>actually i do not know why, do they produce different results?
They can. scanf is unique in how it handles whitespace. A newline is taken to be whitespace, so plenty of people (even experienced C programmers) have trouble with scanf and string data. The most common problem being something like this:
Code:
#include <stdio.h>
int main(void)
{
char s[100];
puts("One");
scanf("%s", s);
puts("Two");
fgets(s, sizeof (s), stdin);
puts("End");
return 0;
}
scanf stops reading at whitespace and leaves the resulting newline character in the input stream. Since fgets stops reading at a newline and the first character it reads is the leftover newline from scanf, it appears as if the call to fgets is skipped completely. Some people like to add a space in the format string of scanf since a space tells scanf to read a length of whitespace. This of course is a hack that rarely works as planned. Other hacks include
Code:
fflush(stdin); /* Undefined */
and
Code:
while (getchar() != '\n')
;
and a better (if cryptic) use of scanf to remove extraneous characters from the stream and then eat the newline if scanf succeeded:
Code:
if (scanf("%99s%*[^\n]", s) >= 0)
getchar();
Note the 99 added to mark the maximum length of acceptable input. This is a good idea in any situation because the vanilla %s format flag is open to buffer overrun if there's too much input.
All of this should tell you that it's usually better to avoid scanf completely and use fgets for all of your interactive input needs. Then you can use a host of parsing techniques (such as sscanf which doesn't suffer the problems of scanf).
