GR

GR said:

How many size allowed filename in the ff_fopen()?

the ff_fopen function is defined in <SDK Install Path>\source\fs\freertos_fat\FreeRTOS-FAT\ff_stdio.c

This is part of the Free RTOS FAT Library, See Description below

https://www.freertos.org/FreeRTOS-Plus/FreeRTOS_Plus_FAT/stdio_API/ff_fopen.html

it states :

If the file could not be opened then NULL is returned and the task's errno is set to indicate the reason. A task can obtain its errno value using the stdioGET_ERRNO() API function.

Link to STD error values below , if the pathname is too long following Error value is set 

www.freertos.org/.../errno.html

FF_FILE * ff_fopen( const char * pcFile,
                    const char * pcMode )
{
    FF_FILE * pxStream = NULL;
    FF_DirHandler_t xHandler;
    FF_Error_t xError;
    uint8_t ucMode;

    /* Insert the current working directory in front of relative paths. */
    pcFile = prvABSPath( pcFile );

    /* Look-up the I/O manager for the file system. */
    if( FF_FS_Find( pcFile, &xHandler ) == pdFALSE )
    {
        stdioSET_ERRNO( pdFREERTOS_ERRNO_ENXIO ); /* No such device or address. */
    }
    else
    {
        /* Now 'xHandler.pcPath' contains an absolute path within the file system.
         * Translate a type string "r|w|a[+]" to +FAT's mode bits. */
        ucMode = FF_GetModeBits( pcMode );

        pxStream = FF_Open( xHandler.pxManager, xHandler.pcPath, ucMode, &xError );
        stdioSET_ERRNO( prvFFErrorToErrno( xError ) );

        #if ( ffconfigUSE_NOTIFY != 0 )
            {
                if( ( pxStream != NULL ) && ( ( ucMode & ( FF_MODE_WRITE | FF_MODE_APPEND ) ) != 0 ) )
                {
                    /*_RB_ Function	name needs updating. */
                    callFileEvents( pcFile, eFileCreate );
                }
            }
        #endif /* ffconfigUSE_NOTIFY */

        #if ( ffconfigDEV_SUPPORT != 0 )
            {
                if( pxStream != NULL )
                {
                    FF_Device_Open( pcFile, pxStream );
                }
            }
        #endif /* ffconfigDEV_SUPPORT */
    }

    return pxStream;
}

FreeRTOS-Plus-FAT Standard APIs

Please check the below link 

https://www.freertos.org/FreeRTOS-Plus/FreeRTOS_Plus_FAT/Embedded_File_System_Configuration.html#ffconfigMAX_FILENAME

fconfigMAX_FILENAME

Sets the maximum length for file names, including the path. Note that the value of this define is directly related to the maximum stack use of the +FAT library. In some API's, a character buffer of size 'ffconfigMAX_FILENAME' will be declared on stack.

From File : source\fs\freertos_fat\FreeRTOS-FAT\include\FreeRTOSFATConfigDefaults.h

/* Sets the maximum length for file names, including the path.
 * Note that the value of this define is directly related to the maximum stack
 * use of the +FAT library. In some API's, a character buffer of size
 * 'ffconfigMAX_FILENAME' will be declared on stack. */
    #define ffconfigMAX_FILENAME    129

Hope this helps

Regards

Anshu