Linux/POSIX ABI是否定义int类型大小？涉及ABI兼容性疑问

Great question—this touches on a key intersection of C standard flexibility, POSIX requirements, and system ABI guarantees, which is super important for cross-compiler and cross-process compatibility.

Let’s break this down step by step:

1. C标准的模糊性 #

First, the C standard (C99, C11, etc.) does not define a fixed size for int—it only mandates that:

• sizeof(int) >= sizeof(short)
• int must be able to hold values from at least -32767 to 32767 (so a minimum of 16 bits).
  This leaves room for compiler/platform variation in theory, but practice on UNIX/Linux systems is much stricter.

2. POSIX与系统ABI的强制约定 #

While POSIX doesn’t explicitly spell out the size of int, it imposes indirect constraints that, combined with system ABI (Application Binary Interface) rules, lock in int to 4 bytes (32 bits) on both 32-bit and 64-bit UNIX/Linux systems:

• File descriptor usage: POSIX specifies that file descriptors are represented as int values. For inter-process communication (like passing fds via UNIX sockets’ cmsghdr structure), the size of int must be consistent across all processes and compiled code—otherwise, the data layout in control messages would break, leading to corrupted file descriptors or crashes.
• System ABI compliance: All major UNIX/Linux systems (including Linux, BSD, Solaris) define their ABIs (e.g., System V ABI for x86-64, i386 System V ABI) to set int as 4 bytes, regardless of the system’s word size. Compilers targeting these systems must adhere to the ABI—so GCC, Clang, and other compilers will all use 4-byte int for these platforms, eliminating ABI incompatibility between them.

3. Practical confirmation #

As you found, this is a consistent practice across UNIX/Linux platforms: both 32-bit and 64-bit systems use 4-byte int. This isn’t just a convention—it’s a hard requirement for system-level compatibility, so you won’t encounter compilers deviating from this on these systems.

In short: While the C standard leaves int size flexible, POSIX-compatible UNIX/Linux systems enforce a 4-byte int via their ABIs, ensuring that file descriptor handling (including cmsghdr transfers) works reliably across compilers and processes.

内容的提问来源于stack exchange，提问作者Hauleth