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Below is the list of POSIX system level API calls supported in UCRT
Functions _exit access chdir chmod close creat dup dup2 execl execle execlp execv execve execvp getcwd getpid lfind localeconv lsearch lseek memccpy mkdir open putenv read rmdir setlocale strdup strnlen strtoimax strtoumax swab toascii tzset umask unlink wcsdup wcsnlen wcstoimax wcstoumax write -
Below are the interesting APIs common to both POSIX and UCRT
Functions _exit _tolower _toupper access asctime assert atexit bsearch calloc chdir chmod clearerr clock close creat ctime difftime dup dup2 execl execle execlp execv execve execvp exit fclose fdopen feof ferror fflush fgetc fgetpos fgets fgetwc fgetws fileno fopen fprintf fputc fputs fputwc fputws fread free freopen fscanf fseek fsetpos ftell fwide fwprintf fwrite fwscanf getc getchar getcwd getenv getpid getwc getwchar gmtime lfind localeconv localtime longjmp lsearch lseek malloc mblen mbrlen mbrtowc mbsrtowcs mbstowcs mbtowc memccpy memchr memcmp memcpy memmove memset mkdir mktime open perror printf putc putchar putenv puts putwc putwchar qsort raise rand read remove rename rewind rmdir scanf setbuf setjmp setlocale setvbuf signal signbit snprintf sprintf srand sscanf strcat strchr strcmp strcoll strcpy strcspn strdup strerror strftime strlen strncat strncmp strncpy strnlen strpbrk strrchr strspn strstr strtod strtof strtoimax strtok strtol strtold strtoll strtoul strtoull strtoumax strxfrm swab swprintf swscanf system tempnam time tmpfile tmpnam toascii tolower toupper towctrans towlower towupper tzset umask ungetc ungetwc unlink va_copy va_end va_start vfprintf vfscanf vfwprintf vfwscanf vprintf vscanf vsnprintf vsprintf vsscanf vswprintf vswscanf vwprintf vwscanf wcrtomb wcscat wcschr wcscmp wcscoll wcscpy wcscspn wcsdup wcsftime wcslen wcsncat wcsncmp wcsncpy wcsnlen wcspbrk wcsrchr wcsrtombs wcsspn wcsstr wcstod wcstof wcstoimax wcstok wcstol wcstold wcstoll wcstombs wcstoul wcstoull wcstoumax wcsxfrm wctob wctomb wctrans wctype wmemchr wmemcmp wmemcpy wmemmove wmemset wprintf write