.SH SYNOPSIS
.nf
.B #include <unistd.h>
-.PP
+.P
.BI "int execve(const char *" pathname ", char *const _Nullable " argv [],
.BI " char *const _Nullable " envp []);
.fi
This causes the program that is currently being run by the calling process
to be replaced with a new program, with newly initialized stack, heap,
and (initialized and uninitialized) data segments.
-.PP
+.P
\fIpathname\fP must be either a binary executable, or a script
starting with a line of the form:
-.PP
+.P
.in +4n
.EX
\fB#!\fP\fIinterpreter \fP[optional-arg]
.EE
.in
-.PP
+.P
For details of the latter case, see "Interpreter scripts" below.
-.PP
+.P
.I argv
is an array of pointers to strings passed to the new program
as its command-line arguments.
(Thus, in the new program,
.I argv[argc]
will be NULL.)
-.PP
+.P
.I envp
is an array of pointers to strings, conventionally of the form
.BR key=value ,
The
.I envp
array must be terminated by a NULL pointer.
-.PP
+.P
This manual page describes the Linux system call in detail;
for an overview of the nomenclature and the many, often preferable,
standardised variants of this function provided by libc,
.B PATH
environment variable, see
.BR exec (3).
-.PP
+.P
The argument vector and environment can be accessed by the
new program's main function, when it is defined as:
-.PP
+.P
.in +4n
.EX
int main(int argc, char *argv[], char *envp[])
.EE
.in
-.PP
+.P
Note, however, that the use of a third argument to the main function
is not specified in POSIX.1;
according to POSIX.1,
the environment should be accessed via the external variable
.BR environ (7).
-.PP
+.P
.BR execve ()
does not return on success, and the text, initialized data,
uninitialized data (bss), and stack of the calling process are overwritten
according to the contents of the newly loaded program.
-.PP
+.P
If the current program is being ptraced, a \fBSIGTRAP\fP signal is sent to it
after a successful
.BR execve ().
-.PP
+.P
If the set-user-ID bit is set on the program file referred to by
\fIpathname\fP,
then the effective user ID of the calling process is changed
Similarly, if the set-group-ID bit is set on the program file,
then the effective group ID of the calling
process is set to the group of the program file.
-.PP
+.P
The aforementioned transformations of the effective IDs are
.I not
performed (i.e., the set-user-ID and set-group-ID bits are ignored)
or
.IP \[bu]
the calling process is being ptraced.
-.PP
+.P
The capabilities of the program file (see
.BR capabilities (7))
are also ignored if any of the above are true.
-.PP
+.P
The effective user ID of the process is copied to the saved set-user-ID;
similarly, the effective group ID is copied to the saved set-group-ID.
This copying takes place after any effective ID changes that occur
because of the set-user-ID and set-group-ID mode bits.
-.PP
+.P
The process's real UID and real GID, as well as its supplementary group IDs,
are unchanged by a call to
.BR execve ().
-.PP
+.P
If the executable is an a.out dynamically linked
binary executable containing
shared-library stubs, the Linux dynamic linker
is called at the start of execution to bring
needed shared objects into memory
and link the executable with them.
-.PP
+.P
If the executable is a dynamically linked ELF executable, the
interpreter named in the PT_INTERP segment is used to load the needed
shared objects.
.IP \[bu]
The floating-point environment is reset to the default (see
.BR fenv (3)).
-.PP
+.P
The process attributes in the preceding list are all specified
in POSIX.1.
The following Linux-specific process attributes are also
.B CLONE_FILES
flag of
.BR clone (2).
-.PP
+.P
Note the following further points:
.IP \[bu] 3
All threads other than the calling thread are destroyed during an
.SS Interpreter scripts
An interpreter script is a text file that has execute
permission enabled and whose first line is of the form:
-.PP
+.P
.in +4n
.EX
\fB#!\fP\fIinterpreter \fP[optional-arg]
.EE
.in
-.PP
+.P
The
.I interpreter
must be a valid pathname for an executable file.
-.PP
+.P
If the
.I pathname
argument of
specifies an interpreter script, then
.I interpreter
will be invoked with the following arguments:
-.PP
+.P
.in +4n
.EX
\fIinterpreter\fP [optional-arg] \fIpathname\fP arg...
.EE
.in
-.PP
+.P
where
.I pathname
is the pathname of the file specified as the first argument of
.\" See the P - preserve-argv[0] option.
