cant write to file within call handler interface

hello

i am not able to write to file until the pl call
handler interface. this is the template which i am
following

PG_FUNCTION_INFO_V1(my_call_handler);

Datum
my_call_handler(PG_FUNCTION_ARGS)
{...my code...
int fd = open("filename",O_WRONLY);write(fd,buffer,strlen(buffer) + 1);
//fsync(fd).......i tried this as well but                   did'nt work

}


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