Learning Forum

[muhammaddanishmasood]

I am having trouble loading a UDF that writes data after defined timestep. The UDF compiles without any errors, but Fluent refuses to load it whenever the code attempts to save data. The strange part is that this happens even when using a single processor in the Fluent Launcher. I am using ANSYS 2025 R2 Student version.

If I remove the file-writing part, the UDF loads and runs normally.

Here is the UDF:

 

#include "udf.h"

#include

#define WRITE_FREQUENCY 10

DEFINE_EXECUTE_AT_END(write_flow_data)

int current_ts = N_TIME; if (current_ts % WRITE_FREQUENCY == 0 && current_ts > 0)

  #if RP_HOST char filename[256];

sprintf(filename, "flowfield_ts_%d.dat", current_ts);

Message("[UDF] Writing data file: %s\n", filename);

Write_Data(filename);

Message("[UDF] Done writing file.\n"); #endif

I also tried the alternative version:

 

#if RP_HOST RP_Write_Data(filename); #endif

Both versions compile correctly, but when I try to load the library Fluent reports:

 

ERROR: The UDF library you are trying to load (libudf\win64\2ddp_node\libudf.dll) is not compiled for parallel use

This is confusing because:

• I launch Fluent in Serial mode

• The UDF only fails when data-writing functions are present

• Without the Write_Data/RP_Write_Data call, the same UDF loads fine

[Rob]

I assume it's actually compiled, loaded and hooked into Fluent without any errors? Are there any warnings? Is time really an integer? 

[muhammaddanishmasood]

Right now, I am only checking for integral values. There are no problem while building it. But on loading, I get the error: 
Copyright 1987-2025 ANSYS, Inc. All Rights Reserved. Compiler and linker: Microsoft Visual C++

# Generating ud_io1.h UDF_timestep.c

# Generating udf_names.c because of makefile UDF_timestep.obj udf_names.c

# Linking libudf.dll because of makefile user_nt.udf udf_names.obj UDF_timestep.obj Creating library libudf.lib and object libudf.exp

Done.

DESKTOP-G8ISVJC:

Opening library “C:\Fluent_UDF_Test\libudf”… ERROR: The UDF library you are trying to load (libudf) is not compiled for parallel use on the current platform (win64). The system cannot find the file specified. C:\\Fluent_UDF_Test\\libudf\\win64\\2ddp_host\\libudf.dll Done.

 

[Rob]

So it's not built. Try compiling with the built in CLANG compiler and see what comes up. 

[muhammaddanishmasood]

I get this error while using the built i compiler.

Compiler path: "C:\PROGRA~1\ANSYSI~1\ANSYSS~1\v252\fluent"\ntbin\clang\bin\clang-cl

Linker path: "C:\PROGRA~1\ANSYSI~1\ANSYSS~1\v252\fluent"\ntbin\clang\bin\lld-link

C sources: ['MYUDF3.c']

lld-link: error: undefined symbol: Write_Data

>>> referenced by MYUDF3.obj:(write_flow_data)

scons: *** [libudf.dll] Error 1

Creating user_nt.udf file for 2ddp_node ...

But I can build it using Visual code compiler:

Compiler and linker: Microsoft Visual C++

# Generating ud_io1.h

MYUDF3.c

# Generating udf_names.c because of makefile MYUDF3.obj

udf_names.c

# Linking libudf.dll because of makefile user_nt.udf udf_names.obj MYUDF3.obj

Creating library libudf.lib and object libudf.exp

 

Done.

 

[Rob]

So the built in compiler has a problem. Are you compiling the VS version in Fluent or outside Fluent? Note, a .c file can compile as it's gramatically correct but still fail to build for Fluent if it's not appropriate. 

[muhammaddanishmasood]

I am running inside fluent.

 

[Rob]

Have you read https://ansyshelp.ansys.com/public/account/Secured?returnurl=/Views/Secured/corp/v252/en/flu_udf/flu_udf_sec_parallel_printing_reading.html%23flu_udf_sec_parallel_reading  ?