man3/exec.3

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.\"     @(#)exec.3	6.4 (Berkeley) 4/19/91
.\"
.\" Converted for Linux, Mon Nov 29 11:12:48 1993, faith@cs.unc.edu
.\" Updated more for Linux, Tue Jul 15 11:54:18 1997, pacman@cqc.com
.\" Modified, 24 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added note on casting NULL
.\"
.TH EXEC 3  2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
execl, execlp, execle, execv, execvp, execvpe \- execute a file
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.B extern char **environ;
.PP
.BI "int execl(const char *" pathname ", const char *" arg ", ..."
.B  "                /*, (char *) NULL */);"
.BI "int execlp(const char *" file ", const char *" arg ", ..."
.B  "                /*, (char *) NULL */);"
.BI "int execle(const char *" pathname ", const char *" arg ", ..."
.BI "                /*, (char *) NULL, char *const " envp "[] */);"
.BI "int execv(const char *" pathname ", char *const " argv "[]);"
.BI "int execvp(const char *" file ", char *const " argv "[]);"
.BI "int execvpe(const char *" file ", char *const " argv \
"[], char *const " envp "[]);"
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR execvpe ():
.nf
    _GNU_SOURCE
.fi
.SH DESCRIPTION
The
.BR exec ()
family of functions replaces the current process image with a new process
image.
The functions described in this manual page are layered on top of
.BR execve (2).
(See the manual page for
.BR execve (2)
for further details about the replacement of the current process image.)
.PP
The initial argument for these functions is the name of a file that is
to be executed.
.PP
The functions can be grouped based on the letters following the "exec" prefix.
.\"
.SS l - execl(), execlp(), execle()
The
.I "const char\ *arg"
and subsequent ellipses can be thought of as
.IR arg0 ,
.IR arg1 ,
\&...,
.IR argn .
Together they describe a list of one or more pointers to null-terminated
strings that represent the argument list available to the executed program.
The first argument, by convention, should point to the filename associated
with the file being executed.
The list of arguments
.I must
be terminated by a null pointer,
and, since these are variadic functions, this pointer must be cast
.IR "(char\ *) NULL" .
.PP
By contrast with the 'l' functions, the 'v' functions (below) specify the
command-line arguments of the executed program as a vector.
.\"
.SS v - execv(), execvp(), execvpe()
The
.I "char\ *const argv[]"
argument is an array of pointers to null-terminated strings that
represent the argument list available to the new program.
The first argument, by convention, should point to the filename
associated with the file being executed.
The array of pointers
.I must
be terminated by a null pointer.
.SS e - execle(), execvpe()
The environment of the new process image is specified via the argument
.IR envp .
The
.I envp
argument is an array of pointers to null-terminated strings and
.I must
be terminated by a null pointer.
.PP
All other
.BR exec ()
functions (which do not include 'e' in the suffix)
take the environment for the new process
image from the external variable
.I environ
in the calling process.
.SS p - execlp(), execvp(), execvpe()
These functions duplicate the actions of the shell in
searching for an executable file
if the specified filename does not contain a slash (/) character.
The file is sought in the colon-separated list of directory pathnames
specified in the
.B PATH
environment variable.
If this variable isn't defined, the path list defaults to
a list that includes the directories returned by
.IR confstr(_CS_PATH)
(which typically returns the value "/bin:/usr/bin")
and possibly also the current working directory;
see NOTES for further details.
.PP
.BR execvpe ()
searches for the program using the value of
.B PATH
from the caller's environment, not from the
.I envp
argument.
.PP
If the specified filename includes a slash character, then
.B PATH
is ignored, and the file at the specified pathname is executed.
.PP
In addition, certain errors are treated specially.
.PP
If permission is denied for a file (the attempted
.BR execve (2)
failed with the error
.BR EACCES ),
these functions will continue searching the rest of the search path.
If no other file is found, however,
they will return with
.I errno
set to
.BR EACCES .
.PP
If the header of a file isn't recognized (the attempted
.BR execve (2)
failed with the error
.BR ENOEXEC ),
these functions will execute the shell
.RI ( /bin/sh )
with the path of the file as its first argument.
(If this attempt fails, no further searching is done.)
.PP
All other
.BR exec ()
functions (which do not include 'p' in the suffix)
take as their first argument a (relative or absolute) pathname
that identifies the program to be executed.
.SH RETURN VALUE
The
.BR exec ()
functions return only if an error has occurred.
The return value is \-1, and
.I errno
is set to indicate the error.
.SH ERRORS
All of these functions may fail and set
.I errno
for any of the errors specified for
.BR execve (2).
.SH VERSIONS
The
.BR execvpe ()
function first appeared in glibc 2.11.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR execl (),
.BR execle (),
.BR execv ()
T}	Thread safety	MT-Safe
T{
.BR execlp (),
.BR execvp (),
.BR execvpe ()
T}	Thread safety	MT-Safe env
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.PP
The
.BR execvpe ()
function is a GNU extension.
.SH NOTES
The default search path (used when the environment
does not contain the variable \fBPATH\fR)
shows some variation across systems.
It generally includes
.I /bin
and
.IR /usr/bin
(in that order) and may also include the current working directory.
On some other systems, the current working is included after
.I /bin
and
.IR /usr/bin ,
as an anti-Trojan-horse measure.
The glibc implementation long followed the traditional default where
the current working directory is included at the start of the search path.
However, some code refactoring during the development of glibc 2.24
.\" glibc commit 1eb8930608705702d5746e5491bab4e4429fcb83
caused the current working directory to be dropped altogether
from the default search path.
This accidental behavior change is considered mildly beneficial,
and won't be reverted.
.PP
The behavior of
.BR execlp ()
and
.BR execvp ()
when errors occur while attempting to execute the file is historic
practice, but has not traditionally been documented and is not specified by
the POSIX standard.
BSD (and possibly other systems) do an automatic
sleep and retry if
.B ETXTBSY
is encountered.
Linux treats it as a hard
error and returns immediately.
.PP
Traditionally, the functions
.BR execlp ()
and
.BR execvp ()
ignored all errors except for the ones described above and
.B ENOMEM
and
.BR E2BIG ,
upon which they returned.
They now return if any error other than the ones
described above occurs.
.SH BUGS
Before glibc 2.24,
.BR execl ()
and
.BR execle ()
employed
.BR realloc (3)
internally and were consequently not async-signal-safe,
in violation of the requirements of POSIX.1.
.\" https://sourceware.org/bugzilla/show_bug.cgi?id=19534
This was fixed in glibc 2.24.
.\"
.SS Architecture-specific details
On sparc and sparc64,
.BR execv ()
is provided as a system call by the kernel
(with the prototype shown above)
for compatibility with SunOS.
This function is
.I not
employed by the
.BR execv ()
wrapper function on those architectures.
.SH SEE ALSO
.BR sh (1),
.BR execve (2),
.BR execveat (2),
.BR fork (2),
.BR ptrace (2),
.BR fexecve (3),
.BR system (3),
.BR environ (7)