C程序获取sizeof(buff)时崩溃，多线程分段词频统计求助

Hey there, let's break down why you're hitting that crash when calling the word_count function, and how to fix it:

Possible Causes & Fixes #

• Issue 1: buff is a pointer, not an array (invalid sizeof calculation)
  The original word_count function might have been written to work with a fixed-size stack array, where sizeof(buff) returns the actual array size. But in your code, if you're passing a dynamically allocated pointer (like char* buff = malloc(segment_size)) to the function, sizeof(buff) will only return the size of the pointer itself (8 bytes on 64-bit systems), not the actual buffer length. This leads to out-of-bounds memory access later in the function, which triggers the crash.

  Fix:
  Modify the word_count function signature to explicitly accept the buffer size as a parameter, instead of relying on sizeof:

  void word_count(char* buff, size_t buff_len, /* Add your frequency tracking params here */)
  

  When calling the function, pass the actual segment size you used to split the file. Use buff_len to control how much of the buffer the function processes.

• Issue 2: Invalid buff pointer (dangling/wild pointer)
  If your getCurrentSegmentWordcount function returns a pointer to a stack-allocated buffer (which gets destroyed once the function exits) or you accidentally free the buffer before word_count runs, the buff pointer becomes invalid. Even a seemingly harmless sizeof(buff) check (if it's a variable-length array, which uses runtime sizeof) can trigger a crash when accessing invalid memory.

  Fix:

  • For segment buffers, use dynamic allocation (malloc/calloc) instead of stack arrays, so the memory persists until you explicitly free it after the thread finishes.
  • Double-check that you're not freeing the buffer memory before the corresponding thread completes its word_count execution.

• Issue 3: Thread parameter mishandling
  If you're passing buffer-related data to threads via stack variables (like a local struct), the stack memory might be reused before the thread starts running, corrupting the buff pointer in the parameter.

  Fix:
  Allocate a separate parameter struct for each thread using malloc, which holds the buff pointer, buffer size, and any frequency tracking structures. Once the thread finishes, free this struct to avoid memory leaks.

Debugging Tips #

• Use GDB to set a breakpoint at the crash location: Check if buff points to valid memory, and compare the value of sizeof(buff) to your expected segment size.
• Verify that your segment buffers are properly null-terminated (if the original word_count function expects this) or that you're passing the exact segment length if it processes raw bytes without relying on a null terminator.
• Cross-reference the original word_count implementation to see if it has hidden assumptions (like fixed buffer size) that your multi-threaded setup isn't satisfying.

内容的提问来源于stack exchange，提问作者tom44