src/OS/os.Configuring

   1 $Cambridge: exim/src/OS/os.Configuring,v 1.1 2004/10/06 15:07:39 ph10 Exp $
   2
   3 Configuring Exim for different Operating Systems
   4 ------------------------------------------------
   5
   6 These notes describe the way in which Exim is configured at the C program level
   7 for different operating systems. The normal configuration options that apply
   8 independently of the operating system are specified by creating files in the
   9 Local directory, as described in chapter 4 of the manual.
  10
  11 These notes cover the os.* files in the OS directory, and contain information
  12 for people who want to port the program to some new OS, or to modify the
  13 configuration for an existing port. If you are just wanting to compile Exim on
  14 a system that it already knows about, you do not need to read further unless
  15 there are problems.
  16
  17 The os.c-<ostype> files
  18 -----------------------
  19
  20 There may be an os.c-<ostype> file for each operating system, but for many of
  21 them it is not necessary. No error occurs is there isn't one. There is a
  22 generic file called os.c which contains code that is common to two or more OS
  23 for setting a restarting or a non-restarting signal, for computing the load
  24 average, and for finding all the network interface addresses. A few OS have
  25 their own individual code for one or more of these. When they do, the code is
  26 put into an os.c-<ostype> file, which also defines a macro such as
  27 OS_RESTARTING_SIGNAL (for example) to cut out the common code in the generic
  28 os.c.
  29
  30 The os.h-<ostype> files
  31 -----------------------
  32
  33 For each OS that Exim knows about, there is an os.h-<ostype> file, where
  34 <ostype> is the OS name. The relevant file is included as a C header file for
  35 all Exim compilation by pointing a symbolic link called os.h at it from the
  36 build directory. The settings are as follows:
  37
  38 The select() function
  39 ---------------------
  40
  41 There is a difference in the data type for the second argument to the select()
  42 function in some OS. The macro SELECT_ARG2_TYPE can be used to define the type.
  43 If it is not defined in os.h, then it is defaulted to fs_set in exim.h.
  44
  45 The dn_expand() function
  46 ------------------------
  47
  48 There is a difference in the data type for the fourth argument to the
  49 dn_expand() function in some OS. The macro DN_EXPAND_ARG4_TYPE can be used to
  50 define the type. If it is not defined in os.h, then it is defaulted to char *
  51 in exim.h.
  52
  53 The h_errno variable
  54 --------------------
  55
  56 If NEED_H_ERRNO is defined, then a definition of the form
  57
  58 extern int h_errno
  59
  60 is included in the compiled code of Exim.
  61
  62 The strerror() function
  63 -----------------------
  64
  65 Most systems provide the ANSI standard strerror() function; older systems may
  66 instead have an errlist[] variable in which to look up error texts. Defining
  67 STRERROR_FROM_ERRLIST causes Exim to build its own strerror() function that
  68 mimics the ANSI function by lookup up the error code in errlist.
  69
  70 Truncating files
  71 ----------------
  72
  73 The fcntl() option for truncating the length of a file is called F_FREESP in
  74 most systems; in some, however, it is called O_TRUNC. Some os.h files define
  75 F_FREESP to be O_TRUNC for this reason.
  76
  77 Finding local interfaces
  78 ------------------------
  79
  80 The SIOCGIFCONF ioctl for finding local interfaces behaves differently on BSD
  81 systems. It returns a vector of ifreq blocks containing sockaddr structures
  82 that can be longer than their sizeof definition, making the returned ifreq
  83 blocks longer than their sizeof definitions. BSD sockaddrs structures contain
  84 an sa_len field giving the actual size. To cope with difference, there is a
  85 macro called HAVE_SA_LEN. If it is defined, code that works on BSD systems is
  86 used. Otherwise, the objects returned by SIOCGIFCONF are assumed to be of
  87 length sizeof(struct ifreq).
  88
  89 On some operating systems, the SIOCGIFCONF ioctl returns the IP addresses
  90 with the list of interfaces, and there is no need to call SIOCGIFADDR for each
  91 individual address. Mostly, making the second call does no harm, but on Linux
  92 when there are IP aliases, it causes things to go wrong. This also happens on
  93 BSDI. Therefore, there is now a macro to cut it out, called
  94 SIOCGIFCONF_GIVES_ADDR.
  95
  96 Note that, if IPv6 support is configured, Exim cannot find the IPv6 addresses
  97 on local interfaces by itself. You need to set the local_interfaces option in
  98 this situation.
  99
 100 Computing load averages
 101 -----------------------
 102
 103 There are several different ways that load averages are computed. One-off code
 104 is put in the os.c-<ostype>, but several OS use similar methods, and these
 105 are coded in the generic os.c, using a number of parameters to make variations
 106 between OS.
