# tracefs_cpu_open_mapped

## NAME



           tracefs_cpu_open_mapped, tracefs_cpu_is_mapped,
           tracefs_mapped_is_supported, tracefs_cpu_map, tracefs_cpu_unmap -
           Memory mapping of the ring buffer
    


## SYNOPSIS



           **#include <tracefs.h>**
    
           bool **tracefs_cpu_is_mapped**(struct tracefs_cpu *tcpu);
           bool **tracefs_mapped_is_supported**(void);
           int **tracefs_cpu_map**(struct tracefs_cpu *tcpu);
           void **tracefs_cpu_unmap**(struct tracefs_cpu *tcpu);
           struct tracefs_cpu * **tracefs_cpu_open_mapped**(struct tracefs_instance *instance,
                                                       int cpu, bool nonblock);
    


## DESCRIPTION



           If the trace_pipe_raw supports memory mapping, this is usually a
           more efficient method to stream data from the kernel ring buffer
           than by reading it, as it does not require copying the memory that
           is being read.
    
           If memory mapping is supported by the kernel and the application
           asks to use the memory mapping via either **tracefs_cpu_map()** or by
           **tracefs_cpu_open_mapped()** then the functions tracefs_cpu_read(3)
           and tracefs_cpu_read_buf(3) will use the mapping directly instead
           of calling the read system call.
    
           Note, mapping will cause tracefs_cpu_buffered_read(3) and
           tracefs_cpu_buffered_read_buf(3) to act just like
           tracefs_cpu_read(3) and tracefs_cpu_read_buf(3) respectively as it
           doesn’t make sense to use a splice pipe when mapped. The kernel
           will do a copy for splice reads on mapping, and then another copy
           in the function when it can avoid the copying if the ring buffer
           is memory mapped.
    
           If the _tcpu_ is memory mapped it will also force
           tracefs_cpu_write(3) and tracefs_cpu_pipe(3) to copy from the
           mapping instead of using splice. Thus care must be used when
           determining to map the ring buffer or not, and why it does not get
           mapped by default.
    
           The **tracefs_cpu_is_mapped()** function will return true if _tcpu_
           currently has its ring buffer memory mapped and false otherwise.
           This does not return whether or not that the kernel supports
           memory mapping, but that can usually be determined by calling
           **tracefs_cpu_map()**.
    
           The **tracefs_mapped_is_supported()** returns true if the ring buffer
           can be memory mapped.
    
           The **tracefs_cpu_map()** function will attempt to map the ring buffer
           associated to _tcpu_ if it is not already mapped.
    
           The **tracefs_cpu_unmap()** function will unmap the ring buffer
           associated to _tcpu_ if it is mapped.
    
           The **tracefs_cpu_open_mapped()** is equivalent to calling
           tracefs_cpu_open(3) followed by **tracefs_cpu_map()** on the returned
           _tcpu_ of tracefs_cpu_open(3). Note, this will still succeed if the
           mapping fails, in which case it acts the same as
           tracefs_cpu_open(3). If knowing if the mapping succeed or not,
           **tracefs_cpu_is_mapped()** should be called on the return _tcpu_.
    


## RETURN VALUE



           **tracefs_cpu_is_mapped()** returns true if the given _tcpu_ has its
           ring buffer memory mapped or false otherwise.
    
           **tracefs_mapped_is_supported()** returns true if the tracing ring
           buffer can be memory mapped or false if it cannot be or an error
           occurred.
    
           **tracefs_cpu_map()** returns 0 on success and -1 on error in mapping.
           If 0 is returned then **tracefs_cpu_is_mapped()** will return true
           afterward, or false if the mapping failed.
    
           **tracefs_cpu_open_mapped()** returns an allocated tracefs_cpu on
           success of creation regardless if it succeed in mapping the ring
           buffer or not. It returns NULL for the same reasons
           tracefs_cpu_open(3) returns NULL. If success of mapping is to be
           known, then calling **tracefs_cpu_is_mapped()** afterward is required.
    


## EXAMPLE



               #include <stdlib.h>
               #include <ctype.h>
               #include <tracefs.h>
    
               static void read_subbuf(struct tep_handle *tep, struct kbuffer *kbuf)
               {
                       static struct trace_seq seq;
                       struct tep_record record;
                       int missed_events;
    
                       if (seq.buffer)
                               trace_seq_reset(&seq);
                       else
                               trace_seq_init(&seq);
    
                       while ((record.data = kbuffer_read_event(kbuf, &record.ts))) {
                               record.size = kbuffer_event_size(kbuf);
                               missed_events = kbuffer_missed_events(kbuf);
                               if (missed_events) {
                                       printf("[MISSED EVENTS");
                                       if (missed_events > 0)
                                               printf(": %d]\n", missed_events);
                                       else
                                               printf("]\n");
                               }
                               kbuffer_next_event(kbuf, NULL);
                               tep_print_event(tep, &seq, &record,
                                               "%s-%d %6.1000d\t%s: %s\n",
                                               TEP_PRINT_COMM,
                                               TEP_PRINT_PID,
                                               TEP_PRINT_TIME,
                                               TEP_PRINT_NAME,
                                               TEP_PRINT_INFO);
                               trace_seq_do_printf(&seq);
                               trace_seq_reset(&seq);
                       }
               }
    
               int main (int argc, char **argv)
               {
                       struct tracefs_cpu *tcpu;
                       struct tep_handle *tep;
                       struct kbuffer *kbuf;
                       bool mapped;
                       int cpu;
    
                       if (argc < 2 || !isdigit(argv[1][0])) {
                               printf("usage: %s cpu\n\n", argv[0]);
                               exit(-1);
                       }
    
                       cpu = atoi(argv[1]);
    
                       tep = tracefs_local_events(NULL);
                       if (!tep) {
                               perror("Reading trace event formats");
                               exit(-1);
                       }
    
                       tcpu = tracefs_cpu_open_mapped(NULL, cpu, 0);
                       if (!tcpu) {
                               perror("Open CPU 0 file");
                               exit(-1);
                       }
    
                       /*
                        * If this kernel supports mapping, use normal read,
                        * otherwise use the piped buffer read, although if
                        * the mapping succeeded, tracefs_cpu_buffered_read_buf()
                        * acts the same as tracefs_cpu_read_buf(). But this is just
                        * an example on how to use tracefs_cpu_is_mapped().
                        */
                       mapped = tracefs_cpu_is_mapped(tcpu);
                       if (!mapped)
                               printf("Was not able to map, falling back to buffered read\n");
                       while ((kbuf = mapped ? tracefs_cpu_read_buf(tcpu, true) :
                                       tracefs_cpu_buffered_read_buf(tcpu, true))) {
                               read_subbuf(tep, kbuf);
                       }
    
                       kbuf = tracefs_cpu_flush_buf(tcpu);
                       if (kbuf)
                               read_subbuf(tep, kbuf);
    
                       tracefs_cpu_close(tcpu);
                       tep_free(tep);
    
                       return 0;
               }
    


## FILES



               **tracefs.h**
                       Header file to include in order to have access to the library APIs.
               **-ltracefs**
                       Linker switch to add when building a program that uses the library.
    


## LICENSE



           libtracefs is Free Software licensed under the GNU LGPL 2.1
    


## RESOURCES



           **https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/ **
    


## COPYING



           Copyright (C) 2022 Google, Inc. Free use of this software is
           granted under the terms of the GNU Public License (GPL).
    


## NOTES



            1. rostedt@goodmis.org
               mailto:rostedt@goodmis.org
    
            2. linux-trace-devel@vger.kernel.org
               mailto:linux-trace-devel@vger.kernel.org
    


* * *