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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |EXAMPLE |FILES |SEE ALSO |AUTHOR |REPORTING BUGS |LICENSE |RESOURCES |COPYING |NOTES |COLOPHON | |
LIBTRACEFS(3) libtracefs ManualLIBTRACEFS(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> inttracefs_cpu_read_size(struct tracefs_cpu *tcpu); inttracefs_cpu_read(struct tracefs_cpu *tcpu, void *buffer, boolnonblock); inttracefs_cpu_buffered_read(struct tracefs_cpu *tcpu, void *buffer, boolnonblock); inttracefs_cpu_write(struct tracefs_cpu *tcpu, intwfd, boolnonblock); inttracefs_cpu_stop(struct tracefs_cpu *tcpu); inttracefs_cpu_flush(struct tracefs_cpu *tcpu, void *buffer); inttracefs_cpu_flush_write(struct tracefs_cpu *tcpu, intwfd); inttracefs_cpu_pipe(struct tracefs_cpu *tcpu, intwfd, boolnonblock);
This set of APIs can be used to read the raw data from the trace_pipe_raw files in the tracefs file system. Thetracefs_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. Thetracefs_cpu_read()reads the trace_pipe_raw files associated totcpu intobuffer.buffer must be at least the size of the sub buffer of the ring buffer, which is returned bytracefs_cpu_read_size(). Ifnonblock is set, and there’s no data available, it will return immediately. Otherwise depending on howtcpu was opened, it will block. Iftcpu was opened with nonblock set, then thisnonblock will make no difference. Thetracefs_cpu_buffered_read()is basically the same astracefs_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 andtracefs_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 withnonblock set. Calls totracefs_cpu_flush()should be done to read the rest of the file at the end of the trace. Thetracefs_cpu_write()will pipe the data from the trace_pipe_raw file associated withtcpu into thewfd file descriptor. Ifnonblock is set, then it will not block on if there’s nothing to write. Note, it will only write sub buffer size data towfd. Calls to tracefs_cpu_flush_write() are needed to write out the rest. Thetracefs_cpu_stop()will attempt to unblock a task blocked ontcpu 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. Thetracefs_cpu_flush()reads the trace_pipe_raw file associated by thetcpu and puts it intobuffer, which must be the size of the sub buffer which is retrieved. bytracefs_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. Thetracefs_cpu_flush_write()same astrace_cpu_flush()except it takes a file descriptorwfd to flush the data into. Thetracefs_cpu_pipe()is similar totracefs_cpu_write()but thewfd file descriptor must be a pipe. This call is an optimization oftracefs_cpu_write()that uses two calls tosplice(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 twosplice(2) system calls andtracefs_cpu_pipe()can simply use a single call towfd.
Thetracefs_cpu_open()returns a struct tracefs_cpu descriptor that can be used by the other functions or NULL on error. Thetracefs_cpu_read_size()returns the minimum size of the buffers to be used withtracefs_cpu_read(),tracefs_cpu_buffered_read()andtracefs_cpu_flush(). Returns negative on error. Thetracefs_cpu_read()returns the number of bytes read, or negative on error. Thetracefs_cpu_buffered_read()returns the number of bytes read or negative on error. Thetracefs_cpu_write()returns the number of bytes written to the file or negative on error. Thetracefs_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. Thetracefs_cpu_flush()returns the number of bytes read or negative on error. Thetracefs_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 thelibtracefs (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 isnot part of the original manual page), send a mail to man-pages@man7.orglibtracefs 1.8.1 01/02/2025LIBTRACEFS(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 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |