MKTEMP(3) | Library Functions Manual | MKTEMP(3) |
char *
mktemp(char *template);
int
mkstemp(char *template);
char *
mkdtemp(char *template);
X
' s appended to it, for example /tmp/temp.XXXXXX. The trailing ‘X
' s are replaced with the current process number and/or a unique letter combination. The number of unique file names mktemp() can return depends on the number of ‘X
' s provided. Although the NetBSD implementation of the functions will accept any number of trailing ‘X
' s, for portability reasons one should use only six. Using six ‘X
' s will result in mktemp() testing roughly 26 ** 6 (308915776) combinations.The mkstemp() function makes the same replacement to the template and creates the template file, mode 0600, returning a file descriptor opened for reading and writing. This avoids the race between testing for a file's existence and opening it for use.
The mkdtemp() function is similar to mkstemp(), but it creates a mode 0700 directory instead and returns the path.
Please note that the permissions of the file or directory being created are subject to the restrictions imposed by the umask(2) system call. It may thus happen that the created file is unreadable and/or unwritable.
For instance, code of this form:
char sfn[15] = ""; FILE *sfp; strlcpy(sfn, "/tmp/ed.XXXXXX", sizeof sfn); if (mktemp(sfn) == NULL || (sfp = fopen(sfn, "w+")) == NULL) { fprintf(stderr, "%s: %s\n", sfn, strerror(errno)); return (NULL); } return (sfp);
should be rewritten like this:
char sfn[15] = ""; FILE *sfp; int fd = -1; strlcpy(sfn, "/tmp/ed.XXXXXX", sizeof sfn); if ((fd = mkstemp(sfn)) == -1 || (sfp = fdopen(fd, "w+")) == NULL) { if (fd != -1) { unlink(sfn); close(fd); } fprintf(stderr, "%s: %s\n", sfn, strerror(errno)); return (NULL); } return (sfp);
Often one will find code which uses mktemp() very early on, perhaps to globally initialize the template nicely, but the code which calls open(2) or fopen(3) on that filename will occur much later. (In almost all cases, the use of fopen(3) will mean that the flags O_CREAT | O_EXCL are not given to open(2), and thus a symbolic link race becomes possible, hence making necessary the use of fdopen(3) as seen above). Furthermore, one must be careful about code which opens, closes, and then re-opens the file in question. Finally, one must ensure that upon error the temporary file is removed correctly.
There are also cases where modifying the code to use mktemp(), in concert with open(2) using the flags O_CREAT | O_EXCL, is better, as long as the code retries a new template if open(2) fails with an errno of EEXIST.
The mktemp(), mkstemp() and mkdtemp() functions may also set errno to any value specified by the stat(2) function.
The mkstemp() function may also set errno to any value specified by the open(2) function.
The mkdtemp() function may also set errno to any value specified by the mkdir(2) function.
The mkstemp() function appeared in 4.4BSD.
The mkdtemp() function appeared in NetBSD 1.4.
For this reason, ld(1) will output a warning message whenever it links code that uses mktemp().
The mkdtemp() function is nonstandard and should not be used if portability is required.
April 29, 2010 | NetBSD 6.1 |