Previous: Making Special Files, Up: File System Interface

### 14.11 Temporary Files

If you need to use a temporary file in your program, you can use the `tmpfile` function to open it. Or you can use the `tmpnam` (better: `tmpnam_r`) function to provide a name for a temporary file and then you can open it in the usual way with `fopen`.

The `tempnam` function is like `tmpnam` but lets you choose what directory temporary files will go in, and something about what their file names will look like. Important for multi-threaded programs is that `tempnam` is reentrant, while `tmpnam` is not since it returns a pointer to a static buffer.

These facilities are declared in the header file `stdio.h`.

— Function: FILE * tmpfile (void)

> This function creates a temporary binary file for update mode, as if by calling `fopen` with mode `"wb+"`. The file is deleted automatically when it is closed or when the program terminates. (On some other ISO C systems the file may fail to be deleted if the program terminates abnormally).

> This function is reentrant.

> When the sources are compiled with `_FILE_OFFSET_BITS == 64` on a 32-bit system this function is in fact `tmpfile64`, i.e., the LFS interface transparently replaces the old interface.

— Function: FILE * tmpfile64 (void)

> This function is similar to `tmpfile`, but the stream it returns a pointer to was opened using `tmpfile64`. Therefore this stream can be used for files larger then 2^31 bytes on 32-bit machines.

> Please note that the return type is still `FILE *`. There is no special `FILE` type for the LFS interface.

> If the sources are compiled with `_FILE_OFFSET_BITS == 64` on a 32 bits machine this function is available under the name `tmpfile` and so transparently replaces the old interface.

— Function: char * tmpnam (char *result)

> This function constructs and returns a valid file name that does not refer to any existing file. If the result argument is a null pointer, the return value is a pointer to an internal static string, which might be modified by subsequent calls and therefore makes this function non-reentrant. Otherwise, the result argument should be a pointer to an array of at least `L_tmpnam` characters, and the result is written into that array.

> It is possible for `tmpnam` to fail if you call it too many times without removing previously-created files. This is because the limited length of the temporary file names gives room for only a finite number of different names. If `tmpnam` fails it returns a null pointer.

> Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using `tmpnam`, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the `O_EXCL` flag. Using `tmpfile` or `mkstemp` is a safe way to avoid this problem.

— Function: char * tmpnam_r (char *result)

> This function is nearly identical to the `tmpnam` function, except that if result is a null pointer it returns a null pointer.

> This guarantees reentrancy because the non-reentrant situation of `tmpnam` cannot happen here.

> Warning: This function has the same security problems as `tmpnam`.

— Macro: int L_tmpnam

> The value of this macro is an integer constant expression that represents the minimum size of a string large enough to hold a file name generated by the `tmpnam` function.

— Macro: int TMP_MAX

> The macro `TMP_MAX` is a lower bound for how many temporary names you can create with `tmpnam`. You can rely on being able to call `tmpnam` at least this many times before it might fail saying you have made too many temporary file names.

> With the GNU C Library, you can create a very large number of temporary file names. If you actually created the files, you would probably run out of disk space before you ran out of names. Some other systems have a fixed, small limit on the number of temporary files. The limit is never less than `25`.

— Function: char * tempnam (const char *dir, const char *prefix)

> This function generates a unique temporary file name. If prefix is not a null pointer, up to five characters of this string are used as a prefix for the file name. The return value is a string newly allocated with `malloc`, so you should release its storage with `free` when it is no longer needed.

> Because the string is dynamically allocated this function is reentrant.

> The directory prefix for the temporary file name is determined by testing each of the following in sequence. The directory must exist and be writable.

> - The environment variable `TMPDIR`, if it is defined. For security reasons this only happens if the program is not SUID or SGID enabled.
> - The dir argument, if it is not a null pointer.
> - The value of the `P_tmpdir` macro.
> - The directory `/tmp`.

> This function is defined for SVID compatibility.

> Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using `tempnam`, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the `O_EXCL` flag. Using `tmpfile` or `mkstemp` is a safe way to avoid this problem.

— SVID Macro: char * P_tmpdir

> This macro is the name of the default directory for temporary files.

Older Unix systems did not have the functions just described. Instead they used `mktemp` and `mkstemp`. Both of these functions work by modifying a file name template string you pass. The last six characters of this string must be ‘`XXXXXX`’. These six ‘`X`’s are replaced with six characters which make the whole string a unique file name. Usually the template string is something like ‘`/tmp/prefixXXXXXX`’, and each program uses a unique prefix.

NB: Because `mktemp` and `mkstemp` modify the template string, you must not pass string constants to them. String constants are normally in read-only storage, so your program would crash when `mktemp` or `mkstemp` tried to modify the string. These functions are declared in the header file `stdlib.h`.

— Function: char * mktemp (char *template)

> The `mktemp` function generates a unique file name by modifying template as described above. If successful, it returns template as modified. If `mktemp` cannot find a unique file name, it makes template an empty string and returns that. If template does not end with ‘`XXXXXX`’, `mktemp` returns a null pointer.

> Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using `mktemp`, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the `O_EXCL` flag. Using `mkstemp` is a safe way to avoid this problem.

— Function: int mkstemp (char *template)

> The `mkstemp` function generates a unique file name just as `mktemp` does, but it also opens the file for you with `open` (see Opening and Closing Files). If successful, it modifies template in place and returns a file descriptor for that file open for reading and writing. If `mkstemp` cannot create a uniquely-named file, it returns `-1`. If template does not end with ‘`XXXXXX`’, `mkstemp` returns `-1` and does not modify template.

> The file is opened using mode `0600`. If the file is meant to be used by other users this mode must be changed explicitly.

Unlike `mktemp`, `mkstemp` is actually guaranteed to create a unique file that cannot possibly clash with any other program trying to create a temporary file. This is because it works by calling `open` with the `O_EXCL` flag, which says you want to create a new file and get an error if the file already exists.

— Function: char * mkdtemp (char *template)

> The `mkdtemp` function creates a directory with a unique name. If it succeeds, it overwrites template with the name of the directory, and returns template. As with `mktemp` and `mkstemp`, template should be a string ending with ‘`XXXXXX`’.

> If `mkdtemp` cannot create an uniquely named directory, it returns `NULL` and sets errno appropriately. If template does not end with ‘`XXXXXX`’, `mkdtemp` returns `NULL` and does not modify template. errno will be set to `EINVAL` in this case.

> The directory is created using mode `0700`.

The directory created by `mkdtemp` cannot clash with temporary files or directories created by other users. This is because directory creation always works like `open` with `O_EXCL`. See Creating Directories.

The `mkdtemp` function comes from OpenBSD.