Why do I receive linker errors when compiling a fixed-point Simulink S-functions?

I wrote a fixed-point S-function that uses functions from the Fixed-Point S-Function API, such as:

• ssRegisterDataTypeFxpBinaryPoint
• ssRegisterDataTypeFxpSlopeBias
• ssRegisterDataTypeHalfPrecision
• ssRegisterDataTypeFxpScaledDouble
• ssRegisterDataTypeFxpFSlopeFixExpBias
• ssFxpSetU32BitRegionCompliant
• ssFxpGetU32BitRegionCompliant
• ssFxpSetU32BitRegion
• ssFxpGetU32BitRegion

When I compile the S-function using MEX, I receive the following error:

ERROR: >> mex sfun_user_fxp_asr.c
Building with 'Microsoft Visual C++ 2022 (C)'.
Error using mex
Creating library sfun_user_fxp_asr.lib and object sfun_user_fxp_asr.exp
sfun_user_fxp_asr.obj : error LNK2019: unresolved external symbol ssRegisterDataTypeFxpBinaryPoint referenced in function mdlSetInputPortDataType
sfun_user_fxp_asr.obj : error LNK2019: unresolved external symbol ssGetDataTypeIsFxpFltApiCompat referenced in function isDataTypeSupported
sfun_user_fxp_asr.obj : error LNK2019: unresolved external symbol ssGetDataTypeStorageContainCat referenced in function isDataTypeSupported
sfun_user_fxp_asr.obj : error LNK2019: unresolved external symbol ssGetDataTypeFxpWordLength referenced in function mdlSetInputPortDataType
sfun_user_fxp_asr.obj : error LNK2019: unresolved external symbol ssGetDataTypeFxpIsSigned referenced in function mdlSetInputPortDataType
sfun_user_fxp_asr.obj : error LNK2019: unresolved external symbol ssGetDataTypeIsScalingPow2 referenced infunction isDataTypeSupported
sfun_user_fxp_asr.obj : error LNK2019: unresolved external symbol ssGetDataTypeFractionLength referenced in function mdlSetInputPortDataType
sfun_user_fxp_asr.mexw64 : fatal error LNK1120: 7 unresolved externals

Answer 1

[MathWorks Support Team]

To compile a fixed-point S-function, additional steps must be used compared to standard C-MEX S-functions.

1. In your S-function, include "fixedpoint.c" and "fixedpoint.h". For more information, see: 

Structure of the S-Function

2. Pass an extra argument, "-lfixedpoint", to the MEX command. For example:

>> mex('sfun_user_fxp_asr.c','-lfixedpoint')

Passing the extra argument should resolve the linker errors.

The documentation on that topic is located at: Creating MEX-Files