Readability
Most important quality of source code is readability. More important than efficiency.
Avoid too long lines
Limit lines in your program to 80 characters long whenever possible.
It improves readability.
If your lines tend to be longer than that think of redesigning your program.
Naming types using _t suffix
POSIX standard reserves _t suffix for itself, so naming types using _t suffix is discouraged.
typedef struct { whatever } my_struct_t; DiscouragedUsing booleans in C
Using booleans in C
Sequence points
Sequence point is a point in a computer program where all side effects are guaranteed to have been performed.
C FAQ: What is a sequence point?
Sequence point (wikipedia)