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File: libc.info, Node: Temporary Files, Prev: 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)
Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd
lock | *Note POSIX Safety Concepts::.
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)
Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd
lock | *Note POSIX Safety Concepts::.
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 than 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)
Preliminary: | MT-Unsafe race:tmpnam/!result | AS-Unsafe | AC-Safe
| *Note POSIX Safety Concepts::.
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)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
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)
Preliminary: | MT-Safe env | AS-Unsafe heap | AC-Unsafe mem | *Note
POSIX Safety Concepts::.
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)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
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)
Preliminary: | MT-Safe | AS-Safe | AC-Safe fd | *Note POSIX Safety
Concepts::.
The 'mkstemp' function generates a unique file name just as
'mktemp' does, but it also opens the file for you with 'open'
(*note 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)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
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'. *Note Creating
Directories::.
The 'mkdtemp' function comes from OpenBSD.
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