NAME

mkdtemp, mkostemp, mkstemp — create a unique directory or file

SYNOPSIS

[CX] #include <stdlib.h>

char *mkdtemp(char *template);
int mkostemp(char *template, int flag);
int mkstemp(char *template);

DESCRIPTION

The mkdtemp() function shall create a directory with a unique name derived from template. The application shall ensure that the string provided in template is a pathname ending with at least six trailing 'X' characters. The mkdtemp() function shall modify the contents of template by replacing six or more 'X' characters at the end of the pathname with the same number of characters from the portable filename character set. The characters shall be chosen such that the resulting pathname does not duplicate the name of an existing file at the time of the call to mkdtemp(). The mkdtemp() function shall use the resulting pathname to create the new directory as if by a call to:

mkdir(pathname, S_IRWXU)

The mkstemp() function shall create a regular file with a unique name derived from template and return a file descriptor for the file open for reading and writing. The application shall ensure that the string provided in template is a pathname ending with at least six trailing 'X' characters. The mkstemp() function shall modify the contents of template by replacing six or more 'X' characters at the end of the pathname with the same number of characters from the portable filename character set. The characters shall be chosen such that the resulting pathname does not duplicate the name of an existing file at the time of the call to mkstemp(). The mkstemp() function shall use the resulting pathname to create the file, and obtain a file descriptor for it, as if by a call to:

open(pathname, O_RDWR|O_CREAT|O_EXCL, S_IRUSR|S_IWUSR)

By behaving as if the O_EXCL flag for open() is set, the function prevents any possible race condition between testing whether the file exists and opening it for use.

The mkostemp() function shall be equivalent to the mkstemp() function, except that the flag argument can contain additional flags to be used as if by open(). Behavior is unspecified if the flag argument contains more than the following flags:

O_APPEND

Set append mode.

O_CLOEXEC

Set the FD_CLOEXEC file descriptor flag.

O_CLOFORK

Set the FD_CLOFORK file descriptor flag.

O_DSYNC

^[SIO] Write according to the synchronized I/O data integrity completion.

O_RSYNC

^[SIO] Synchronized read I/O operations.

O_SYNC

^[XSI|SIO] Write according to synchronized I/O file integrity completion.

RETURN VALUE

Upon successful completion, the mkdtemp() function shall return the value of template. Otherwise, it shall return a null pointer and shall set errno to indicate the error.

Upon successful completion, the mkstemp() function shall return an open file descriptor. Otherwise, it shall return -1 and shall set errno to indicate the error.

ERRORS

These functions shall fail if:

[EINVAL]

The string pointed to by template does not end in "XXXXXX".

The mkostemp() function may fail if:

[EINVAL]

The value of the flag argument is invalid.

Additional error conditions for the mkdtemp() function are defined in mkdir. Additional error conditions for the mkstemp() and mkostemp() functions are defined in open.

EXAMPLES

Generating a Pathname

The following example creates a file with a 10-character name beginning with the characters "file" and opens the file for reading and writing. The value returned as the value of fd is a file descriptor that identifies the file.

#include <stdlib.h>
...
char template[] = "/tmp/fileXXXXXX";
int fd;


fd = mkstemp(template);

APPLICATION USAGE

It is possible to run out of letters.

Portable applications should pass exactly six trailing 'X's in the template and no more; implementations may treat any additional trailing 'X's as either a fixed or replaceable part of the template. To be sure of only passing six, a fixed string of at least one non-'X' character should precede the six 'X's.

Since 'X' is in the portable filename character set, some of the replacement characters can be 'X's, leaving part (or even all) of the template effectively unchanged.

RATIONALE

The O_CLOEXEC and O_CLOFORK flags of mkostemp() are necessary to avoid a data race in multi-threaded applications. Without O_CLOFORK, a file descriptor is leaked into a child process created by one thread in the window between another thread creating a temporary file descriptor with mkstemp() and then using fcntl() to set the FD_CLOFORK flag. Without O_CLOEXEC, a temporary file descriptor intentionally inherited by child processes is similarly leaked into an executed program if FD_CLOEXEC is not set atomically.

Implementations are encouraged to have mkdtemp(), mkostemp(), and mkstemp() report an [EILSEQ] error if the last component of the pathname in template contains any bytes that have the encoded value of a <newline> character.

FUTURE DIRECTIONS

None.

SEE ALSO

getpid, mkdir, open, tmpfile, tmpnam

XBD <stdlib.h>

CHANGE HISTORY

First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Issue 7

Austin Group Interpretation 1003.1-2001 #143 is applied.

SD5-XSH-ERN-168 is applied, clarifying file permissions upon creation.

The mkstemp() function is moved from the XSI option to the Base.

The mkdtemp() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 1.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0380 [291], XSH/TC1-2008/0381 [324], and XSH/TC1-2008/0382 [291] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0215 [567,669] is applied.

Issue 8

Austin Group Defect 251 is applied, encouraging implementations to disallow the creation of filenames containing any bytes that have the encoded value of a <newline> character.

Austin Group Defects 411, 1318, and 1350 are applied, adding mkostemp().

Austin Group Defect 652 is applied, adding the [EINVAL] error for mkstemp().

Austin Group Defect 1734 is applied, replacing the error conditions specified only for mkdtemp() with a reference to mkdir.