man3/exec.3

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  32 .\"     @(#)exec.3      6.4 (Berkeley) 4/19/91
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  34 .\" Converted for Linux, Mon Nov 29 11:12:48 1993, faith@cs.unc.edu
  35 .\" Updated more for Linux, Tue Jul 15 11:54:18 1997, pacman@cqc.com
  36 .\" Modified, 24 Jun 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  37 .\"     Added note on casting NULL
  38 .\"
  39 .TH EXEC 3  1993-11-29 "BSD MANPAGE" "Linux Programmer's Manual"
  40 .SH NAME
  41 execl, execlp, execle, execv, execvp \- execute a file
  42 .SH SYNOPSIS
  43 .B #include <unistd.h>
  44 .sp
  45 .B extern char **environ;
  46 .sp
  47 .BI "int execl(const char *" path ", const char *" arg ", ...);"
  48 .br
  49 .BI "int execlp(const char *" file ", const char *" arg ", ...);"
  50 .br
  51 .BI "int execle(const char *" path ", const char *" arg ,
  52 .br
  53 .BI "           ..., char * const " envp "[]);"
  54 .br
  55 .BI "int execv(const char *" path ", char *const " argv "[]);"
  56 .br
  57 .BI "int execvp(const char *" file ", char *const " argv "[]);"
  58 .SH DESCRIPTION
  59 The
  60 .BR exec ()
  61 family of functions replaces the current process image with a new process
  62 image.  The functions described in this manual page are front-ends for the
  63 function
  64 .BR execve (2).
  65 (See the manual page for
  66 .BR execve ()
  67 for detailed information about the replacement of the current process.)
  68 .PP
  69 The initial argument for these functions is the pathname of a file which is
  70 to be executed.
  71 .PP
  72 The
  73 .I "const char *arg"
  74 and subsequent ellipses in the
  75 .BR execl (),
  76 .BR execlp (),
  77 and
  78 .BR execle ()
  79 functions can be thought of as
  80 .IR arg0 ,
  81 .IR arg1 ,
  82 \&...,
  83 .IR argn .
  84 Together they describe a list of one or more pointers to null-terminated
  85 strings that represent the argument list available to the executed program.
  86 The first argument, by convention, should point to the filename associated
  87 with the file being executed.  The list of arguments
  88 .I must
  89 be terminated by a NULL
  90 pointer, and, since these are variadic functions, this pointer must be cast
  91 .BR "(char *) NULL" .
  92 .PP
  93 The
  94 .BR execv ()
  95 and
  96 .BR execvp ()
  97 functions provide an array of pointers to null-terminated strings that
  98 represent the argument list available to the new program.  The first
  99 argument, by convention, should point to the filename associated with the
 100 file being executed.  The array of pointers
 101 .I must
 102 be terminated by a NULL pointer.
 103 .PP
 104 The
 105 .BR execle ()
 106 function also specifies the environment of the executed process by following
 107 the NULL
 108 pointer that terminates the list of arguments in the parameter list or the
 109 pointer to the argv array with an additional parameter.  This additional
 110 parameter is an array of pointers to null-terminated strings and
 111 .I must
 112 be terminated by a NULL pointer.
 113 The other functions take the environment for the new process
 114 image from the external variable
 115 .I environ
 116 in the current process.
 117 .PP
 118 Some of these functions have special semantics.
 119 .PP
 120 The functions
 121 .BR execlp ()
 122 and
 123 .BR execvp ()
 124 will duplicate the actions of the shell in searching for an executable file
 125 if the specified filename does not contain a slash (/) character.  The
 126 search path is the path specified in the environment by the
 127 .B PATH
 128 variable.  If this variable isn't specified, the default path
 129 ``:/bin:/usr/bin'' is used.  In addition, certain
 130 errors are treated specially.
 131 .PP
 132 If permission is denied for a file (the attempted
 133 .BR execve ()
 134 returned
 135 .BR EACCES ),
 136 these functions will continue searching the rest of the search path.  If no
 137 other file is found, however, they will return with the global variable
 138 .I errno
 139 set to
 140 .BR EACCES .
 141 .PP
 142 If the header of a file isn't recognized (the attempted
 143 .BR execve ()
 144 returned
 145 .BR ENOEXEC ),
 146 these functions will execute the shell with the path of the file as its
 147 first argument.  (If this attempt fails, no further searching is done.)
 148 .SH "RETURN VALUE"
 149 If any of the
 150 .BR exec ()
 151 functions returns, an error will have occurred.  The return value is \-1,
 152 and the global variable
 153 .I errno
 154 will be set to indicate the error.
 155 .SH FILES
 156 .I /bin/sh
 157 .SH ERRORS
 158 All of these functions may fail and set
 159 .I errno
 160 for any of the errors specified for the library function
 161 .BR execve (2).
 162 .SH "SEE ALSO"
 163 .BR sh (1),
 164 .BR execve (2),
 165 .BR fork (2),
 166 .BR ptrace (2),
 167 .BR fexecve (3),
 168 .BR environ (5)
 169 .SH COMPATIBILITY
 170 On some other systems the default path (used when the environment
 171 does not contain the variable \fBPATH\fR) has the current working
 172 directory listed after
 173 .I /bin
 174 and
 175 .IR /usr/bin ,
 176 as an anti-Trojan-horse measure. Linux uses here the
 177 traditional "current directory first" default path.
 178 .PP
 179 The behavior of
 180 .BR execlp ()
 181 and
 182 .BR execvp ()
 183 when errors occur while attempting to execute the file is historic
 184 practice, but has not traditionally been documented and is not specified by
 185 the POSIX standard. BSD (and possibly other systems) do an automatic
 186 sleep and retry if ETXTBSY is encountered. Linux treats it as a hard
 187 error and returns immediately.
 188 .PP
 189 Traditionally, the functions
 190 .BR execlp ()
 191 and
 192 .BR execvp ()
 193 ignored all errors except for the ones described above and
 194 .B ENOMEM
 195 and
 196 .BR E2BIG ,
 197 upon which they returned.  They now return if any error other than the ones
 198 described above occurs.
 199 .SH "CONFORMING TO"
 200 .BR execl (),
 201 .BR execv (),
 202 .BR execle (),
 203 .BR execlp ()
 204 and
 205 .BR execvp ()
 206 conform to
 207 IEEE Std1003.1-88 (``POSIX.1'').