Today I Learned: openat()

This post is originally published on yoursunny.com blog https://yoursunny.com/t/2021/openat/

fopen and open

In C programming language, the <stdio.h> header supplies functions for file input and output.
To open a file, we usually use the fopen function.
It is defined by the C language standard and works in every operating system.

Working at a lower level, there's also the open function.
It is a system call provided by the Linux kernel and exposed through glibc.

Both fopen and open have an input parameter: the file pathname, as a NUL-terminated string.
These two functions are declared like this:

FILE* fopen(const char* filename, const char* mode);

int open(const char* pathname, int flags);

If the file we want to access is in the current working directory, or we have the full pathname of the file as a string, this is easy to use.
However, sometimes we want to access a file relative to another directory, and the above API isn't so easy to use.

Directory Path + Filename

One such occasion is in my NDNph library: I wanted to use a directory in the filesystem as a persistent key-value store, where object keys are used as filenames, and object value is written as file content.
The API for this key-value store looks like this:

typedef struct KV KV;

/**
 * @brief Open a key-value store at specified path
 * @param[out] kv key-value store object
 * @param dir directory pathname
 * @return whether success
 */
bool KV_Open(KV* kv, const char* dir);

/**
 * @brief Write @p value to file @p dir "/" @p key
 * @param kv key-value store object
 * @param key filename
 * @param value file content
 * @param size size of file content
 * @return whether success
 */
bool KV_Save(KV* kv, const char* key, const uint8_t* value, size_t size);

Since fopen and open want the file pathname as a single string, I have to concatenate dir and key in KV_Save.
This in turn requires saving a copy of dir in KV_Open function.

typedef struct KV
{
  char* dir;
} KV;

bool KV_Open(KV* kv, const char* dir)
{
  struct stat st;
  if (stat(dir, &st) != 0 || !S_ISDIR(st.st_mode)) {
    return false;
  }
  kv->dir = strdup(dir);
  return true;
}

bool KV_Save(KV* kv, const char* key, const uint8_t* value, size_t size)
{
  char pathname[PATH_MAX];
  int res = snprintf(pathname, sizeof(pathname), "%s/%s", kv->dir, key);
  if (res < 0 || res >= sizeof(pathname)) {
    return false;
  }

  int fd = open(pathname, O_WRONLY | O_CREAT, 0644);
  if (fd < 0) {
    return false;
  }

  // TODO write and close file
}

openat

This week, I came across a new function: openat.
It operates in the same way as open, except that it supports specifying a relative pathname interpreted relative to another directory, which is represented by a file descriptor.

The function signature of openat is:

int openat(int dirfd, const char* pathname, int flags);

This allows me to simplify the key-value store:

typedef struct KV
{
  int dirfd;
} KV;

bool KV_Open(KV* kv, const char* dir)
{
  kv->dirfd = open(dir, O_RDONLY | O_DIRECTORY);
  return kv->dirfd >= 0;
}

bool KV_Save(KV* kv, const char* key, const uint8_t* value, size_t size)
{
  int fd = openat(kv->dirfd, key, O_WRONLY | O_CREAT, 0644);
  if (fd < 0) {
    return false;
  }

  // TODO write and close file
}

KV_Open opens the directory as a file descriptor with the open function.
The O_DIRECTORY flag ensures we are opening a directory instead of a regular file.
It's no longer necessary to save a copy of the directory path.

KV_Save calls openat with the directory file descriptor and the filename key.
It's no longer necessary to perform string concatenation.
The code is 5 lines shorter than the open-based solution.

Conclusion and Code Download

This article introduces Linux openat syscall that I recently discovered.
The openat function enables resolving a filename or relative path, relative to another directory that is not the current working directory.
It can do so without requiring manual string concatenation.

Code samples (whole program including load/save/delete functions):

• open.c: key-value store implemented with open
• openat.c: key-value store implemented with openat
• view on GitHub Gist

Caution: this proof-of-concept code assumes that key is a valid filename.
It cannot safely handle untrusted and potentially malicious input.