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Integrate C Code in Custom S-Functions

You can integrate C code in custom S-functions to work with Simulink® Desktop Real-Time™ applications.


Do not use the Analog Input, Analog Output, Digital Input, or Digital Output blocks as starting points for creating custom blocks.

Supported C Functions

You can use ANSI® C functions that do not use the operating system in your custom blocks. The following includes a partial list of supported functions:

  • Console I/Oprintf

    The printf function sends output to the MATLAB® Command Window when it is called from the real-time application.

  • Data conversionabs, atof, atoi, atol, itoa, labs, ltoa, strtod, strtol, strtoul, ultoa

  • Memory allocationcalloc, free, malloc

    Memory allocation is not an operation that can be done in real time. To work with a Simulink Desktop Real-Time application, memory management must occur before real-time simulation begins. Simulation switches into real time after mdlStart, so you can allocate memory in mdlInitializeSizes or mdlStart. You cannot allocate memory in any function after mdlStart, such as mdlOutputs or mdlUpdate.

  • Memory manipulation_memccpy, memcpy, memchr, memcmp, _memicmp, memmove, memset

  • Character string manipulationstrcat, strchr, strcmp, strcpy, strcspn, _strdup, _stricmp, strlen, _strlwr, strncat, strncmp, strncpy, _strnset, strpbrk, strrchr, _strrev, _strset, strspn, strstr, strtok, strupr

  • Mathematicalacos, asin, atan, atan2, ceil, cos, cosh, div, exp, fabs, floor, fmod, frexp, ldexp, ldiv, log, log10, max, min, modf, pow, rand, sin, sinh, sqrt, srand, tan, tanh, uldiv

  • Character class tests and conversionisalnum, isalpha, _isascii, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, isxdigit, isxupper, isxlower, _toascii, tolower, toupper

  • Searching and sortingbsearch, qsort

  • Dummy functionsexit, fprintf

Unsupported C Functions

If you create your own custom blocks, use only C functions that Simulink Desktop Real-Time supports. Simulink Desktop Real-Time does not support functions that use the operating system. This category includes functions from vendor-supplied block libraries for the operating system, which are also not supported.

The following list includes many, but not all, of the unsupported functions:

  • File I/Ofopen, freopen, fclose, fread, fwrite, fputs, fputc, fgets, fgetc, gets, getc, getchar, puts, putc, putchar, fflush, setbuf, setvbuf

  • Console I/Ofprintf, sprintf, vfprintf, vprintf, vsprintf, fscanf, scanf, sscanf

  • Process managementspawn, exit, abort, atexit

  • Signals and exceptionssignal, longimp, raise

  • Time functionsclock, time, difftime, asctime, ctime, difftime, gmtime, localtime, mktime, strftime

  • Operating system API functions No operating system API functions, such as Win64 functions, are supported.

Incompatibility with Operating System API Calls

The Simulink Desktop Real-Time kernel intercepts the interrupt from the system clock. It then reprograms the system clock to operate at a higher frequency for running your real-time application. At the original clock frequency, it sends an interrupt to the operating system to allow software that uses the operating system API to run.

As a result, software that uses the operating system API, such as Win64 functions, cannot be executed as a component of your real-time application. Software you use to write I/O drivers must not make calls to the operating system API.

I/O Register Access from S-Functions Limitation

Operating system drivers can access I/O registers only from the real-time kernel and not from the Simulink software. To prevent drivers from attempting to access I/O registers from Simulink S-functions, enter code fragments like the following:

/* we are in real-time kernel, do board I/O */
/* we are in Simulink, don't do board I/O */