trick in scanf

libjingle has a very tricky use of scanf and printf in httpcommon.cc

"%.*s" used in printf and "%*s%n %n%*s%n" used in scanf.

no offical explainations found so far.But you can figure out what they mean if you run the following exmaple code.

#include

int main()

{

  char path[] = "/test.html";

  printf("%s %.*s HTTP/%s\n", "Get", sizeof(path),path, "1.1");

 

  char line[256];

  sprintf(line,"%s %.*s HTTP/%s", "Get", sizeof(path),path, "1.1");

 

  int vmajor, vminor;

  int vend, dstart, dend;

  int ret = sscanf(line, "%*s%n %n%*s%n HTTP/%lu.%lu", &vend, &dstart, &dend, &vmajor, &vminor);

  

  printf("result: %d ",ret);

  printf("vend = %d ", vend);

  printf("dstart = %d ", dstart);

  printf("dend = %d ", dend);

  printf("vmajor = %d ", vmajor);

  printf("vminor = %d ", vminor);

  

 system("pause");

}

Results :

Get /test.html HTTP/1.1

result: 2 vend = 3 dstart = 4 dend = 14 vmajor = 1 vminor = 1 请按任意键继续. .