The various standards dictate which header file must define
    each of the standard functions. This is the included header file
    documented with each function prototype in the Reference Section
    of this manual.

    However, many of the functions defined by the standards already
    existed on several operating systems and were defined in
    different header files. This is especially true on OpenVMS
    systems with the header files <processes.h>, <unixio.h>, and
    <unixlib.h>.

    So, to provide upward compatibility for these functions, their
    prototypes are duplicated in both the expected header file as
    well as the header file defined by the standards.

    Duplicated Prototypes lists these functions.

    Table B-1 Duplicated Prototypes

                 Duplicated
    Function     in            Standard says

    access       <unixio.h>    <unistd.h>
    alarm        <signal.h>    <unistd.h>
    bcmp         <string.h>    <strings.h>
    bcopy        <string.h>    <strings.h>
    bzero        <string.h>    <strings.h>
    chdir        <unixio.h>    <unistd.h>
    chmod        <unixio.h>    <stat.h>
    chown        <unixio.h>    <unistd.h>
    close        <unixio.h>    <unistd.h>
    creat        <unixio.h>    <fcntl.h>
    ctermid      <stdio.h>     <unistd.h>
    cuserid      <stdio.h>     <unistd.h>
    dirname      <string.h>    <libgen.h>
    dup          <unixio.h>    <unistd.h>
    dup2         <unixio.h>    <unistd.h>
    ecvt         <unixlib.h>   <stdlib.h>
    execl        <processes.h> <unistd.h>
    execle       <processes.h> <unistd.h>
    execlp       <processes.h> <unistd.h>
    execv        <processes.h> <unistd.h>
    execve       <processes.h> <unistd.h>
    execvp       <processes.h> <unistd.h>
    _exit        <stdlib.h>    <unistd.h>
    fcvt         <unixlib.h>   <stdlib.h>
    ffs          <string.h>    <strings.h>
    fsync        <stdio.h>     <unistd.h>
    ftime        <time.h>      <timeb.h>
    gcvt         <unixlib.h>   <stdlib.h>
    getcwd       <unixlib.h>   <unistd.h>
    getegid      <unixlib.h>   <unistd.h>
    getenv       <unixlib.h>   <stdlib.h>
    geteuid      <unixlib.h>   <unistd.h>
    getgid       <unixlib.h>   <unistd.h>
    getopt       <stdio.h>     <unistd.h>
    getpid       <unixlib.h>   <unistd.h>
    getppid      <unixlib.h>   <unistd.h>
    getuid       <unixlib.h>   <unistd.h>
    index        <string.h>    <strings.h>
    isatty       <unixio.h>    <unistd.h>
    lseek        <unixio.h>    <unistd.h>
    mkdir        <unixlib.h>   <stat.h>
    mktemp       <unixio.h>    <stdlib.h>
    nice         <stdlib.h>    <unistd.h>
    open         <unixio.h>    <fcntl.h>
    pause        <signal.h>    <unistd.h>
    pipe         <processes.h> <unistd.h>
    read         <unixio.h>    <unistd.h>
    rindex       <string.h>    <strings.h>
    sbrk         <stdlib.h>    <unistd.h>
    setgid       <unixlib.h>   <unistd.h>
    setuid       <unixlib.h>   <unistd.h>
    sleep        <signal.h>    <unistd.h>
    strcasecmp   <string.h>    <strings.h>
    strncasecmp  <string.h>    <strings.h>
    system       <processes.h> <stdlib.h>
    times        <time.h>      <times.h>
    umask        <stdlib.h>    <stat.h>
    vfork        <processes.h> <unistd.h>
    wait         <processes.h> <wait.h>
    write        <unixio.h>    <unistd.h>