c - 如何计算文件中的换行符，但不计算只是换行符的行？

Question

为了正确解析输入，我需要能够计算文件中的行数。但是，我不想计算只是换行符的行。为了帮助解决这个问题，我创建了以下函数:

int countLinesInFile(char *filename) {
  int newlines = 0;

  if (access(filename,F_OK) != -1)
    error("File not found",0);

  FILE *input = fopen(filename,"r");

  int size = 256 * 4;
  char buffer[size];
  while ((fgets(buffer,sizeof(buffer),input)) != EOF) {
    printf("Read a string");
    if (buffer == "\n")
      continue;
    newlines++;
  }

  fclose(input);
  return newlines;
}

在文件顶部，我有以下内容:

#include <stdio.h>
#include <unistd.h>

当我运行程序并尝试计算行数时，它出现段错误。使用 valgrind，我可以看到以下内容:

==6632== Invalid read of size 4
==6632==    at 0x4EA8E6B: fgets (in /usr/lib64/libc-2.24.so)
==6632==    by 0x402219: countLinesInFile (in [executable])
[other information about program, does not seem relevant]
==6632==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==6632== 
==6632== 
==6632== Process terminating with default action of signal 11 (SIGSEGV)
==6632==  Access not within mapped region at address 0x0
==6632==    at 0x4EA8E6B: fgets (in /usr/lib64/libc-2.24.so)
==6632==    by 0x402219: countLinesInFile (in [executable])
[other information about program, does not seem relevant]
==6632==  If you believe this happened as a result of a stack
==6632==  overflow in your program's main thread (unlikely but
==6632==  possible), you can try to increase the size of the
==6632==  main thread stack using the --main-stacksize= flag.
==6632==  The main thread stack size used in this run was 8388608.
==6632== 
==6632== HEAP SUMMARY:
==6632==     in use at exit: 475 bytes in 16 blocks
==6632==   total heap usage: 19 allocs, 3 frees, 3,075 bytes allocated
==6632== 
==6632== LEAK SUMMARY:
==6632==    definitely lost: 0 bytes in 0 blocks
==6632==    indirectly lost: 0 bytes in 0 blocks
==6632==      possibly lost: 0 bytes in 0 blocks
==6632==    still reachable: 475 bytes in 16 blocks
==6632==         suppressed: 0 bytes in 0 blocks
==6632== Rerun with --leak-check=full to see details of leaked memory
==6632== 
==6632== For counts of detected and suppressed errors, rerun with: -v
==6632== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

我试图在 while 循环的开头放置一行“printf("Reading file")”。这段代码不执行，我相信 fgets 是问题所在。不幸的是，我不知道这个问题是什么。

我已验证我尝试读取的文件中确实包含正确的文本，并且不为空。

我创建的函数是该任务的正确方法吗？如果是这样，我可能遇到什么问题？以后如何避免这个问题？

更新:这对我来说确实是一个愚蠢的错误。我正在使用 Valgrind 运行该程序，它看起来没有使用可执行文件的目录，这意味着它找不到该文件。感谢您的帮助。

Answer 1

有两件事:首先，一旦无法读取更多行，fgets 就会返回 NULL，而不是 EOF。因此，条件应该是 while(fgets(...) != NULL) 或短 while(fgets(...))。其次，buffer == "\n"比较两个指针到字符，即比较两个内存地址。而且不太可能有任何东西具有与字符串文字 "\n" 相同的内存地址。因此，比较字符，即 buffer[0]=='\n' 或 buffer[0]!='\n'。我认为您可以轻松地摆脱 continue 语句，使代码如下所示:

  while (fgets(buffer,sizeof(buffer),input)) {
    if (buffer[0] != '\n') {
      newlines++;
    }
  }