Both Microsoft and Borland provide lists of functions that are reentrant. There appear to be a number of additional functions that are reentrant except when they report error messages via DOS.
Reentrant Functions for Microsoft C 5.1
abs labs memset strcmpi strnset atoi lfind mkdir strcpy strrchr atol lsearch movedata stricmp strrev bsearch memccpy putch strlen strset chdir memcpy strcat strncmp srstr getpid memcmp segread strncat strupr halloc memcpy strcat strncmp swab hfree memicmp strchr strncpy tolower itoa memmove strcmp strnicmp toupperReentrant Functions for Borland C++ 3.0
Note: /oi option in-line functions
memchr memcmp memset stpcpy strcat strchr strcmp strcpy strlen strncat strncmp strncpy strnset strrchr rotl rotr fabs alloca