What Determines the Maximum Size of a Go Slice?

Maximum Length of a Slice in Go

This question explores the maximum size of a slice that can be created in Go and the factors that determine the limit.

In Go, slices are used to store ordered collections of values. The length of a slice refers to the number of elements it contains, and its capacity refers to the maximum number of elements it can hold without needing to allocate additional memory.

The type int determines the size of a slice in Go. Depending on the target build, the size of an int can be 32-bit or 64-bit. This means that the maximum capacity of a slice is determined by the size of the default integer on the target build.

When attempting to create a slice with a size greater than the maximum capacity allowed by the integer size, different error messages can occur depending on the conditions.

• "fatal error: runtime: out of memory": This error indicates that the system is unable to allocate the necessary memory for the slice. This typically occurs when the slice size is too large for the available physical memory.
• "panic: runtime error: makeslice: len out of range": This error occurs when the slice size is greater than the maximum allowed by the type of the slice index, regardless of whether memory is an issue.

For example, in a 64-bit Linux OS with 4GB of memory, the following slice creation fails with "fatal error: runtime: out of memory":

r := make([]bool, math.MaxUint32)

Meanwhile, the following slice creation fails with "panic: runtime error: makeslice: len out of range":

r := make([]bool, math.MaxInt64)

This is because math.MaxUint32 is a 32-bit unsigned integer, while math.MaxInt64 is a 64-bit signed integer. The Go runtime performs safety checks to ensure that the size of the slice does not exceed the maximum allowed by the index type.

Interestingly, if the slice element type has zero size, such as struct{}, it is possible to create a slice with a size of math.MaxInt64:

r := make([]struct{}, math.MaxInt64)

This is because the zero-sized element type takes no memory, and the runtime check for memory allocation is not triggered.

In summary, the maximum size of a slice in Go is determined by the size of the type used for the index of the slice. For a slice with a non-zero-sized element type, the maximum size is limited by the available physical memory, while for a slice with a zero-sized element type, the maximum size is limited only by the size of the index type.

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