const char*与字符串字面量使用差异：Linux下Modbus串口连接异常排查

Hey there, let's break down why you're hitting this issue—it's all about C-style string termination!

First, a quick recap of how C handles strings: any function expecting a const char* parameter assumes the string ends with a null character ('\0'). When you pass the string literal "/dev/ttyS4", the compiler automatically adds this null terminator behind the scenes. That's why this approach works flawlessly.

But when you define your array like this:

const char mbPort[] = {'/', 'd', 'e', 'v', '/', 't', 't', 'y', 'S', '4'};

You're creating an array exactly 10 characters long—with no null terminator at the end. When modbus_new_rtu tries to read this "string", it keeps scanning past the end of your array until it hits a random null byte in memory. On Linux, this almost certainly results in a garbage serial port path the system can't recognize—hence the connection failure.

Now, why does this work on Windows? It's pure luck. Your Windows array:

const char mbPort[] = { '\\', '\\', '.', '\\', '\\', 'C', 'O', 'M', '5' };

Is 9 characters long. By chance, the memory immediately after this array in your Windows program might happen to be a null byte. This isn't guaranteed though—your Windows code could break unexpectedly if your code changes or memory layout shifts.

Fixes to Try #

There are two simple, reliable ways to fix this on Linux (and make your Windows code more robust too):

1. Add the null terminator explicitly to your array:

   const char mbPort[] = {'/', 'd', 'e', 'v', '/', 't', 't', 'y', 'S', '4', '\0'};
   

2. Initialize the array with a string literal (the easiest and most readable option):

   const char mbPort[] = "/dev/ttyS4";
   

   This behaves exactly like passing the literal directly—the compiler automatically appends the required '\0' for you.

Quick Verification #

If you want to confirm this is the root cause, use strlen() to check the length of both versions:

• For the literal or properly terminated array: strlen(mbPort) will return 10 (the correct length of "/dev/ttyS4").
• For your original array without '\0': strlen(mbPort) will return a random number longer than 10, since it's reading past the array bounds.

This is a classic C string gotcha—easy to overlook, but once you know about null terminators, it all clicks into place.

内容的提问来源于stack exchange，提问作者Ksawery