EXEC(2)                                                   EXEC(2)

     NAME
          execl, execv, execle, execve, execlp, execvp, exec, exece,
          environ - execute a file

     SYNOPSIS
          execl(name, arg0, arg1, ..., argn, 0)
          char *name, *arg0, *arg1, ..., *argn;

          execv(name, argv)
          char *name, *argv[];

          execle(name, arg0, arg1, ..., argn, 0, envp)
          char *name, *arg0, *arg1, ..., *argn, *envp[];

          execve(name, argv, envp)
          char *name, *argv[], *envp[];

          extern char **environ;

     DESCRIPTION
          Exec in all its forms overlays the calling process with the
          named file, then transfers to the entry point of the core
          image of the file.  There can be no return from a successful
          exec; the calling core image is lost.

          Files remain open across exec unless explicit arrangement
          has been made; see ioctl(2). Ignored/held signals remain
          ignored/held across these calls, but signals that are caught
          (see signal(2)) are reset to their default values.

          Each user has a real user ID and group ID and an effective
          user ID and group ID.  The real ID identifies the person
          using the system; the effective ID determines his access
          privileges.  Exec changes the effective user and group ID to
          the owner of the executed file if the file has the `set-
          user-ID' or `set-group-ID' modes.  The real user ID is not
          affected.

          The name argument is a pointer to the name of the file to be
          executed.  If the first two bytes of that file are the ASCII
          string `#!', then the first line of the file is taken to be
          ASCII and determines the name of the program to execute.
          The first nonblank string following `#!' in that line is
          substituted for name. Any second string, separated from the
          first by blanks or tabs, is inserted between the first two
          arguments (arguments 0 and 1) passed to the invoked file.

          The argument pointers arg0, arg1, ... or the pointers in
          argv address null-terminated strings.  Conventionally argu-
          ment 0 is the name of the file.

     EXEC(2)                                                   EXEC(2)

          Execl is useful when a known file with known arguments is
          being called; the arguments to execl are the character
          strings constituting the file and the arguments.  A 0 argu-
          ment must end the argument list.

          Execv is useful when the number of arguments is unknown in
          advance; the arguments to execv are the name of the file to
          be executed and a vector of strings containing the argu-
          ments.  The last argument string must be followed by a 0
          pointer.

          When a C program is executed, it is called as follows:

               main(argc, argv, envp)
               int argc;
               char **argv, **envp;

          where argc is the argument count and argv is an array of
          character pointers to the arguments themselves.  As indi-
          cated, argc is conventionally at least one and the first
          member of the array points to a string containing the name
          of the file.

          Argv is directly usable in another execv because argv[argc]
          is 0.

          Envp is a pointer to an array of strings that constitute the
          environment of the process.  Each string consists of a name,
          a `=', and a null-terminated value; or a name, a pair of
          parentheses (), a value bracketed by { and }, and by a null.
          The array of pointers is terminated by a null pointer.  The
          shell sh(1) passes an environment entry for each global
          shell variable defined when the program is called.  See
          environ(5) for some conventionally used names.

          The C run-time start-off routine places a copy of envp in
          the global cell environ, which is used by execv and execl to
          pass the environment to any subprograms executed by the cur-
          rent program.  The exec routines use lower-level routines as
          follows to pass an environment explicitly:

               execve(file, argv, environ);
               execle(file, arg0, arg1, . . . , argn, 0, environ);

          Execlp and execvp are called with the same arguments as
          execl and execv, but duplicate the shell's actions in
          searching for an executable file in a list of directories.
          The directory list is obtained from the environment.  Exect
          is the same as execve, except it arranges for a stop to
          occur on the first instruction of the new core image for the
          benefit of tracers, see proc(4).

     EXEC(2)                                                   EXEC(2)

     FILES
          /bin/sh  shell, invoked if command file found by execlp or
          execvp

     SEE ALSO
          fork(2), environ(5)

     DIAGNOSTICS
          If the file cannot be found, if it is not executable, if it
          does not start with a valid magic number (see a.out(5)), if
          maximum memory is exceeded, or if the arguments require too
          much space, a return constitutes the diagnostic; the return
          value is -1.  Even for the super-user, at least one of the
          execute-permission bits must be set for a file to be exe-
          cuted.

     BUGS
          If execvp is called to execute a file that turns out to be a
          shell command file, and if it is impossible to execute the
          shell, the values of argv[0] and argv[-1] will be modified
          before return.
          The path search of execlp and execvp does not extend to
          names substituted by `#!'.