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Error using mex G:\Matlab_​WS\code_te​st\Assignm​ent1\Add_T​wo.c:3:1: warning: return type defaults to 'int' [-Wimplicit-int] Add_Two(U16 a, U16 b)

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Trying to understand mex compilation with an example, but getting compilation error. Below are the commands that I used to compile. Mex file is generated but its compilation failed.
def = legacy_code('initialize');
def.SourceFiles = {'Add_Two.c'};
def.HeaderFiles = {'Add_Two.h'};
def.SFunctionName = 'Add_Two_SFun';
def.OutputFcnSpec = 'uint32 y1 = Add_Two(uint16 u1, uint16 u2)';
legacy_code('sfcn_cmex_generate', def);
>> legacy_code('compile', def)
### Start Compiling Add_Two_SFun
mex('-IG:\Matlab_WS\code_test\Assignment1', '-c', '-outdir', 'C:\Users\Shafi\AppData\Local\Temp\tp548d5365_ef4a_45c6_9aec_e200ba5a55ff', 'G:\Matlab_WS\code_test\Assignment1\Add_Two.c')
Building with 'MinGW64 Compiler (C)'.
Error using mex
G:\Matlab_WS\code_test\Assignment1\Add_Two.c:3:1: warning: return type defaults to 'int'
Add_Two(U16 a, U16 b)
G:\Matlab_WS\code_test\Assignment1\Add_Two.c:3:1: error: conflicting types for 'Add_Two'
In file included from G:\Matlab_WS\code_test\Assignment1\Add_Two.c:1:0:
G:\Matlab_WS\code_test\Assignment1/Add_Two.h:11:5: note: previous declaration of 'Add_Two' was here
U32 Add_Two(U16 a, U16 b);
Error in legacycode.LCT/compile
Error in legacycode.LCT.legacyCodeImpl
Error in legacy_code (line 101)
[varargout{1:nargout}] = legacycode.LCT.legacyCodeImpl(action, varargin{1:end});
Input files:
#include <Add_Two.h>
Add_Two(U16 a, U16 b)
U32 add_result;
U32 output;
add_result = (U32)a + (U32)b;
if(THOUSAND > add_result)
output = add_result;
output = SUM_DEFAULT;
return output;
#ifndef ADD_TWO_H_
#define ADD_TWO_H_
typedef unsigned int U16;
typedef unsigned long int U32;
#define THOUSAND (U32)1000
#define SUM_DEFAULT (U32)25
U32 Add_Two(U16 a, U16 b);

Accepted Answer

Andrew Janke
Andrew Janke on 31 Jan 2020
Edited: Andrew Janke on 31 Jan 2020
See this line where the function is actually defined in Add_Two.c?
Add_Two(U16 a, U16 b)
You need to add an explicit "`U32`" as its return type there.
U32 Add_Two(U16 a, U16 b)
Andrew Janke
Andrew Janke on 31 Jan 2020
Heheh. Happens to the best of us.
You need to keep that function declaration in there: because it is returning something besides a regular int, you need the explicit prototype in there to tell the C compiler what type of value to expect coming back from the function.

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