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man pages section 3: Basic Library Functions

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Updated: July 2017



mkstemp, mkstemps - make a unique file name from a template and open the file


#include <stdlib.h>

int mkstemp(char *template);
int mkstemps(char *template, int slen);


The mkstemp() function replaces the contents of the string pointed to by template by a unique file name, and returns a file descriptor for the file open for reading and writing. The function thus prevents any possible race condition between testing whether the file exists and opening it for use. The string in template should look like a file name with six trailing 'X's; mkstemp() replaces each 'X' with a character from the portable file name character set. The characters are chosen such that the resulting name does not duplicate the name of an existing file.

The mkstemps() function behaves the same as mkstemp(), except it permits a suffix to exist in the template. The template should be of the form /tmp/tmpXXXXXXsuffix. The slen parameter specifies the length of the suffix string.

Return Values

Upon successful completion, mkstemp() and mkstemps() return an open file descriptor. Otherwise −1 is returned if no suitable file could be created.


The mkstemp() and mkstemps() functions can set errno to the same values as lstat(2) and open(2).


It is possible to run out of letters.

The mkstemp() function does not check to determine whether the file name part of template exceeds the maximum allowable file name length.

The mkstemp() function is frequently used to create a temporary file that will be removed by the application before the application terminates.

The tmpfile(3C) function is preferred over the mkstemp() function for creating temporary files when control over the path and name of the file is not needed.

The mkstemp() function has a transitional interface for 64-bit file offsets. See lf64(5).


See attributes(5) for descriptions of the following attributes:

Interface Stability
For mkstemp(), see standards(5).

See Also

getpid(2), open(2), tmpfile(3C), mktemp(3C), attributes(5), lf64(5), standards(5)