1. Program Execution in Linux
David Ferry, Chris Gill
CSE 422S - Operating Systems Organization
Washington University in St. Louis
St. Louis, MO 63143

2. Creating an Executable File
//Source code
#include <stdio.h>
int foo = 20;
int main( int argc, char* argv[]){
printf(“Hello, world!\n”);
return 0; }
Compiler
Relocatable Object file: D foo
T main
U puts
Linker
Executable file
Two stages:
Compilation
Linking
The compiler translates source code to machine code.
The linker connects binary files to libraries to create an executable.
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The Symbol Table
Program binaries have a symbol table that keep track of data and code: Example: The linker must resolve undefined symbols before the program can be run!
int foo = 10;
int bar = 20;
int main( int argc, char* argv[] ){
printf(“Hello, world!\n”);
return 0; }
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4. Static vs. Dynamic Linking
Static linking – required code and data is copied into executable at compile time Dynamic linking – required code and data is linked to executable at runtime
my_program.o
Static:
Dynamic:
Program code
my_program.o
libc.so
Program code
Program Data
Library Code
Program Data
Library Code
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Parts of a Program
Virtual Address Space
A program has two components:
Data
Code
Either component may be:
static (fixed at compile time)
dynamic (linked at run time)
The compiler creates static sections as part of a binary.
The linker links dynamic sections from other binaries.
0xc000_0000
Stack
Memory Map Segment
Heap
.bss
.data
.text
0x0000_0000
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6. CSE 422S – Operating Systems Organization
Program Segmentation
Virtual Address Space
Static code:
.text segment
Dynamic code:
Memory map segment
Static data:
.data segment (initialized)
Dynamic data:
Initialized at runtime:
Stack
Heap
.bss
0xc000_0000
Stack
Memory Map Segment
Heap
.bss
.data
.text
0x0000_0000
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7. Running a Statically Linked Program
A statically linked program is entirely self-contained:
The loader creates a valid process by loading a binary image into memory
On Linux, execve() system call
The C runtime initializes the process to execute normal C code
Usually called crt0.o
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The C Runtime
Initializes the C stack and heap
Sets up argc and argv
Calls user-specified program constructors and destructors
Does C library intialization
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9. Running a Statically Linked Program
User forks() an existing process to get a new process space
execve() reads program into memory
Starts executing at _start() in the C runtime, which sets up environment
C runtime eventually calls main()
After main returns, C runtime does some cleanup
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10. Running a Dynamically Linked Program
Some functions and data do not exist in process space at runtime
The dynamic linker (called ld) maps these into the memory map segment on-demand
Stack
Memory Map Segment
Heap
.bss
.data
.text
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11. CSE 422S – Operating Systems Organization
Linking at Runtime
At compile time:
The linker (ld) is embedded in program
Addresses of dynamic functions are replaced with calls to the linker
At runtime the linker does lazy-binding:
Program runs as normal until it encounters an unresolved function
Program jumps to linker
Linker maps shared library into address space and replaces the unresolved address with the resolved address
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12. Runtime Linker Implementation
Uses a procedure link table (PLT) to do lazy binding
Stack
//Source code
#include <stdio.h>
int foo = 20;
int main( int argc, char* argv[]){
printf(“Hello, world!\n”);
return 0; }
Heap
.bss
Procedure Link Table (PLT)
.data
linker_stub()
.text
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13. Runtime Linker Implementation
Uses a procedure link table (PLT) to do lazy binding
Stack
//Source code
#include <stdio.h>
int foo = 20;
int main( int argc, char* argv[]){
printf(“Hello, world!\n”);
return 0; }
Library with printf() function
Heap
.bss
Procedure Link Table (PLT)
.data
library printf()
.text
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14. Static vs. Dynamic Linking
Does not need to look up libraries at runtime
Does not need extra PLT indirection
Replicates disk space
Dynamic:
Less disk space (7K vs 571K for hello world)
Shared libraries already in memory and in hot cache
Incurs lookup and indirection overheads
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15. Executable File Format
The current binary file format is called ELF - Executable and Linking Format
First part of file is the ELF Header, which defines contents of the rest of the file
Segments contain data & code needed at runtime
Sections contain linking & relocation data
Adds additional segments past .text, .data, etc.: .rodata – read-only data .debug – debugging symbol table and more…
GCC adds it’s own sections…
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16. CSE 422S – Operating Systems Organization
Binary File Utilities
nm – prints symbol table
objdump – prints all binary data
readelf – prints ELF data
pmap – prints memory map of a running process
ldd – prints dynamic library dependencies of a binary
strip – strips symbol data from a binary
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