc
I try install netbeans 7.4 for html5 and php develop version on Linux Mint 15 Olivia.But installer throw error wthi next message:
* Error in
`/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200': double free or
corruption (out): 0x089f5720 *
======= Backtrace: ========= /lib/i386-linux-gnu/libc.so.6(+0x767e2)[0xb765d7e2]
/lib/i386-linux-gnu/libc.so.6(+0x77530)[0xb765e530]
/lib/i386-linux-gnu/libz.so.1(+0xd8fb)[0xb77a88fb]
/lib/i386-linux-gnu/libz.so.1(deflateEnd+0x3c)[0xb77a01dc]
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200[0x8057911]
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200[0x8057ad6]
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200[0x804eee0]
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200[0x8058adc]
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200[0x8048f87]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf5)[0xb7600935]
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200[0x8048fad]
======= Memory map: ======== 08048000-0805d000 r-xp 00000000 08:08 526424 /usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200
0805d000-0805e000 r--p 00015000 08:08 526424
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200 0805e000-0805f000
rw-p 00016000 08:08 526424
/usr/lib/jvm/java-7-openjdk-i386/jre/bin/unpack200 0805f000-08060000
rw-p 00000000 00:00 0 089ac000-08a16000 rw-p 00000000 00:00 0
[heap] b75ac000-b75c7000 r-xp 00000000 08:01 393963
/lib/i386-linux-gnu/libgcc_s.so.1 b75c7000-b75c8000 r--p 0001a000
08:01 393963 /lib/i386-linux-gnu/libgcc_s.so.1 b75c8000-b75c9000
rw-p 0001b000 08:01 393963 /lib/i386-linux-gnu/libgcc_s.so.1
b75e5000-b75e7000 rw-p 00000000 00:00 0 b75e7000-b7795000 r-xp
00000000 08:01 397441 /lib/i386-linux-gnu/libc-2.17.so
b7795000-b7797000 r--p 001ae000 08:01 397441
/lib/i386-linux-gnu/libc-2.17.so b7797000-b7798000 rw-p 001b0000 08:01
397441 /lib/i386-linux-gnu/libc-2.17.so b7798000-b779b000 rw-p
00000000 00:00 0 b779b000-b77b2000 r-xp 00000000 08:01 394091
/lib/i386-linux-gnu/libz.so.1.2.7 b77b2000-b77b3000 r--p 00016000
08:01 394091 /lib/i386-linux-gnu/libz.so.1.2.7 b77b3000-b77b4000
rw-p 00017000 08:01 394091 /lib/i386-linux-gnu/libz.so.1.2.7
b77cd000-b77d2000 rw-p 00000000 00:00 0 b77d2000-b77d3000 r-xp
00000000 00:00 0 [vdso] b77d3000-b77f3000 r-xp 00000000 08:01
397442 /lib/i386-linux-gnu/ld-2.17.so b77f3000-b77f4000 r--p
0001f000 08:01 397442 /lib/i386-linux-gnu/ld-2.17.so
b77f4000-b77f5000 rw-p 00020000 08:01 397442
/lib/i386-linux-gnu/ld-2.17.so bfa28000-bfa49000 rw-p 00000000 00:00 0
[stack]