This is documentation for Mathematica 8, which was
based on an earlier version of the Wolfram Language.
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Portability of MathLink Programs

The Mathematica side of a MathLink connection is set up to work exactly the same on all computer systems. But inevitably there are differences between external programs on different computer systems.
For a start, different computer systems almost always require different executable binaries. When you call Install, therefore, you must be sure that prog corresponds to a program that can be executed on your particular computer system.
Install["file"]try to execute file directly
Install["file",LinkProtocol->"type"]use the specified protocol for low-level data transport
$SystemIDidentify the type of computer system being used
Install["dir"]try to execute a file with a name of the form dir/$SystemID/dir

Installing programs on different computer systems.

Mathematica follows the convention that if prog is an ordinary file, then Install will just try to execute it. But if prog is a directory, then Mathematica will look for a subdirectory of that directory whose name agrees with the current value of $SystemID, and will then try to execute a file named prog within that subdirectory.
mcc -o prog ... put compiled code in the file prog in the current directory
mcc -xo prog ... put compiled code in prog/$SystemID/prog

Typical Unix commands for compiling external programs.

Even though the executable binary of an external program is inevitably different on different computer systems, it can still be the case that the source code in a language such as C from which this binary is obtained can be essentially the same.
But to achieve portability in your C source code there are several points that you need to watch.
For a start, you should never make use of extra features of the C language or C runtime libraries that happen to be provided on a particular system, but are not part of standard C. In addition, you should try to avoid dealing with segmented or otherwise special memory models.
The include file mathlink.h contains standard C prototypes for all the functions in the MathLink library.
MLPutInteger32()MLGetInteger32()integer corresponding to C type , that is, 32 bits
MLPutInteger16()MLGetInteger16()integer of type , that is, 16 bits
MLPutInteger64()MLGetInteger64()64-bit integer
MLPutReall64()MLGetReal64()IEEE double-precision real number, corresponding to the C-language type
MLPutReal32()MLGetReal32()IEEE single-precision real number, corresponding to the C-language type
MLPutReal128()MLGetReal128()IEEE quad-precision real number

MathLink functions that use specific C types.

If you are going to call MathLink library functions in a portable way, it is essential that you use the same types as they do.
If your programs correctly match the argument types for the MathLink library functions, you do not have to worry about C type differences between computer systems. MathLink automatically converts the C types to the appropriate sizes for each platform. MathLink also swaps bytes as needed to correctly transfer numbers across platforms, and it converts between floating-point number formats with the smallest possible loss of precision.
MLPutString(stdlink,char*s)put a null-terminated C character string
MLPutUnicodeString(stdlink,unsigned short*s,int n)
put a string encoded in terms of 16-bit UCS-2 Unicode characters
MLPutByteString(stdlink,unsigned char*s,int n)
put a string containing only 8-bit character codes
MLPutUTF8String(stdlink, const unsigned char*s,int n)put a string of UTF-8 encoded Unicode characters
MLPutUTF16String(stdlink, const unsigned short*s,int n)put a string of UTF-16 encoded Unicode characters
MLPutUTF32String(stdlink, const unsigned int*s,int n)put a string of UTF-32 encoded Unicode characters
MLGetString(stdlink,char**s)get a null-terminated C character string
MLGetUnicodeString(stdlink,unsigned short**s,long*n)
get a string encoded in terms of 16-bit UCS-2 Unicode characters
MLGetByteString(stdlink,unsigned char**s,long*n,long spec)
get a string containing only 8-bit character codes, using spec as the code for all 16-bit characters
MLGetUTF8String(stdlink, const unsigned char**s,int*m,int*n)get a string of UTF-8 encoded Unicode characters
MLGetUTF16String(stdlink, const unsigned short**s,int*m,int*n)get a string of UTF-16 encoded Unicode characters
MLGetUTF32String(stdlink, const unsigned int**s,int*n)get a string of UTF-32 encoded Unicode characters

Manipulating general strings.

In simple C programs, it is typical to use strings that contain only ordinary ASCII characters. But in Mathematica it is possible to have strings containing all sorts of special characters. These characters are specified within Mathematica using Unicode character codes, as discussed in "Raw Character Encodings".
C language strings typically use only 8 bits to store the code for each character. UCS-2 encoded strings, however, require 16 bits. As a result, the functions MLPutUnicodeString() and MLGetUnicodeString() work with arrays of integers. The same is true of UTF-16 encoded strings and the corresponding functions MLPutUTF16String() and MLGetUTF16String().
UTF-32 encoded strings require 32 bits for each character, and the corresponding functions MLPutUTF32String() and MLGetUTF32String() work with arrays of unsigned int integers.
If you know that your program will not have to handle special characters, then you may find it convenient to use MLPutByteString() and MLGetByteString(). These functions represent all characters directly using 8-bit character codes. If a special character is sent from Mathematica, then it will be converted by MLGetByteString() to a fixed code that you specify.
may need to be different on different computer systems

A point to watch in creating portable MathLink programs.

Computer systems and compilers that have C runtime libraries based on the Unix model allow MathLink programs to have a main program of the form which simply calls MLMain.
Some computer systems or compilers may however require main programs of a different form. You should realize that you can do whatever initialization you want inside before calling MLMain(). Once you have called MLMain(), however, your program will effectively go into an infinite loop, responding to requests from Mathematica until the link to it is closed.