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char *file_name;
FILE *file;

/* Initialize file_name */

if (!fgets(file_name, sizeof(file_name), stdin)#include <stdio.h>
 
void func(const char *file_name) {
  /* Handle error */
}

FILE *file;

  if ((file = fopen(file_name, "wb")) == NULL) {
    /* Handle error */
  }

  /* Operate on file. */

  fclose(file);
}

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#ifdef O_NOFOLLOW
  #define OPEN_FLAGS O_NOFOLLOW | O_NONBLOCK
#else
  #define OPEN_FLAGS O_NONBLOCK
#endif

/* ... */

void func(const char *file_name) {
  struct stat orig_st;
  struct stat open_st;
  int fd;
  int flags;
char *file_name;

/* Initialize file_name */

if (!fgets(file_name, sizeof(file_name), stdin)) {
  /* Handle error */
}

if ((lstat(file_name, &orig_st) != 0)
 ||
      (!S_ISREG(orig_st.st_mode)))
 {
    /* Handle error */
  }

  /* A TOCTOU race condition exists here, see below. */

  fd = open(file_name, OPEN_FLAGS | O_WRONLY);
  if (fd == -1) {
    /* Handle error */
  }

  if (fstat(fd, &open_st) != 0) {
    /* Handle error */
  }

  if ((orig_st.st_mode != open_st.st_mode) ||
      (orig_st.st_ino  != open_st.st_ino) ||
      (orig_st.st_dev  != open_st.st_dev)) {
    /* File was tampered with. */
  }

  /* Optional: drop the O_NONBLOCK now that we are sure
  *   this is a good file. */
  if ((flags = fcntl(fd, F_GETFL)) == -1) {
    /* Handle error */
  }

  if (fcntl(fd, F_SETFL, flags & ~O_NONBLOCK) != 0) {
    /* Handle error */
  }

  /* Operate on file */

  close(fd);
}

#include <Windows.h>
 
void func(const TCHAR *file_name) {
  HANDLE hFile = CreateFile(file_name,
                            GENERIC_READ | GENERIC_WRITE, 0, 
                            NULL, OPEN_EXISTING,
                            FILE_ATTRIBUTE_NORMAL, NULL);
  if (hFile == INVALID_HANDLE_VALUE) {
    /* Handle error */
  } else if (GetFileType(hFile) != FILE_TYPE_DISK) {
    /* Handle error */
    CloseHandle(hFile);
  } else {
    /* Operate on file */
    CloseHandle(hFile);
  }
}