The threads in an application must cooperate and synchronize when sharing the data and the resources of the process.
A problem arises when multiple threads call functions that manipulate an object. In a single-threaded world, synchronizing access to such objects is not a problem. However, as Example 9–3 illustrates, synchronization is a concern with multithreaded code. Note that the printf(3S) function is safe to call for a multithreaded program. This example illustrates what could happen if printf() were not safe.
Example 9-3 printf() Problem/* thread 1: */
printf("go to statement reached");
/* thread 2: */
printf("hello world");
printed on display:
go to hello