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     The exec functions load and execute new child processes. When the
     call is successful in DOS, the child process is placed in the
     memory previously occupied by the calling process. Under OS/2,
     calling an exec function is equivalent to calling the
     corresponding function with the P_NOWAITO argument specified,
     followed by a call to the exit function. Sufficient memory must be
     available for loading and executing the child process.
 
     All of the functions in the exec family use the same exec
     function. The letter(s) at the end of the function name determine
     the specific variation, as listed below:
 
     Letter     Variation
 
     p          Uses the PATH environment variable to find the file to
                be executed.
 
     l          Command-line arguments are passed individually to the
                exec function.
 
     v          Command-line arguments are passed to the exec function
                as an array of pointers.
 
     e          An array of pointers to environment arguments is
                explicitly passed to the child process.
 
     The <cmdname> argument specifies the file to be executed as the
     child process. It can specify a full path (from the root), a
     partial path (from the current working directory), or just a file
     name.
 
     If <cmdname> does not have a file-name extension or does not end
     with a period (.), the exec function searches for the named file.
     If the search is unsuccessful, it tries the same base name first
     with the extension .COM, then with the extension .EXE. If
     <cmdname> has an extension, only that extension is used in the
     search. If <cmdname> ends with a period, the exec function
     searches for <cmdname> with no extension.
 
     The execlp, execlpe, execvp, and execvpe routines search for
     <cmdname> (using the same procedures) in the directories specified
     by the PATH environment variable. If <cmdname> contains a drive
     specifier or any slashes (i.e., if it is a relative path name),
     the exec call searches only for the specified file and no path
     searching is done. Note that the DOS APPEND command cannot be used
     with the EXEC functions.
 
     Arguments are passed to the new process by giving one or more
     pointers to character strings as arguments in the exec call. These
     character strings form the argument list for the child process.
     The combined length of the strings forming the argument list for
     the new process must not exceed 128 bytes (in real mode only). The
     terminating null character (\0) for each string is not included
     in the count, but space characters (inserted automatically to
     separate the arguments) are counted.
 
     The argument pointers can be passed as separate arguments (execl,
     execle, execlp, and execlpe) or as an array of pointers (execv,
     execve, execvp, and execvpe). At least one argument, <arg0>, must
     be passed to the child process (which sees it as <argv[0]>).
     Usually, this argument is a copy of the <cmdname> argument. (A
     different value does not produce an error.)
 
     Under versions of DOS earlier than 3.0, the passed value of <arg0>
     is not available for use in the child process. However, under OS/2
     and DOS versions 3.0 and later, the <cmdname> is available as
     <arg0>.
 
     The execl, execle, execlp, and execlpe calls are typically used
     when the number of arguments is known in advance. The <arg0>
     argument is usually a pointer to <cmdname>. The <arg1> through
     <argn> arguments point to the character strings forming the new
     argument list. A NULL pointer must follow <argn> to mark the end
     of the argument list.
 
     The execv, execve, execvp, and execvpe calls are useful when the
     number of arguments to the new process is variable. Pointers to
     the arguments are passed as an array, <argv>. The <argv[0]>
     argument is usually a pointer to <cmdname>. The <argv[1]> through
     <argv[n]> arguments point to the character strings forming the new
     argument list. The <argv[n+1]> argument must be a NULL pointer to
     mark the end of the argument list.
 
     Files that are open when an exec call is made remain open in the
     new process. In the execl, execlp, execv, and execvp calls, the
     child process inherits the environment of the parent. The execle,
     execlpe, execve, and execvpe calls allow you to alter the
     environment for the child process by passing a list of environment
     settings through the <envp> argument. The <envp> argument is an
     array of character pointers, each element of which (except for the
     final element) points to a null-terminated string defining an
     environment variable. Such a string usually has the form
 
          NAME=value
 
     where NAME is the name of an environment variable and <value> is
     the string value to which that variable is set. (Note that <value>
     is not enclosed in double quotation marks.) The final element of
     the <envp> array should be NULL. When <envp> itself is NULL, the
     child process inherits the environment settings of the parent
     process.
 
     A program executed with one of the exec functions is always loaded
     into memory as if the "maximum allocation" field in the program's
     .EXE file header is set to the default value of 0FFFFH. If you use
     the EXEMOD utility to change the maximum allocation field of a
     program, the program's behavior when invoked with one of the exec
     functions may be different from when it is invoked directly from
     the operating-system command line or with one of the spawn
     functions.
 
     The exec calls do not preserve the translation modes of open
     files. If the child process must use files inherited from the
     parent, the setmode routine should be used to set the translation
     mode of these files to the desired mode.
 
     You must explicitly flush (using fflush or flushall) or close any
     stream prior to the exec function call.
 
     Signal settings are not preserved in child processes created by
     calls to exec routines. The signal settings are reset to the
     default in the child process.
 
     Because of differences in DOS versions 2.0 and 2.1, child
     processes generated by the exec family of functions (or by the
     equivalent spawn functions with the P_OVERLAY argument) can cause
     fatal system errors when they exit. If you are running DOS 2.0 or
     2.1, you must upgrade to DOS version 3.0 or later in order to use
     these functions.
 
     Bound programs cannot use the exec family of functions in real
     mode.
 
     Return Value
 
     The exec functions do not normally return to the calling process.
     If an exec function returns, an error has occurred and the return
     value is -1. The errno variable is set to E2BIG, EACCES, EMFILE,
     ENOENT, ENOEXEC, or ENOMEM.
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