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.
FILE * tmpfile (void) ¶Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd lock | See 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.
FILE * tmpfile64 (void) ¶Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem fd lock | See 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.
char * tmpnam (char *result) ¶Preliminary: | MT-Unsafe race:tmpnam/!result | AS-Unsafe | AC-Safe | See 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.
char * tmpnam_r (char *result) ¶Preliminary: | MT-Safe | AS-Safe | AC-Safe | See 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.
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.
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.
char * tempnam (const char *dir, const char *prefix) ¶Preliminary: | MT-Safe env | AS-Unsafe heap | AC-Unsafe mem | See 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.
TMPDIR, if it is defined. For security
reasons this only happens if the program is not SUID or SGID enabled.
P_tmpdir macro.
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.
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.
char * mktemp (char *template) ¶Preliminary: | MT-Safe | AS-Safe | AC-Safe | See 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.
int mkstemp (char *template) ¶Preliminary: | MT-Safe | AS-Safe | AC-Safe fd | See 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
(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.
char * mkdtemp (char *template) ¶Preliminary: | MT-Safe | AS-Safe | AC-Safe | See 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.
See Creating Directories.
The mkdtemp function comes from OpenBSD.