Why are local variables not set to zero?

Question

Since global and static variables are initialized to 0 by default, why are local variables not initialized to 0 by default as well?

Answer 1

Because such zero-initializations take execution time. It would make your program significantly slower. Each time you call a function, the program would have to execute pointless overhead code, which sets the variables to zero.

Static variables persist for the whole lifetime of the program, so there you can afford the luxuary to zero-initialize them, because they are only initialized once. While locals are initialized in runtime.

It is not uncommon in realtime systems to enable a compiler option which stops the zero initialization of static storage objects as well. Such an option makes the program non-standard, but also makes it start up faster.

Answer 2

This is because global and static variables live in different memory regions than local variables.

• uninitialized static and global variables live in the .bss segment, which is a memory region that is guaranteed to be initialized to zero on program startup, before the program enters `main'

• explicitly initialized static and global variables are part of the actual application file, their value is determined at compile-time and loaded into memory together with the application

• local variables are dynamically generated at runtime, by growing the stack. If your stack grows over a memory region that holds garbage, then your uninitialized local variables will contain garbage (garbage in, garbage out).

Answer 3

Because that would take time, and it's not always the case that you need them to be zero.

The allocation of local variables (typically on the CPU's hardware stack) is very fast, much less than one instruction per variable and basically independent of the size of the variables.

So any initialization code (which generally would not be independent of the size of the variables) would add a relatively massive amount of overhead, compared to the allocation, and since you cannot be sure that the initialization is needed, it would be very disruptive when optimizing for performance.

Global/static variables are different, they generally live in a segment of the program's binary that is set to 0 by the program loader anyway, so you get that "for free".