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  2009-02-22 "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 argument list or the
 111 pointer to the
 112 .I argv
 113 array with an additional argument.
 114 This additional
 115 argument is an array of pointers to null-terminated strings and
 116 .I must
 117 be terminated by a NULL pointer.
 118 The other functions take the environment for the new process
 119 image from the external variable
 120 .I environ
 121 in the current process.
 122 .SS Special semantics for execlp() and execvp()
 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 (2)
 140 failed with the error
 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
 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 (2)
 151 failed with the error
 152 .BR ENOEXEC ),
 153 these functions will execute the shell
 154 .RI ( /bin/sh )
 155 with the path of the file as its first argument.
 156 (If this attempt fails, no further searching is done.)
 157 .SH "RETURN VALUE"
 158 If any of the
 159 .BR exec ()
 160 functions returns, an error will have occurred.
 161 The return value is \-1, and
 162 .I errno
 163 will be set to indicate the error.
 164 .SH ERRORS
 165 All of these functions may fail and set
 166 .I errno
 167 for any of the errors specified for the library function
 168 .BR execve (2).
 169 .SH "CONFORMING TO"
 170 POSIX.1-2001.
 171 .SH NOTES
 172 On some other systems the default path (used when the environment
 173 does not contain the variable \fBPATH\fR) has the current working
 174 directory listed after
 175 .I /bin
 176 and
 177 .IR /usr/bin ,
 178 as an anti-Trojan-horse measure.
 179 Linux uses here the
 180 traditional "current directory first" default path.
 181 .PP
 182 The behavior of
 183 .BR execlp ()
 184 and
 185 .BR execvp ()
 186 when errors occur while attempting to execute the file is historic
 187 practice, but has not traditionally been documented and is not specified by
 188 the POSIX standard.
 189 BSD (and possibly other systems) do an automatic
 190 sleep and retry if
 191 .B ETXTBSY
 192 is encountered.
 193 Linux treats it as a hard
 194 error and returns immediately.
 195 .PP
 196 Traditionally, the functions
 197 .BR execlp ()
 198 and
 199 .BR execvp ()
 200 ignored all errors except for the ones described above and
 201 .B ENOMEM
 202 and
 203 .BR E2BIG ,
 204 upon which they returned.
 205 They now return if any error other than the ones
 206 described above occurs.
 207 .SH "SEE ALSO"
 208 .BR sh (1),
 209 .BR execve (2),
 210 .BR fork (2),
 211 .BR ptrace (2),
 212 .BR fexecve (3),
 213 .BR environ (7)