Visibility - static¶
Visibility¶
Compilation unit: the entity seen by one compiler call
The C file that is being compiled
All included header files
Result is usually one object file
Symbol resolution:
By compiler inside one compilation unit
By linker across multiple compilation units
Among symbols that the linker sees
A Somewhat Contrived Example (1)¶
Two compilation units linked into an executable …
main.c¶#include <stdio.h>
extern float avg(
int *begin, int *end);
int main(void)
{
int array[] =
{ 1, 2, 3, 4, 5 };
printf("%f\n",
avg(array, array+3));
return 0;
}
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avg.c¶int sum(int *begin, int *end)
{
int sum = 0;
while (begin < end)
sum += *begin++;
return sum;
}
float avg(int *begin, int *end)
{
return
(float)sum(begin, end) /
(end-begin);
}
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A Somewhat Contrived Example (2)¶
Function sum() in avg.c is globally visible
Anybody could declare it and use it
Linker will resolve it (that’s his job)
Name could clash with another symbol in another compilation unit
Linker error (“duplicate symbol”) when linking statically
Subtle bug when using shared libraries
Innocent reader has to think twice
“Can I modify the function without telling anybody?”
⟶ Ambiguity that needs resolution!
The static Keyword (Hooray!)¶
Solution: static - restrict visibility to the compilation unit
avg.c¶static int sum(
int *begin, int *end)
{
int sum = 0;
while (begin < end)
sum += *begin++;
return sum;
}
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