Question

I have a c program that is trying to read in a file. Using the access() command it says the file is there but fopen() returns NULL and errno says the file does not exist.

A truncated snippet of code:

  FILE *fp;
  char *filename = strdup(git_dir);
  strcat(filename, "/HEAD");

  printf(git_dir);
  printf(":");
  printf(filename);
  printf(":");

  if (access(filename, F_OK)) {
    printf("Y U NO OPEN:");
  }

  fp = fopen(filename, "r");
  if (fp == NULL) {
    printf(strerror(errno));
    return;
  }

As you might tell from the code, this is trying to open the .git/HEADS file of a git repository. The particular repository this is failing on was cloned into a subdir of another repository and then added as a submodule. I do not have problems that were cloned by running the git submodule update command.

The above code does not print “Y U NO OPEN:” but it does print out the strerror(). I printed the filename to the screen and opened the file with less and it opens fine. This program is generating my zsh prompt, so I imagine it is being run by my user, and the permissions on the file are

-rw-r--r-- 1 ben users   23 Jun 30 13:32 HEAD

Any suggestions?

Answer 1

You can’t do this:

char *filename = strdup(git_dir);
strcat(filename, "/HEAD");

You’re appending the string “/HEAD” to filename, but filename only has room for the content of git_dir. Thus you’re writing past the buffer, overwriting memory, and possibly causing havoc, and all kinds of unpredictable behavior can occur.

Do this instead:

char *filename = malloc(strlen(git_dir) + 6); //length of /HEAD + null terminator
strcpy(filename, git_dir);
strcat(filename, "/HEAD");

Keep in mind that stdout is normally line-buffered as well. This means that if you do printf("Y U NO OPEN:"); , you might not see the output immediately. Print a newline, \n, to flush the output.