Can I assume the size of long int is always 4 bytes?

Question

Is it always true that long int (which as far as I understand is a synonym for long) is 4 bytes?

Can I rely on that? If no

Answer 1

When we first implemented C on ICL Series 39 hardware, we took the standard at its word and mapped the data types to the natural representation on that machine architecture, which was short = 32 bits, int = 64 bits, long = 128 bits.

But we found that no serious C applications worked; they all assumed the mapping short = 16, int = 32, long = 64, and we had to change the compiler to support that.

So whatever the official standard says, for many years everyone has converged on long = 64 bits and it's not likely to change.

Answer 2

Short answer: No! You cannot make fixed assumptions on the size of long int. Because, the standard (C standard or POSIX) does not document the size of long int (as repeatedly emphasized). Just to provide a counter example to your belief, most of the 64 bit systems have long of size 64! To maximize portability use sizeof appropriately.

Use sizeof(long int) to check the size, it returns the size of long in bytes. The value is system or environment dependent; meaning, the compiler determines the size based on the hardware and OS.

Answer 3

The compiler determines the size based on the type of hardware and OS.

So, assumptions should not be made regarding the size.

Answer 4

As pointed out by @delnan, POSIX implementations keep the size of long and int as unspecified and it often differs between 32 bit and 64 bit systems.

The length of long is mostly hardware related (often matching the size of data registers on the CPU and sometimes other software related issues such as OS design and ABI interfacing).

To ease your mind, sizeof isn't a function, but a compiler directive*, so your code isn't using operations when using sizeof - it's the same as writing a number, only it's portable.

use:

sizeof(long int)

* As Dave pointed out in the comments, sizeof will be computed at runtime when it's impossible to compute the value during compilation, such as when using variable length arrays.

Also, as pointed out in another comment, sizeof takes into consideration the padding and alignment used by the implementation, meaning that the actual bytes in use could be different then the size in memory (this could be important when bit shifting).

If you're looking for specific byte sized variables, consider using a byte array or (I would assume to be supported) the types defined by C99 in stdint.h - as suggested by @dbush.

Answer 5

Use code sizeof(long int) and check the size. It will give you the size of long int in bytes on the system you're working currently. The answer of your question in particular is NO. It is nowhere guaranteed in C or in POSIX or anywhere.

Answer 6

The standard says nothing about the size of long int, so it is dependent on the environment which you are using.

To get the size of long int on your environment you can use the sizeof operator and get the size of long int. Something like

sizeof(long int)

C standard only requires the following points about the sizes of types

• int >= 16 bits,
• long >= 32 bits,
• long long (since C99) >= 64 bits
• sizeof(char) <= sizeof(short) <= sizeof(int) <= sizeof(long) <= sizeof(long long)
• sizeof(char) == 1
• CHAR_BIT >= 8

The remaining are implementations defined, so it's not surprised if one encountered some systems where int has 18/24/36/60 bits, one's complement signed form, sizeof(char) == sizeof(short) == sizeof(int) == sizeof(long) == 4, 48-bit long or 9-bit char like Exotic architectures the standards committees care about and List of platforms supported by the C standard

The point about long int above is completely wrong. Most Linux/Unix implementations define long as a 64-bit type but it's only 32 bits in Windows because they use different data models (have a look at the table here 64-bit computing), and this is regardless of 32 or 64-bit OS version.

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