|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | EXAMPLE | FILES | SEE ALSO | AUTHOR | REPORTING BUGS | LICENSE | RESOURCES | COPYING | NOTES | COLOPHON |
|
|
|
LIBTRACEFS(3) libtracefs Manual LIBTRACEFS(3)
tracefs_cpu_read_size, tracefs_cpu_read,
tracefs_cpu_buffered_read, tracefs_cpu_write, tracefs_cpu_stop,
tracefs_cpu_flush, tracefs_cpu_flush_write, tracefs_cpu_pipe -
Reading trace_pipe_raw data
#include <tracefs.h>
int tracefs_cpu_read_size(struct tracefs_cpu *tcpu);
int tracefs_cpu_read(struct tracefs_cpu *tcpu, void *buffer, bool nonblock);
int tracefs_cpu_buffered_read(struct tracefs_cpu *tcpu, void *buffer, bool nonblock);
int tracefs_cpu_write(struct tracefs_cpu *tcpu, int wfd, bool nonblock);
int tracefs_cpu_stop(struct tracefs_cpu *tcpu);
int tracefs_cpu_flush(struct tracefs_cpu *tcpu, void *buffer);
int tracefs_cpu_flush_write(struct tracefs_cpu *tcpu, int wfd);
int tracefs_cpu_pipe(struct tracefs_cpu *tcpu, int wfd, bool nonblock);
This set of APIs can be used to read the raw data from the
trace_pipe_raw files in the tracefs file system.
The tracefs_cpu_read_size() returns the subbuffer size of the
trace_pipe_raw. This returns the minimum size of the buffer that
is passed to the below functions.
The tracefs_cpu_read() reads the trace_pipe_raw files associated
to tcpu into buffer. buffer must be at least the size of the sub
buffer of the ring buffer, which is returned by
tracefs_cpu_read_size(). 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() is basically the same as
tracefs_cpu_read() 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() should be done to read the rest of the file at
the end of the trace.
The tracefs_cpu_write() will pipe the data from the trace_pipe_raw
file associated with tcpu into the wfd file descriptor. If
nonblock is set, then it will not block on if there’s nothing to
write. Note, it will only write sub buffer size data to wfd. Calls
to tracefs_cpu_flush_write() are needed to write out the rest.
The tracefs_cpu_stop() will attempt to unblock a task blocked on
tcpu reading it. On older kernels, it may not do anything for the
pipe reads, as older kernels do not wake up tasks waiting on the
ring buffer. Returns 0 if it definitely woke up any possible
waiters, but returns 1 if it is not sure it worked and waiters may
need to have a signal sent to them.
The tracefs_cpu_flush() 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. by tracefs_cpu_read_size(). 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.
The tracefs_cpu_flush_write() same as trace_cpu_flush() except it
takes a file descriptor wfd to flush the data into.
The tracefs_cpu_pipe() is similar to tracefs_cpu_write() but the
wfd file descriptor must be a pipe. This call is an optimization
of tracefs_cpu_write() that uses two calls to splice(2) in order
to connect the trace_pipe_raw file descriptor with the write file
descriptor. splice(2) requires that one of the passed in file
descriptors is a pipe. If the application wants to pass the data
to an existing pipe, there’s no reason for there to be two
splice(2) system calls and tracefs_cpu_pipe() can simply use a
single call to wfd.
The tracefs_cpu_open() returns a struct tracefs_cpu descriptor
that can be used by the other functions or NULL on error.
The tracefs_cpu_read_size() returns the minimum size of the
buffers to be used with tracefs_cpu_read(),
tracefs_cpu_buffered_read() and tracefs_cpu_flush(). Returns
negative on error.
The tracefs_cpu_read() returns the number of bytes read, or
negative on error.
The tracefs_cpu_buffered_read() returns the number of bytes read
or negative on error.
The tracefs_cpu_write() returns the number of bytes written to the
file or negative on error.
The tracefs_cpu_stop() returns zero if any waiters were guaranteed
to be woken up from waiting on input, or returns one if this is an
older kernel that does not supply that guarantee, and a signal may
need to be sent to any waiters. Returns negative on error.
The tracefs_cpu_flush() returns the number of bytes read or
negative on error.
The tracefs_cpu_flush_write() returns the number of bytes written
to the file or negative on error.
#define _LARGEFILE64_SOURCE
#include <stdlib.h>
#include <ctype.h>
#include <pthread.h>
#include <unistd.h>
#include <tracefs.h>
struct thread_data {
struct tracefs_cpu *tcpu;
int done;
int fd;
};
static void *thread_run(void *arg)
{
struct thread_data *data = arg;
struct tracefs_cpu *tcpu = data->tcpu;
int fd = data->fd;
int ret;
while (!data->done) {
ret = tracefs_cpu_write(tcpu, fd, false);
printf("wrote %d\n", ret);
}
return NULL;
}
int main (int argc, char **argv)
{
struct tracefs_instance *instance;
struct thread_data data;
pthread_t thread;
char *file;
int secs = 10;
int cpu;
int ret;
if (argc < 3 || !isdigit(argv[1][0])) {
printf("usage: %s cpu file_destination [sleep secs]\n\n", argv[0]);
exit(-1);
}
cpu = atoi(argv[1]);
file = argv[2];
if (argc > 3)
secs = atoi(argv[3]);
instance = tracefs_instance_create("cpu_write");
if (!instance) {
perror("create instance");
exit(-1);
}
memset(&data, 0, sizeof(data));
data.tcpu = tracefs_cpu_open(instance, cpu, 0);
if (!data.tcpu) {
perror("Open instance");
exit(-1);
}
data.fd = open(file, O_WRONLY | O_CREAT | O_TRUNC | O_LARGEFILE, 0644);
if (data.fd < 0) {
perror(file);
exit(-1);
}
pthread_create(&thread, NULL, thread_run, &data);
sleep(secs);
data.done = 1;
printf("stopping\n");
ret = tracefs_cpu_stop(data.tcpu);
printf("joining %d\n", ret);
pthread_join(thread, NULL);
tracefs_trace_off(instance);
do {
ret = tracefs_cpu_flush_write(data.tcpu, data.fd);
printf("flushed %d\n", ret);
} while (ret > 0);
tracefs_trace_on(instance);
tracefs_cpu_close(data.tcpu);
close(data.fd);
return 0;
}
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.
tracefs_cpu_open(3) tracefs_cpu_close(3) tracefs_cpu_read_buf(3)
tracefs_cpu_buffered_read_buf(3) tracefs_cpu_flush_buf(3)
libtracefs(3), libtraceevent(3), trace-cmd(1)
Steven Rostedt <rostedt@goodmis.org[1]>
Report bugs to <linux-trace-devel@vger.kernel.org[2]>
libtracefs is Free Software licensed under the GNU LGPL 2.1
https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/
Copyright (C) 2022 Google, Inc. Free use of this software is
granted under the terms of the GNU Public License (GPL).
1. rostedt@goodmis.org
mailto:rostedt@goodmis.org
2. linux-trace-devel@vger.kernel.org
mailto:linux-trace-devel@vger.kernel.org
This page is part of the libtracefs (Linux kernel trace file
system library) project. Information about the project can be
found at ⟨https://www.trace-cmd.org/⟩. If you have a bug report
for this manual page, see ⟨https://www.trace-cmd.org/⟩. This page
was obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-06-02.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
libtracefs 1.8.1 01/02/2025 LIBTRACEFS(3)
Pages that refer to this page: tracefs_cpu_open(3), tracefs_cpu_open_mapped(3), tracefs_cpu_read_buf(3), tracefs_instance_get_name(3)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|