Memory layout and Linux layout of C Programs on linux

Memory layout and Linux layout of C Programs on linux

Before studying this issue carefully, I think the C program only consists of code segments, stacks, and stacks in the memory. Only when I was asked this question a few days ago did I find that my impression was incomplete.

In this article, we analyze the variables and function addresses in a C program to analyze the layout of the C program in memory.

First, we will briefly introduce the Memory Distribution of C Programs on Linux.

In general, from the low address to the high address distribution:

• Program code segment and read-only data segment
  • Program code and string constants are stored here
• Readable and writable Data Segment
  • Global variables. Static variables are stored here.
• Data heap
  • The Dynamic Allocation in the program exists.
• Shared Library
  • The shared library loaded by the program is loaded in this place.
• Data Stack
  • Local variables, function parameters, and function return addresses are stored here.
• Kernel address
  • Operating system kernel memory space

The C language code is as follows (var_memory.c)

 1 #include <stdio.h> 2 #include <stdlib.h> 3  4 #include <sys/types.h> 5 #include <unistd.h> 6 #include <errno.h> 7 #include <argp.h> 8  9 int global = 123;10 static int static_global = 123;11 12 int main()13 {14     char *string_literal = "hello";15     static int static_local = 456;16     char local_array[100] = {'a', 'b', 'c'};17     int *malloced = (int *)malloc(10*sizeof(int));18 19     pid_t pid = getpid();20     printf("pid: %ld\n", (long)pid);21 22     // string_literaling literal and function in this module are loaded23     // into readonly and excutable segment24     printf("%.8p: addr of main\n", &main);25     printf("%.8p: addr of local string_literal literal\n", string_literal);26     puts("");27 28     // static and global variables are allocated in data segment29     printf("%.8p: addr of global int\n", &global);30     printf("%.8p: addr of global static int\n", &static_global);31     printf("%.8p: addr of local static int\n", &static_local);32     puts("");33 34     // allocated in heap35     printf("%.8p: addr of local malloc\n", malloced);36     puts("");37 38     // errno is from a library39     printf("%.8p: addr of errno\n", &errno);40     puts("");41 42     // printf is from a library43     printf("%.8p: addr of printf\n", &printf);44     puts("");45 46     printf("%.8p: addr of  argp_program_verstatic_localon_hook\n", & argp_program_version_hook);47     puts("");48 49     // local variable in stack50     printf("%.8p: addr of local pid\n", &pid);51     printf("%.8p: addr of local array\n", local_array);52     puts("");53 54     scanf("%s", local_array);55     return 0;56 }

Linux Command Line

make var_memory.c

Generate the var_memory executable file and run

./var_memory

The output is as follows:

 1 pid: 4760 2 0x004b26dd: addr of main 3 0x004b29d0: addr of local string_literal literal 4  5 0x004b4008: addr of global int 6 0x004b400c: addr of global static int 7 0x004b4010: addr of local static int 8  9 0x00d30160: addr of local malloc10 11 0xb7db86b4: addr of errno12 13 0xb7e0b1c0: addr of printf14 15 0xb7f8f798: addr of  argp_program_verstatic_localon_hook16 17 0xbf85518c: addr of local pid18 0xbf855198: addr of local array

Open another terminal window and run the command according to the pid output above

Pmap 4760 # process id output above shall prevail

Output:

 1 4760:   ./var_memory 2 004b2000      4K r-x-- var_memory 3 004b3000      4K r---- var_memory 4 004b4000      4K rw--- var_memory 5 00d30000    136K rw---   [ anon ] 6 b7db8000      8K rw---   [ anon ] 7 b7dba000   1852K r-x-- libc-2.26.so 8 b7f89000      4K ----- libc-2.26.so 9 b7f8a000      8K r---- libc-2.26.so10 b7f8c000      4K rw--- libc-2.26.so11 b7f8d000     12K rw---   [ anon ]12 b7fbb000      8K rw---   [ anon ]13 b7fbd000     12K r----   [ anon ]14 b7fc0000      8K r-x--   [ anon ]15 b7fc2000    148K r-x-- ld-2.26.so16 b7fe7000      4K r---- ld-2.26.so17 b7fe8000      4K rw--- ld-2.26.so18 bf836000    132K rw---   [ stack ]19  total     2352K

We can see from the output of running the C language program and pmap.

• The main function (0x004b26dd) and String constant (0x004b29d0) are allocated in the memory area.
  • 004b2000 4 K r-x -- var_memory. rx indicates read-only and executable.
• Global variable (0x004b4008), global static variable static_global (0x004b400c), local static variable (0x004b4010) is allocated in the Data Segment
  • 004b4000 4 K rw --- var_memory, where rw indicates read and write.
• The space allocated using malloc (0x00d30160) is in heap
  • 00d30000 136 K rw --- [anon]
• The global variable errno (0xb7db86b4) provided by the shared library is allocated to the Data Segment of the shared library.
  • B7db8000 8 K rw --- [anon]
• The printf (0xb7e0b1c0) function provided by the shared library is allocated to the shared library code segment.
  • B7dba000 1852 K r-x -- libc-2.26.so
• Local variables (0xbf85518c, 0xbf855198) are allocated to the stack.
• Bf836000132 K rw --- [stack]

I hope this article will help you with the layout of C Programs in the memory.