Node:Temporary Files, Previous:Making Special Files, Up:File System Interface
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) | Function |
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 * tmpfile64 (void) | Function |
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 If the sources are compiled with |
char * tmpnam (char *result) | Function |
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 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 |
char * tmpnam_r (char *result) | Function |
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
Warning: This function has the same security problems as
|
int L_tmpnam | Macro |
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 | Macro |
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 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 |
char * tempnam (const char *dir, const char *prefix) | Function |
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.
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 |
char * P_tmpdir | SVID Macro |
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.
Note: 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.
char * mktemp (char *template) | Function |
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 |
int mkstemp (char *template) | Function |
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 |
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) | Function |
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 The directory is created using mode |
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.