.\" Documentation/admin-guide/binfmt-misc.rst
.\" https://www.kernel.org/doc/html/latest/admin-guide/binfmt-misc.html
-.PP
+.P
For portable use,
.I optional-arg
should either be absent, or be specified as a single word (i.e., it
should not contain white space); see NOTES below.
-.PP
+.P
Since Linux 2.6.28,
.\" commit bf2a9a39639b8b51377905397a5005f444e9a892
the kernel permits the interpreter of a script to itself be a script.
.I <limits.h>
or available at run time using the call
.IR "sysconf(_SC_ARG_MAX)" ).
-.PP
+.P
Before Linux 2.6.23, the memory used to store the
environment and argument strings was limited to 32 pages
(defined by the kernel constant
.BR MAX_ARG_PAGES ).
On architectures with a 4-kB page size,
this yields a maximum size of 128\ kB.
-.PP
+.P
On Linux 2.6.23 and later, most architectures support a size limit
derived from the soft
.B RLIMIT_STACK
.SH VERSIONS
POSIX does not document the #! behavior, but it exists
(with some variations) on other UNIX systems.
-.PP
+.P
On Linux,
.I argv
and
.\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
.\" Bug rejected (because fix would constitute an ABI change).
.\"
-.PP
+.P
POSIX.1 says that values returned by
.BR sysconf (3)
should be invariant over the lifetime of a process.
Since Linux 5.1,
.\" commit 6eb3c3d0a52dca337e327ae8868ca1f44a712e02
the limit is 255 characters.
-.PP
+.P
The semantics of the
.I optional-arg
argument of an interpreter script vary across implementations.
and white spaces in
.I optional-arg
are used to delimit the arguments.
-.PP
+.P
Linux (like most other modern UNIX systems)
ignores the set-user-ID and set-group-ID bits on scripts.
.SH STANDARDS
.\" conditions EAGAIN, EINTR, ELIBACC, ENOLINK, EMULTIHOP; POSIX does not
.\" document ETXTBSY, EPERM, EFAULT, ELOOP, EIO, ENFILE, EMFILE, EINVAL,
.\" EISDIR or ELIBBAD error conditions.
-.PP
+.P
With UNIX\ V6, the argument list of an
.BR exec ()
call was ended by 0,
.BR execve ()
does is arrange for an existing process (the calling process)
to execute a new program.
-.PP
+.P
Set-user-ID and set-group-ID processes can not be
.BR ptrace (2)d.
-.PP
+.P
The result of mounting a filesystem
.I nosuid
varies across Linux kernel versions:
some will just ignore the set-user-ID and set-group-ID bits and
.BR exec ()
successfully.
-.PP
+.P
In most cases where
.BR execve ()
fails, control returns to the original executable image,
error that can occur (since Linux 3.1) when calling
.BR execve ()
is as follows.
-.PP
+.P
The
.B EAGAIN
error can occur when a
.\" commit 909cc4ae86f3380152a18e2a3c44523893ee11c4
the resource limit was not imposed on processes that
changed their user IDs.)
-.PP
+.P
Since Linux 3.1, the scenario just described no longer causes the
.BR set*uid ()
call to fail,
.BR set*uid ()
+
.BR execve ().
-.PP
+.P
If the resource limit was not still exceeded at the time of the
.BR execve ()
call
.SH EXAMPLES
The following program is designed to be execed by the second program below.
It just echoes its command-line arguments, one per line.
-.PP
+.P
.in +4n
.\" SRC BEGIN (myecho.c)
.EX
.EE
.\" SRC END
.in
-.PP
+.P
This program can be used to exec the program named in its command-line
argument:
-.PP
+.P
.in +4n
.\" SRC BEGIN (execve.c)
.EX
.EE
.\" SRC END
.in
-.PP
+.P
We can use the second program to exec the first as follows:
-.PP
+.P
.in +4n
.EX
.RB "$" " cc myecho.c \-o myecho"
argv[2]: world
.EE
.in
-.PP
+.P
We can also use these programs to demonstrate the use of a script
interpreter.
To do this we create a script whose "interpreter" is our
.I myecho
program:
-.PP
+.P
.in +4n
.EX
.RB "$" " cat > script"
.RB "$" " chmod +x script"
.EE
.in
-.PP
+.P
We can then use our program to exec the script:
-.PP
+.P
.in +4n
.EX
.RB "$" " ./execve ./script"
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