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

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 not, could it be true for a POSIX based OS?

The standards say nothing regarding the exact size of any integer types aside from char. Typically, long is 32-bit on 32-bit systems and 64-bit on 64-bit systems.

The standard does however specify a minimum size. From section 5.2.4.2.1 of the C Standard:

1 The values given below shall be replaced by constant expressions suitable for use in #if preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the following shall be replaced by expressions that have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. Their implementation-defined values shall be equal or greater in magnitude (absolute value) to those shown, with the same sign.

...

• minimum value for an object of type long int

  LONG_MIN -2147483647 // −(2^31−1)

• maximum value for an object of type long int

  LONG_MAX +2147483647 // 2^31−1

This says that a long int must be a minimum of 32 bits, but may be larger. On a machine where CHAR_BIT is 8, this gives a minimum byte size of 4. However on machine with e.g. CHAR_BIT equal to 16, a long int could be 2 bytes long.

Here's a real-world example. For the following code:

#include <stdio.h>

int main ()
{
    printf("sizeof(long) = %zu\n", sizeof(long));
    return 0;
}

Output on Debian 7 i686:

sizeof(long) = 4

Output on CentOS 7 x64:

sizeof(long) = 8

So no, you can't make any assumptions on size. If you need a type of a specific size, you can use the types defined in stdint.h. It defines the following types:

• int8_t: signed 8-bit
• uint8_t: unsigned 8-bit
• int16_t: signed 16-bit
• uint16_t: unsigned 16-bit
• int32_t: signed 32-bit
• uint32_t: unsigned 32-bit
• int64_t: signed 64-bit
• uint64_t: unsigned 64-bit

The stdint.h header is described in section 7.20 of the standard, with exact width types in section 7.20.1.1. The standard states that these typedefs are optional, but they exist on most implementations.

No, neither the C standard nor POSIX guarantee this and in fact most Unix-like 64-bit platforms have a 64 bit (8 byte) long.

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.

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.