共享内存系统调用shmflg参数中0标志的含义是什么？

Great question! Let's unpack this clearly, since the nuances of shmget() flags can be easy to miss in the man page if you're not digging for the right details.

First, remember that the shmflg parameter is actually a combination of two types of values:

• Permission bits: Similar to file system permissions (e.g., 0644), which control access to the shared memory segment when it's created.
• Control flags: Special flags like IPC_CREAT, IPC_EXCL, or IPC_NOWAIT that modify the behavior of shmget().

When you pass 0 for shmflg, here's exactly what happens:

• You're telling shmget() that you only want to retrieve an existing shared memory segment associated with the given key. No new segment will be created—if the segment doesn't exist, shmget() will return -1 (an error).
• You aren't specifying any control flags, so none of the special behaviors (like creating a new segment, failing if the segment already exists, or avoiding blocking) are enabled.
• The permission bit portion of shmflg is effectively ignored here, because we aren't creating a new segment. The call will succeed only if your process already has the necessary access permissions to the existing shared memory segment.

Example use case #

This is common in scenarios where one process creates the shared memory segment (using IPC_CREAT + permission bits), and other processes just need to attach to it later. For example:

// Try to get an existing shared memory segment with key 0x1234
int shmid = shmget(0x1234, 0, 0);
if (shmid == -1) {
    // Handle error: segment doesn't exist, or we don't have access
    perror("shmget failed");
}

In this case, passing 0 makes perfect sense—we don't want to create anything, just grab the existing segment's ID.

内容的提问来源于stack exchange，提问作者Yash Jain