1. Concept
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| 1.1 Program Library |
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 A ``program library'' is simply a file containing compiled code (and data) that is to be incorporated later into a program. |
| Program libraries allow programs to be more modular, faster to recompile, and easier to update. |
| Program libraries can be divided into three types: static libraries, shared libraries, and dynamically loaded (DL) libraries. |
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| 1.2 Static and DL Libraries |
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| Static libraries. |
– Static libraries are simply a collection of ordinary object files
– Static libraries aren't used as often as they once were, because of the advantages of shared libraries. |
| DL libraries. |
– DL libraries are libraries that are loaded at times other than during the startup of a program.
– DL libraries provide a mechanism for shared code and data, allowing a developer of shared code/data to upgrade functionality without requiring applications to be re-linked or re-compiled. |
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| 1.3 Symbols and Linkage |
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| External libraries provide a wealth of functionality – example: C standard library. |
| Programs access libraries’ functions and variables via identifiers known as symbols. |
| Header file declarations/prototypes mapped to symbols at compile time. |
| Symbols are linked to definitions in external libraries during linking. |
| Functions, global variables must be allocated memory before use. |
| Can allocate at compile time (static) or at run time (shared). |
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| Consider the simple hello world program written below: |
#include <stdio.h>
const char msg[] = "Hello, world." ;
int main()
{
puts(msg);
return 0;
}
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– What variables and functions are declared globally?
 msg, main(), puts(), others in stdio.h
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