UC3M

Another consequence derived from the dynamic memory management in C is that memory initialization is only performed if requested through the call to calloc. In other words, when a new memory portion is allocated with a call to malloc, the portion made visible to the program with its actual content. That is, no initial value is assigned. Most likely, the memory contains the information that was stored in its previous use.

This behavior is conceived to obtain the best performance in a program. It is often the case that certain memory portions are requested, but they are then initialized to some specific values from the program itself. In this case, if malloc returns the memory initialized, the task is performed twice with a decrement in the performance. For this reason, only the function calloc performs this initialization. As an example, the next code portion tries to print on the screen as a string the garbage stored in the obtained memory location.

char *string;
string = (char *)malloc(100);
printf("%s\n", string);