 107
 108 Sometimes the load average is not available to unprivileged callers. If
 109 LOAD_AVG_NEEDS_ROOT is set, Exim ensures that it is root before trying to
 110 obtain a load average value.
 111
 112 (1) If HAVE_BSD_GETLOADAVG is defined, Exim uses a simple call to the
 113 getloadavg() function.
 114
 115 (2) If HAVE_KSTAT is defined, Exim uses the kstat package as found in Solaris 2
 116 (but nowhere else as yet). It uses some supplementary definitions:
 117
 118   LOAD_AVG_KSTAT          the kstat to use
 119   LOAD_AVG_KSTAT_MODULE   the module to access
 120   LOAD_AVG_KSTAT_SYMBOL   the symbol containing the value we want
 121   LOAD_AVG_KSTAT_FIELD    the field identity
 122
 123 (3) If HAVE_DEV_KMEM is defined, Exim reads load average values from the
 124 /dev/kmem device. It uses some supplementary definitions:
 125
 126   LOAD_AVG_TYPE           the data type
 127   LOAD_AVG_SYMBOL         the symbol to look up
 128   KERNEL_PATH             the name of the kernel
 129   FSCALE                  a scaling factor
 130
 131 Sometimes FSCALE is defined in system headers so need not be defined in the
 132 os.h-<ostype> file.
 133
 134 Glibc systems and IP options
 135 ----------------------------
 136
 137 The code for inspecting IP options is the same in all OS except for systems
 138 using glibc (e.g. Linux), which uses a different structure to return data from
 139 getsockopt(). To handle this, there is a macro called
 140
 141   GLIBC_IP_OPTIONS
 142
 143 which should be set for Linux (in os.h-Linux) and any other operating system
 144 that uses glibc.
 145
 146 Options for statvfs()
 147 ---------------------
 148
 149 The following settings apply to the compilation of the Exim monitor as well as
 150 to the main Exim binary.
 151
 152 #undefine HAVE_STATFS
 153
 154 Exim has options for checking the amount of space in the spool partition
 155 before accepting a message, and the monitor has the ability to display a
 156 stripchart of the percentage fullness of a particular disc partition, usually
 157 /var/spool/mail. The standard way of finding out the data is to call the
 158 statvfs() function, but some operating systems use statfs() and some may not
 159 have the ability at all. The Exim code uses STATVFS() for this function and
 160 this gets defined appropriately. HAVE_STATFS is defined before including the
 161 os.h file; undefining it suppresses the code for checking a partition in the
 162 main binary, and for monitoring disc partition in the monitor.
 163
 164 When HAVE_STATFS is defined, the distinction between statvfs() and statfs() is
 165 made by checking HAVE_SYS_STATVFS_H. If it is defined, then sys/statvfs.h is
 166 included. Otherwise, STATVFS() is defined as a macro for statfs(), and some
 167 further includes are done, according to the following definitions:
 168
 169 #define HAVE_SYS_MOUNT_H
 170 #define HAVE_VFS_H
 171
 172 Each of those definitions causes the inclusion of the corresponding system
 173 header file in the Exim monitor compilation. For example, the first one causes
 174
 175 #include <sys/mount.h>
 176
 177 to be obeyed. Different systems may require different combinations of these
 178 headers.
 179
 180 The sys/resource.h header
 181 -------------------------
 182
 183 One OS does not have the sys/resource.h header. If NO_SYS_RESOURCE_H is defined
 184 in an os.h-<ostype> file, then the #include for this header is skipped in
 185 exim.h.
 186
 187 The crypt_h header
 188 ------------------
 189
 190 Some OS require crypt.h to be included to get a prototype for the crypt()
 191 function. This is needed only when compiling with AUTH support. If CRYPT_H is
 192 defined, then this header is included.
 193
 194 mmap() support
 195 --------------
 196
 197 The CDB support includes the option of handling file operations by using
 198 mmap()/munmap(). This gives a reasonable performance increase which will
 199 probably scale over multiple processes (since the files are mapped read-only
 200 shared). The vast majority of modern operating systems will support mmap
 201 (certainly in the simplified way that it is being used here). For example any
 202 BSD 4.x derived or POSIX compliant system will support it, as will pretty much
 203 any system using dynamically shared link libraries.
 204
 205 If the OS is believed to support mmap() then the symbol HAVE_MMAP is defined.
 206 Not all systems that support mmap will have had their config files updated to
 207 reflect this. Currently Linux, Sun, BSD and SGI/mips systems have been updated.
 208
 209 *** End ***