Thread: Most Common problems in C

I'd like to make a list of most common problems that newbies to C are facing and possible solutions.
I'll start with:

Problems

1. fflush(stdin)
2. scanf() leaves newline ( related to 1 )
3. void main(void) vs int main(void)
4. not initializing variable before use
5. pointer vs array
6. scanf("%s",&array_name) ;
7. misplacing equality operator( == ) with assignment operator (=)

Code:

    if( i   =   1 )   {       /* trouble! */ 
     if( 0 < n < 10 )  instead of if( n > 0 && n < 10)
     if (x == 5 || 6)  instead of  if (x == 5 || x == 6)

8. Off by One Error ( by Mk27)

Code:

 
     char *copy = malloc(strlen(string));       /* forget +1 for null char*/
     strcpy(copy,string);

9.

Code:

  
       foo_t *p;

       p = malloc( sizeof(*p) );          // good practice 
       
       p = malloc( sizeof(foo_t) );

Solutions

1. fflush(stdin)
2. same as 1
3. void main(void) vs int main(void)
4. Enable compiler warning (for gcc, use -Wall flag)
5. pointer vs array
6. scanf("%s",&array_name);
7. Enable compiler warning.
8.
9. The former method is less accident prone, and is easier to maintain (ie: you don't have to ever change it). If you go back and substitute foo_t for bar_t, you have to chase down all of your malloc calls, and change them too. If you use the first method, you don't have to. ( By quzah )

Please feel free to add other problems you could think of.