std::mt19937 with ARM Compiler 6 uses sys_open and breaks retarget.c

I  tried using empty project (of my own) and copy-pasted your code for main. I also tried enabling retargetting of STDIN and STDOUT via RTE manager as you suggested.

Several things happened:

- size of the binary went to 84 kilobytes with -Oz, even when main is empty. Have no idea how retarget_io.c manages to waste so much space ._.
- project compiled but stalled on BKPT before main with a callchain like _initio -> freopen -> _sys_open. This happens even if main is empty!

This reminded me of an issue that I stumbled in three years ago - community.arm.com/.../using-itm-retarget-with-keil-software-packs

Bloated code size is the reason why I still use a hand-written retarget.cpp (not .c, that was a misprint earlier, sorry) instead of RTE Managing it.

Here it is, more or less:

#if ( (__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050) )

    asm(".global __use_no_semihosting_swi\n");
    
#elif __CC_ARM

    #pragma import(__use_no_semihosting_swi)
    
    namespace std { struct __FILE { int handle;} ; }

#endif

#include <stdio.h>
#include <rt_sys.h>
#include <rt_misc.h>


std::FILE std::__stdout;
std::FILE std::__stdin;
std::FILE std::__stderr;

extern "C"
{ 
    int fputc(int c, FILE *f)
    {
        return ITM_SendChar(c);
    }

    int fgetc(FILE *f)
    {
        char ch = 0;

        return((int)ch);
    }

    int ferror(FILE *f)
    {
        /* Your implementation of ferror */
        return EOF;
    }

    void _ttywrch(int ch)
    {
        ITM_SendChar(ch);            
    }
    
    char *_sys_command_string(char *cmd, int len)
    {
        return NULL;
    }
    
    void _sys_exit(int return_code) 
    {
        while(1);
    }        
}

So 'tuning for armclang' was this line about semihosting - asm(".global __use_no_semihosting_swi\n");

On its own this works very nicely, at least in Simulator (which is all I need at the moment).

I  tried using empty project (of my own) and copy-pasted your code for main. I also tried enabling retargetting of STDIN and STDOUT via RTE manager as you suggested.

Several things happened:

- size of the binary went to 84 kilobytes with -Oz, even when main is empty. Have no idea how retarget_io.c manages to waste so much space ._.
- project compiled but stalled on BKPT before main with a callchain like _initio -> freopen -> _sys_open. This happens even if main is empty!

This reminded me of an issue that I stumbled in three years ago - community.arm.com/.../using-itm-retarget-with-keil-software-packs

Bloated code size is the reason why I still use a hand-written retarget.cpp (not .c, that was a misprint earlier, sorry) instead of RTE Managing it.

Here it is, more or less:

#if ( (__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050) )

    asm(".global __use_no_semihosting_swi\n");
    
#elif __CC_ARM

    #pragma import(__use_no_semihosting_swi)
    
    namespace std { struct __FILE { int handle;} ; }

#endif

#include <stdio.h>
#include <rt_sys.h>
#include <rt_misc.h>


std::FILE std::__stdout;
std::FILE std::__stdin;
std::FILE std::__stderr;

extern "C"
{ 
    int fputc(int c, FILE *f)
    {
        return ITM_SendChar(c);
    }

    int fgetc(FILE *f)
    {
        char ch = 0;

        return((int)ch);
    }

    int ferror(FILE *f)
    {
        /* Your implementation of ferror */
        return EOF;
    }

    void _ttywrch(int ch)
    {
        ITM_SendChar(ch);            
    }
    
    char *_sys_command_string(char *cmd, int len)
    {
        return NULL;
    }
    
    void _sys_exit(int return_code) 
    {
        while(1);
    }        
}

So 'tuning for armclang' was this line about semihosting - asm(".global __use_no_semihosting_swi\n");

On its own this works very nicely, at least in Simulator (which is all I need at the moment).

This explains the original linker error: _sys_open is needed by the iostream functions pulled in from <random>, your retarget.cpp doesn't provide a definition and you disabled semihosting, so the linker complained that it couldn't find a suitable _sys_open. Allowing semihosting pulls in version of _sys_open that uses semihosting, which is why you end up on a breakpoint before main. (If you want to decode the semihosting call, see Chapter 7 in the ARM Compiler Software Development Guide.)

The bulk of your 84KB binary is most likely due to locale and other I/O support. <random> includes <istream> and <ostream>, which is enough to pull in the IO libraries, even if your <random> objects don't use streams.  Large binary sizes aren't really the fault of retarget_io.c -- it's just a very small part of the overall puzzle. 

Hmm, looks like you are correct; removing #include <random> leads to small binary.

Why this include pulls so much unused code that isn't removed even with -LTO and -ffunction-sections - is a whole other story, albeit very interesting :)

Thanks for explanation!

However.. when I used retarget_io.c, provided by RTE Manager, I didn't disable semihosting and still ended up on a breakpoint before main..