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.
  63 The functions described in this manual page are front-ends for the
  64 function
  65 .BR execve (2).
  66 (See the manual page for
  67 .BR execve (2)
  68 for detailed information about the replacement of the current process.)
  69 .PP
  70 The initial argument for these functions is the pathname of a file which is
  71 to be executed.
  72 .PP
  73 The
  74 .I "const char *arg"
  75 and subsequent ellipses in the
  76 .BR execl (),
  77 .BR execlp (),
  78 and
  79 .BR execle ()
  80 functions can be thought of as
  81 .IR arg0 ,
  82 .IR arg1 ,
  83 \&...,
  84 .IR argn .
  85 Together they describe a list of one or more pointers to null-terminated
  86 strings that represent the argument list available to the executed program.
  87 The first argument, by convention, should point to the filename associated
  88 with the file being executed.
  89 The list of arguments
  90 .I must
  91 be terminated by a NULL
  92 pointer, and, since these are variadic functions, this pointer must be cast
  93 .BR "(char *) NULL" .
  94 .PP
  95 The
  96 .BR execv ()
  97 and
  98 .BR execvp ()
  99 functions provide an array of pointers to null-terminated strings that
 100 represent the argument list available to the new program.
 101 The first argument, by convention, should point to the filename
 102 associated with the file being executed.
 103 The array of pointers
 104 .I must
 105 be terminated by a NULL pointer.
 106 .PP
 107 The
 108 .BR execle ()
 109 function also specifies the environment of the executed process by following
 110 the NULL
 111 pointer that terminates the list of arguments in the parameter list or the
 112 pointer to the argv array with an additional parameter.
 113 This additional
 114 parameter is an array of pointers to null-terminated strings and
 115 .I must
 116 be terminated by a NULL pointer.
 117 The other functions take the environment for the new process
 118 image from the external variable
 119 .I environ
 120 in the current process.
 121 .PP
 122 Some of these functions have special semantics.
 123 .PP
 124 The functions
 125 .BR execlp ()
 126 and
 127 .BR execvp ()
 128 will duplicate the actions of the shell in searching for an executable file
 129 if the specified filename does not contain a slash (/) character.
 130 The search path is the path specified in the environment by the
 131 .B PATH
 132 variable.
 133 If this variable isn't specified, the default path
 134 ``:/bin:/usr/bin'' is used.
 135 In addition, certain
 136 errors are treated specially.
 137 .PP
 138 If permission is denied for a file (the attempted
 139 .BR execve ()
 140 returned
 141 .BR EACCES ),
 142 these functions will continue searching the rest of the search path.
 143 If no other file is found, however,
 144 they will return with the global variable
 145 .I errno
 146 set to
 147 .BR EACCES .
 148 .PP
 149 If the header of a file isn't recognized (the attempted
 150 .BR execve ()
 151 returned
 152 .BR ENOEXEC ),
 153 these functions will execute the shell with the path of the file as its
 154 first argument.
 155 (If this attempt fails, no further searching is done.)
 156 .SH "RETURN VALUE"
 157 If any of the
 158 .BR exec ()
 159 functions returns, an error will have occurred.
 160 The return value is \-1,
 161 and the global variable
 162 .I errno
 163 will be set to indicate the error.
 164 .SH FILES
 165 .I /bin/sh
 166 .SH ERRORS
 167 All of these functions may fail and set
 168 .I errno
 169 for any of the errors specified for the library function
 170 .BR execve (2).
 171 .SH "SEE ALSO"
 172 .BR sh (1),
 173 .BR execve (2),
 174 .BR fork (2),
 175 .BR ptrace (2),
 176 .BR fexecve (3),
 177 .BR environ (7)
 178 .SH COMPATIBILITY
 179 On some other systems the default path (used when the environment
 180 does not contain the variable \fBPATH\fR) has the current working
 181 directory listed after
 182 .I /bin
 183 and
 184 .IR /usr/bin ,
 185 as an anti-Trojan-horse measure.
 186 Linux uses here the
 187 traditional "current directory first" default path.
 188 .PP
 189 The behavior of
 190 .BR execlp ()
 191 and
 192 .BR execvp ()
 193 when errors occur while attempting to execute the file is historic
 194 practice, but has not traditionally been documented and is not specified by
 195 the POSIX standard.
 196 BSD (and possibly other systems) do an automatic
 197 sleep and retry if ETXTBSY is encountered.
 198 Linux treats it as a hard
 199 error and returns immediately.
 200 .PP
 201 Traditionally, the functions
 202 .BR execlp ()
 203 and
 204 .BR execvp ()
 205 ignored all errors except for the ones described above and
 206 .B ENOMEM
 207 and
 208 .BR E2BIG ,
 209 upon which they returned.
 210 They now return if any error other than the ones
 211 described above occurs.
 212 .SH "CONFORMING TO"
 213 POSIX.1-2001.