path: root/lib/libc/sys/execve.2

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.Dd $Mdocdate: October 27 2009 $
.Dt EXECVE 2
.Os
.Sh NAME
.Nm execve ,
.Nm exect
.Nd execute a file
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft int
.Fn execve "const char *path" "char *const argv[]" "char *const envp[]"
.Ft int
.Fn exect "const char *path" "char *const argv[]" "char *const envp[]"
.Sh DESCRIPTION
.Fn execve
transforms the calling process into a new process.
The new process is constructed from an ordinary file,
whose name is pointed to by
.Fa path ,
called the
.Em new process file .
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 initialized
with zero data;  see
.Xr a.out 5
and
.Xr elf 5 .
.Pp
An interpreter file begins with a line of the form:
.Bd -filled -offset indent
.Sy #!\&
.Em interpreter
.Bq Em arg
.Ed
.Pp
When an interpreter file is passed to
.Fn execve
the system instead calls
.Fn execve
with the specified
.Em interpreter .
If the optional
.Em arg
is specified, it becomes the first argument to the
.Em interpreter ,
and the original
.Ar path
becomes the second argument;
otherwise,
.Ar path
becomes the first argument.
The original arguments are shifted over to become the subsequent arguments.
The zeroth argument, normally the name of the file being executed, is left
unchanged.
.Pp
The argument
.Fa argv
is a pointer to a null-terminated array of
character pointers to NUL-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 component of
.Fa path ) .
.Pp
The argument
.Fa envp
is also a pointer to a null-terminated array of
character pointers to NUL-terminated strings.
A pointer to this array is normally stored in the global variable
.Va environ .
These strings pass information to the
new process that is not directly an argument to the command (see
.Xr environ 7 ) .
.Pp
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
.Xr close 2
and
.Xr fcntl 2 ) .
Descriptors that remain open are unaffected by
.Fn 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 currently unallocated, these descriptors will be opened to point to
some system file like
.Pa /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.
.Pp
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
.Xr sigaction 2
for more information).
.Pp
If the set-user-ID mode bit of the new process image file is set
(see
.Xr 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
.Xr setuid 2 ) .
The set-user-ID and set-group-ID bits have no effect if the
new process image file is located on a file system mounted with
the nosuid flag.
The process will be started without the new permissions.
.Pp
The new process also inherits the following attributes from
the calling process:
.Pp
.Bl -column parent_process_ID -offset indent -compact
.It process ID Ta see Xr getpid 2
.It parent process ID Ta see Xr getppid 2
.It process group ID Ta see Xr getpgrp 2
.It session ID Ta see Xr getsid 2
.It access groups Ta see Xr getgroups 2
.It working directory Ta see Xr chdir 2
.It root directory Ta see Xr chroot 2
.It control terminal Ta see Xr termios 4
.It resource usages Ta see Xr getrusage 2
.It interval timers Ta see Xr getitimer 2
(unless process image file is setuid or setgid,
in which case all timers are disabled)
.It resource limits Ta see Xr getrlimit 2
.It file mode mask Ta see Xr umask 2
.It signal mask Ta see Xr sigaction 2 ,
.Xr sigsetmask 3
.El
.Pp
When a program is executed as a result of an
.Fn execve
call, it is entered as follows:
.Bd -literal -offset indent
main(int argc, char **argv, char **envp)
.Ed
.Pp
where
.Fa argc
is the number of elements in
.Fa argv
(the
.Dq arg count )
and
.Fa argv
points to the array of character pointers
to the arguments themselves.
.Pp
The
.Fn exect
function is equivalent to
.Fn execve
with the additional property that it executes the file with the program
tracing facilities enabled (see
.Xr ptrace 2 ) .
.Sh RETURN VALUES
As the
.Fn execve
function overlays the current process image
with a new process image the successful call
has no process to return to.
If
.Fn execve
does return to the calling process an error has occurred; the
return value will be \-1 and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn execve
will fail and return to the calling process if:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded
.Dv {NAME_MAX}
characters, or an entire path name exceeded
.Dv {PATH_MAX}
characters.
.It Bq Er ENOENT
The new process file does not exist.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.It Bq Er EACCES
The new process file is not an ordinary file.
.It Bq Er EACCES
The new process file mode denies execute permission.
.It Bq Er EACCES
The new process file is on a filesystem mounted with execution
disabled
.Pf ( Dv MNT_NOEXEC
in
.Ao Pa sys/mount.h Ac ) .
.It Bq Er ENOEXEC
The new process file has the appropriate access
permission, but has an invalid magic number in its header.
.It Bq Er ETXTBSY
The new process file is a pure procedure (shared text)
file that is currently open for writing or reading by some process.
.It Bq Er ENOMEM
The new process requires more virtual memory than
is allowed by the imposed maximum
.Pq Xr getrlimit 2 .
.It Bq Er E2BIG
The number of bytes in the new process's argument list
is larger than the system-imposed limit.
The limit in the system as released is 262144 bytes
.Pf ( Dv NCARGS
in
.Ao Pa sys/param.h Ac ) .
.It Bq Er EFAULT
The new process file is not as long as indicated by
the size values in its header.
.It Bq Er EFAULT
.Fa path ,
.Fa argv ,
or
.Fa envp
point
to an illegal address.
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.It Bq Er ENFILE
During startup of an
.Em interpreter ,
the system file table was found to be full.
.El
.Sh SEE ALSO
.Xr _exit 2 ,
.Xr fork 2 ,
.Xr execl 3 ,
.Xr exit 3 ,
.Xr a.out 5 ,
.Xr elf 5 ,
.Xr environ 7
.Sh STANDARDS
The
.Fn exect
function should not be used in portable applications.
.Sh HISTORY
The
.Fn execve
function call first appeared in
.At v7
and
.Bx 3 .
.Sh CAVEATS
If a program is
.Em setuid
to a non-superuser, but is executed when the real
.Em uid
is
.Dq root ,
then the program has some of the powers of a superuser as well.