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#include <stdio.h>
#include <signal.h>

volatile sig_atomic_t e_flag = 0;

void handler(int signum) {
  e_flag = 1;
}

int main(void) {
  signal(SIGINT, handler);
  while (!e_flag) {}
  puts("Escaped from first while ()");
  e_flag = 0;
  while (!e_flag) {}
  puts("Escaped from second while ()");
  return 0;
}

*nix systems automatically reinstall signal handlers upon handler execution. For example, when compiled with gcc 3.4.4 and executed under Red Hat Linux, the SIGINT is captured both times by handler.

% ./SIG01-A
^C
Escaped from first while ()
^C
Escaped from second while ()
%

However, under Windows systems signal handlers are not automatically reinstalled. For example, when compiled with Microsoft Visual Studio 2005 version 8.0, only the first SIGINT is captured by handler.

> SIG01-A.exe
^C
Escaped from first while ()
^C
>

The second SIGINT executes the default action, which is to terminate program execution.

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References

\[[ISO/IEC 9899-1999TR2|AA. C References#ISO/IEC 9899-1999]\] Section 7.14.1.1, "The {{signal}} function"