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 make a name for a temporary file and then 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.

Function: char * tmpnam (char *result)

This function constructs and returns a file name that is a valid file name and that does not name 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 fixed length of a temporary file name gives room for only a finite number of different names. If tmpnam fails, it returns a null pointer.

Function: char * tmpnam_r (char *result)

This function is nearly identical to the tmpnam function. But it does not allow result to be a null pointer. In the later case a null pointer is returned.

This function is reentrant because the non-reentrant situation of tmpnam cannot happen here.

Macro: int L_tmpnam

The value of this macro is an integer constant expression that represents the minimum allocation size of a string large enough to hold the 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 library, you can create a very large number of temporary file names--if you actually create the files, you will probably run out of disk space before you run 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 filename. 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; 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.

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.

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.

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.

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 section Opening and Closing Files). If successful, it modifies template in place and returns a file descriptor open on that file for reading and writing. If mkstemp cannot create a uniquely-named file, it makes template an empty string and returns -1. If template does not end with `XXXXXX', mkstemp returns -1 and does not modify template.

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 bit, which says you want to always create a new file, and get an error if the file already exists.

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