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MKTEMP(3) Library Functions Manual MKTEMP(3)

mktemp - make a unique file name

char *mktemp(template)
char *template;

Mktemp replaces template by a unique file name, and returns the address of the template. The template should look like a file name with six trailing X's, which will be replaced with the current process id and a unique letter.

getpid(2)

UNIX-7