为什么 fopen 不打开现有文件？ （返回 NULL，错误号 ENOENT）

Question

I have posted this to document my issue, see my self-answer below.

No matter what I try, fopen(...) cannot open the existing file at the path which exists and returns NULL . I am executing the program from a bash script in ~/path . The program file is stored at ~/path/to .

int main(void) {
  const char* filename = "my/file"
  FILE* fp = NULL;
  fp = fopen(filename, "r"); // file is still NULL, segfaults on indirection
  if (!fp) exit(1);
  fclose(fp);
}

Answer 1

fopen(3) is documented as capable of failing:

Otherwise, NULL is returned and errno is set to indicate the error.

So you should at least code:

FILE* fp = fopen(filename, "r");
if (fp == NULL) { perror(filename); exit(EXIT_FAILURE); };

and fopen won't even try to create a file that you open for reading only.

As a rule of thumb, you always need to check against failure of fopen (a minima like above), and report to your user (with the help of errno(3) , perror(3) , strerror(3) -used as strerror(errno) - ...) the reason of that failure. An educated user would be able to manage (perhaps with help from his sysadmin).

ENOENT is documented in errno(3) to mean

ENOENT No such file or directory (POSIX.1-2001).

Typically, this error results when a specified path‐ name does not exist, or one of the components in the directory prefix of a pathname does not exist, or the specified pathname is a dangling symbolic link.

I find that explanation pretty clear. In your case, you probably don't have any path/ directory in your current working directory , or you do have path/to/my/ directory without any file entry, etc (eg path/ exists but without to/ inside it) ....

You could improve your program by showing not only the errno (using strerror(errno) or perror ) but also the working directory. See getcwd(3) . Or you could leave your user to guess it. Your user could have changed the working directory, eg with a cd builtin command of his unix shell .