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@gmail.com>
  37 .\"     Added note on casting NULL
  38 .\"
  39 .TH EXEC 3  2008-07-10 "GNU" "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
  64 .BR execve (2).
  65 (See the manual page for
  66 .BR execve (2)
  67 for further details about the replacement of the current process image.)
  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.
  88 The list of arguments
  89 .I must
  90 be terminated by a NULL
  91 pointer, and, since these are variadic functions, this pointer must be cast
  92 .IR "(char *) NULL" .
  93 .PP
  94 The
  95 .BR execv ()
  96 and
  97 .BR execvp ()
  98 functions provide an array of pointers to null-terminated strings that
  99 represent the argument list available to the new program.
 100 The first argument, by convention, should point to the filename
 101 associated with the file being executed.
 102 The array of pointers
 103 .I must
 104 be terminated by a NULL pointer.
 105 .PP
 106 The
 107 .BR execle ()
 108 function also specifies the environment of the executed process by following
 109 the NULL
 110 pointer that terminates the list of arguments in the parameter list or the
 111 pointer to the argv array with an additional parameter.
 112 This additional
 113 parameter is an array of pointers to null-terminated strings and
 114 .I must
 115 be terminated by a NULL pointer.
 116 The other functions take the environment for the new process
 117 image from the external variable
 118 .I environ
 119 in the current process.
 120 .SS Special semantics for execlp() and execvp()
 121 .PP
 122 The functions
 123 .BR execlp ()
 124 and
 125 .BR execvp ()
 126 will duplicate the actions of the shell in searching for an executable file
 127 if the specified filename does not contain a slash (/) character.
 128 The search path is the path specified in the environment by the
 129 .B PATH
 130 variable.
 131 If this variable isn't specified, the default path
 132 ":/bin:/usr/bin" is used.
 133 In addition, certain
 134 errors are treated specially.
 135 .PP
 136 If permission is denied for a file (the attempted
 137 .BR execve (2)
 138 returned
 139 .BR EACCES ),
 140 these functions will continue searching the rest of the search path.
 141 If no other file is found, however,
 142 they will return with the global variable
 143 .I errno
 144 set to
 145 .BR EACCES .
 146 .PP
 147 If the header of a file isn't recognized (the attempted
 148 .BR execve (2)
 149 returned
 150 .BR ENOEXEC ),
 151 these functions will execute the shell
 152 .RI ( /bin/sh )
 153 with the path of the file as its first argument.
 154 (If this attempt fails, no further searching is done.)
 155 .SH "RETURN VALUE"
 156 If any of the
 157 .BR exec ()
 158 functions returns, an error will have occurred.
 159 The return value is \-1,
 160 and the global variable
 161 .I errno
 162 will be set to indicate the error.
 163 .SH ERRORS
 164 All of these functions may fail and set
 165 .I errno
 166 for any of the errors specified for the library function
 167 .BR execve (2).
 168 .SH "CONFORMING TO"
 169 POSIX.1-2001.
 170 .SH NOTES
 171 On some other systems the default path (used when the environment
 172 does not contain the variable \fBPATH\fR) has the current working
 173 directory listed after
 174 .I /bin
 175 and
 176 .IR /usr/bin ,
 177 as an anti-Trojan-horse measure.
 178 Linux uses here the
 179 traditional "current directory first" default path.
 180 .PP
 181 The behavior of
 182 .BR execlp ()
 183 and
 184 .BR execvp ()
 185 when errors occur while attempting to execute the file is historic
 186 practice, but has not traditionally been documented and is not specified by
 187 the POSIX standard.
 188 BSD (and possibly other systems) do an automatic
 189 sleep and retry if
 190 .B ETXTBSY
 191 is encountered.
 192 Linux treats it as a hard
 193 error and returns immediately.
 194 .PP
 195 Traditionally, the functions
 196 .BR execlp ()
 197 and
 198 .BR execvp ()
 199 ignored all errors except for the ones described above and
 200 .B ENOMEM
 201 and
 202 .BR E2BIG ,
 203 upon which they returned.
 204 They now return if any error other than the ones
 205 described above occurs.
 206 .SH "SEE ALSO"
 207 .BR sh (1),
 208 .BR execve (2),
 209 .BR fork (2),
 210 .BR ptrace (2),
 211 .BR fexecve (3),
 212 .BR environ (7)