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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_open_mapped, tracefs_cpu_is_mapped, tracefs_mapped_is_supported, tracefs_cpu_map, tracefs_cpu_unmap - Memory mapping of the ring buffer
#include <tracefs.h> booltracefs_cpu_is_mapped(struct tracefs_cpu *tcpu); booltracefs_mapped_is_supported(void); inttracefs_cpu_map(struct tracefs_cpu *tcpu); voidtracefs_cpu_unmap(struct tracefs_cpu *tcpu); struct tracefs_cpu *tracefs_cpu_open_mapped(struct tracefs_instance *instance, int cpu, bool nonblock);
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 eithertracefs_cpu_map()or bytracefs_cpu_open_mapped()then the functionstracefs_cpu_read(3) andtracefs_cpu_read_buf(3) will use the mapping directly instead of calling the read system call. Note, mapping will causetracefs_cpu_buffered_read(3) andtracefs_cpu_buffered_read_buf(3) to act just liketracefs_cpu_read(3) andtracefs_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 thetcpu is memory mapped it will also forcetracefs_cpu_write(3) andtracefs_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. Thetracefs_cpu_is_mapped()function will return true iftcpu 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 callingtracefs_cpu_map(). Thetracefs_mapped_is_supported()returns true if the ring buffer can be memory mapped. Thetracefs_cpu_map()function will attempt to map the ring buffer associated totcpu if it is not already mapped. Thetracefs_cpu_unmap()function will unmap the ring buffer associated totcpu if it is mapped. Thetracefs_cpu_open_mapped()is equivalent to callingtracefs_cpu_open(3) followed bytracefs_cpu_map()on the returnedtcpu oftracefs_cpu_open(3). Note, this will still succeed if the mapping fails, in which case it acts the same astracefs_cpu_open(3). If knowing if the mapping succeed or not,tracefs_cpu_is_mapped()should be called on the returntcpu.
tracefs_cpu_is_mapped()returns true if the giventcpu 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 thentracefs_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 reasonstracefs_cpu_open(3) returns NULL. If success of mapping is to be known, then callingtracefs_cpu_is_mapped()afterward is required.
#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; }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_read(3),tracefs_cpu_read_buf(3),tracefs_cpu_buffered_read(3),tracefs_cpu_buffered_read_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)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. | |