EXECVE(2)                 NetBSD System Calls Manual                 EXECVE(2)

NAME
     execve, fexecve -- execute a file

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     execve(const char *path, char *const argv[], char *const envp[]);

     int
     fexecve(int fd, char *const argv[], char *const envp[]);

DESCRIPTION
     The execve() system call transforms the calling process into a new
     process.  The new process is constructed from an ordinary file, whose
     name is pointed to by path, called the new process file.  The fexecve()
     system call is equivalent to execve() except that the file to be executed
     is determined by the file descriptor fd instead of a path.  This file is
     either an executable object file, or a file of data for an interpreter.
     An executable object file consists of an identifying header, followed by
     pages of data representing the initial program (text) and initialized
     data pages.  Additional pages may be specified by the header to be ini-
     tialized with zero data; see elf(5) and a.out(5).

     An interpreter file begins with a line of the form:

           #! interpreter [arg]

     When an interpreter file is execve d, the system actually execve's the
     specified interpreter.  If the optional arg is specified, it becomes the
     first argument to the interpreter, and the name of the originally
     execve'd file becomes the second argument; otherwise, the name of the
     originally execve'd file becomes the first argument.  The original argu-
     ments are shifted over to become the subsequent arguments.  The zeroth
     argument, normally the name of the execve()d file, is left unchanged.
     The interpreter named by interpreter must not itself be an interpreter
     file.  (See script(7) for a detailed discussion of interpreter file exe-
     cution.)

     The argument argv is a pointer to a null-terminated array of character
     pointers to null-terminated character strings.  These strings construct
     the argument list to be made available to the new process.  At least one
     argument must be present in the array; by custom, the first element
     should be the name of the executed program (for example, the last compo-
     nent of path).

     The argument envp is also a pointer to a null-terminated array of charac-
     ter pointers to null-terminated strings.  A pointer to this array is nor-
     mally stored in the global variable environ.  These strings pass informa-
     tion to the new process that is not directly an argument to the command
     (see environ(7)).

     File descriptors open in the calling process image remain open in the new
     process image, except for those for which the close-on-exec flag is set
     (see close(2) and fcntl(2)).  Descriptors that remain open are unaffected
     by execve().

     In the case of a new setuid or setgid executable being executed, if file
     descriptors 0, 1, or 2 (representing stdin, stdout, and stderr) are cur-
     rently unallocated, these descriptors will be opened to point to some
     system file like /dev/null.  The intent is to ensure these descriptors
     are not unallocated, since many libraries make assumptions about the use
     of these 3 file descriptors.

     Signals set to be ignored in the calling process are set to be ignored in
     the new process.  Signals which are set to be caught in the calling
     process image are set to default action in the new process image.
     Blocked signals remain blocked regardless of changes to the signal
     action.  The signal stack is reset to be undefined (see sigaction(2) for
     more information).

     If the set-user-ID mode bit of the new process image file is set (see
     chmod(2)), the effective user ID of the new process image is set to the
     owner ID of the new process image file.  If the set-group-ID mode bit of
     the new process image file is set, the effective group ID of the new
     process image is set to the group ID of the new process image file.  (The
     effective group ID is the first element of the group list.)  The real
     user ID, real group ID and other group IDs of the new process image
     remain the same as the calling process image.  After any set-user-ID and
     set-group-ID processing, the effective user ID is recorded as the saved
     set-user-ID, and the effective group ID is recorded as the saved set-
     group-ID.  These values may be used in changing the effective IDs later
     (see setuid(2)).  The set-ID bits are not honored if the respective file
     system has the nosuid option enabled or if the new process file is an
     interpreter file.  Syscall tracing is disabled if effective IDs are
     changed.

