# tracefs_cpu_read_buf

## NAME



           tracefs_cpu_read_buf, tracefs_cpu_buffered_read_buf,
           tracefs_cpu_flush_buf - Reading trace_pipe_raw data returning a
           kbuffer
    


## SYNOPSIS



           **#include <tracefs.h>**
    
           struct kbuffer * **tracefs_cpu_read_buf**(struct tracefs_cpu *_tcpu_ , bool _nonblock_);
           struct kbuffer * **tracefs_cpu_buffered_read_buf**(struct tracefs_cpu *_tcpu_ , bool _nonblock_);
           struct kbuffer * **tracefs_cpu_flush_buf**(struct tracefs_cpu *_tcpu_);
    


## DESCRIPTION



           This set of APIs can be used to read the raw data from the
           trace_pipe_raw files in the tracefs file system and return a
           kbuffer structure to read it with.
    
           The **tracefs_cpu_read_buf()** reads the trace_pipe_raw files
           associated to _tcpu_ and returns a kbuffer structure that can be
           used to iterate the events. If _nonblock_ is set, and there’s no
           data available, it will return immediately. Otherwise depending on
           how _tcpu_ was opened, it will block. If _tcpu_ was opened with
           nonblock set, then this _nonblock_ will make no difference.
    
           The **tracefs_cpu_buffered_read_buf()** is basically the same as
           **tracefs_cpu_read_buf()** except that it uses a pipe through splice
           to buffer reads. This will batch reads keeping the reading from
           the ring buffer less intrusive to the system, as just reading all
           the time can cause quite a disturbance. Note, one difference
           between this and **tracefs_cpu_read()** is that it will read only in
           sub buffer pages. If the ring buffer has not filled a page, then
           it will not return anything, even with _nonblock_ set. Calls to
           **tracefs_cpu_flush_buf()** or **tracefs_cpu_flush()** should be done to
           read the rest of the file at the end of the trace.
    
           The **tracefs_cpu_flush_buf()** reads the trace_pipe_raw file
           associated by the _tcpu_ and puts it into _buffer_ , which must be the
           size of the sub buffer which is retrieved. This should be called
           at the end of tracing to get the rest of the data. This call will
           convert the file descriptor of trace_pipe_raw into non-blocking
           mode.
    


## RETURN VALUE



           The functions **tracefs_cpu_read_buf()** ,
           tracefs_cpu_buffered_read_buf()* and **tracefs_cpu_flush()** returns a
           kbuffer descriptor that can be iterated over to find the events.
           Note, this descriptor is part of the tracefs_cpu structure and
           should not be freed. It will be freed. It returns NULL on error or
           if nonblock is set and there are no events available. In the case
           of no events, errno will be set with EAGAIN.
    


## EXAMPLE



               #include <stdlib.h>
               #include <ctype.h>
               #include <tracefs.h>
    
               static void read_page(struct tep_handle *tep, struct kbuffer *kbuf)
               {
                       static struct trace_seq seq;
                       struct tep_record record;
    
                       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);
                               kbuffer_next_event(kbuf, NULL);
                               tep_print_event(tep, &seq, &record,
                                               "%s-%d %9d\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;
                       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(NULL, cpu, 0);
                       if (!tcpu) {
                               perror("Open CPU 0 file");
                               exit(-1);
                       }
    
                       while ((kbuf = tracefs_cpu_buffered_read_buf(tcpu, true))) {
                               read_page(tep, kbuf);
                       }
    
                       kbuf = tracefs_cpu_flush_buf(tcpu);
                       if (kbuf)
                               read_page(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
    


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