man-pages/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  2008-08-20 "GNU" "Linux Programmer's Manual"
.SH NAME
execl, execlp, execle, execv, execvp \- execute a file
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.B extern char **environ;
.sp
.BI "int execl(const char *" path ", const char *" arg ", ...);"
.br
.BI "int execlp(const char *" file ", const char *" arg ", ...);"
.br
.BI "int execle(const char *" path ", const char *" arg ,
.br
.BI "           ..., char * const " envp "[]);"
.br
.BI "int execv(const char *" path ", char *const " argv "[]);"
.br
.BI "int execvp(const char *" file ", char *const " argv "[]);"
.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 front-ends for
.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 pathname of a file which is
to be executed.
.PP
The
.I "const char *arg"
and subsequent ellipses in the
.BR execl (),
.BR execlp (),
and
.BR execle ()
functions 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
The
.BR execv ()
and
.BR execvp ()
functions provide 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.
.PP
The
.BR execle ()
function also specifies the environment of the executed process by following
the NULL
pointer that terminates the list of arguments in the argument list or the
pointer to the
.I argv
array with an additional argument.
This additional
argument is an array of pointers to null-terminated strings and
.I must
be terminated by a NULL pointer.
The other functions take the environment for the new process
image from the external variable
.I environ
in the current process.
.SS Special semantics for execlp() and execvp()
.PP
The functions
.BR execlp ()
and
.BR execvp ()
will duplicate the actions of the shell in searching for an executable file
if the specified filename does not contain a slash (/) character.
The search path is the path specified in the environment by the
.B PATH
variable.
If this variable isn't specified, the default path
":/bin:/usr/bin" is used.
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 the global variable
.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.)
.SH "RETURN VALUE"
If any of the
.BR exec ()
functions returns, an error will have occurred.
The return value is \-1,
and the global variable
.I errno
will be set to indicate the error.
.SH ERRORS
All of these functions may fail and set
.I errno
for any of the errors specified for the library function
.BR execve (2).
.SH "CONFORMING TO"
POSIX.1-2001.
.SH NOTES
On some other systems the default path (used when the environment
does not contain the variable \fBPATH\fR) has the current working
directory listed after
.I /bin
and
.IR /usr/bin ,
as an anti-Trojan-horse measure.
Linux uses here the
traditional "current directory first" default path.
.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 "SEE ALSO"
.BR sh (1),
.BR execve (2),
.BR fork (2),
.BR ptrace (2),
.BR fexecve (3),
.BR environ (7)