     The new process also inherits the following attributes from the calling
     process:

           process ID           see getpid(2)
           parent process ID    see getppid(2)
           process group ID     see getpgrp(2)
           access groups        see getgroups(2)
           working directory    see chdir(2)
           root directory       see chroot(2)
           control terminal     see termios(4)
           resource usages      see getrusage(2)
           interval timers      see getitimer(2)
           resource limits      see getrlimit(2)
           file mode mask       see umask(2)
           signal mask          see sigaction(2), sigprocmask(2)

     When a program is executed as a result of an execve() system call, it is
     entered as follows:

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

     where argc is the number of elements in argv (the ``arg count'') and argv
     points to the array of character pointers to the arguments themselves.

     The fexecve() function ignores the file offset of fd.  Since execute per-
     mission is checked by fexecve(), the file descriptor fd need not have
     been opened with the O_EXEC flag.  However, if the file to be executed
     denies read permission for the process preparing to do the exec, the only
     way to provide the fd to fexecve() is to use the O_EXEC flag when opening
     fd.  Note that the file to be executed can not be open for writing.

RETURN VALUES
     As the execve() system call overlays the current process image with a new
     process image the successful call has no process to return to.  If
     execve() does return to the calling process an error has occurred; the
     return value will be -1 and the global variable errno is set to indicate
     the error.

ERRORS
     The execve() system call will fail and return to the calling process if:

     [E2BIG]            The number of bytes in the new process' argument list
                        is larger than the system-imposed limit.  The default
                        compile time limit is 262144 bytes and is specified in
                        the variable NCARGS in <sys/param.h> and get be read
                        from the sysctl(3) MIB variable KERN_ARGMAX.

     [EACCES]           Search permission is denied for a component of the
                        path prefix, the new process file is not an ordinary
                        file, its file mode denies execute permission, or it
                        is on a file system mounted with execution disabled
                        (MNT_NOEXEC in <sys/mount.h>).

     [EAGAIN]           A setuid(7) process has exceeded the current resource
                        limit for the number of processes it is allowed to run
                        concurrently.

     [EFAULT]           The new process file is not as long as indicated by
                        the size values in its header; or the path, argv, or
                        envp arguments point to an illegal address.

     [EIO]              An I/O error occurred while reading from the file sys-
                        tem.

     [ELOOP]            Too many symbolic links were encountered in translat-
                        ing the pathname.

     [ENAMETOOLONG]     A component of a pathname exceeded {NAME_MAX} charac-
                        ters, or an entire path name exceeded {PATH_MAX} char-
                        acters.

     [ENOENT]           The new process file does not exist, or the new
                        process file is a script starting with #! and the
                        script interpreter does not exist.

     [ENOEXEC]          The new process file has the appropriate access per-
                        mission, but has an invalid magic number in its
                        header.

     [ENOMEM]           The new process requires more virtual memory than is
                        allowed by the imposed maximum (getrlimit(2)).

     [ENOTDIR]          A component of the path prefix is not a directory.

     [ETXTBSY]          The new process file is a pure procedure (shared text)
                        file that is currently open for writing or reading by
                        some process.

     In addition, the fexecve() will fail and return to the calling process
     if:

     [EBADF]            The fd argument is not a valid file descriptor open
                        for executing.

SEE ALSO
     _exit(2), fork(2), open(2), execl(3), exit(3), sysctl(3), a.out(5),
     elf(5), environ(7), script(7), mount(8)

STANDARDS
     The execve() system call conforms to IEEE Std 1003.1-2001 (``POSIX.1'').
     with the exception of reopening descriptors 0, 1, and/or 2 in certain
     circumstances.  A future update of the Standard is expected to require
     this behavior, and it may become the default for non-privileged processes
     as well.  The support for executing interpreted programs is an extension.
     The fexecve() system call conforms to The Open Group Extended API Set 2
     specification.

HISTORY
     The execve() function call first appeared in Version 7 AT&T UNIX.  The
     fexecve() system call appeared in NetBSD 10.0.

BUGS
     If a program is setuid to a non-super-user, but is executed when the real
     uid is ``root'', then the program has some of the powers of a super-user
     as well.

NetBSD 9.0                    September 16, 2019                    NetBSD 9.0

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