LIBTRACECMD(3) libtracefs Manual LIBTRACECMD(3)

tracecmd_open, tracecmd_open_fd, tracecmd_open_head, tracecmd_init_data, tracecmd_close - Open and close a trace file.

#include <trace-cmd.h>
struct tracecmd_input *tracecmd_open(const char *file, int flags);
struct tracecmd_input *tracecmd_open_fd(int fd, int flags);
struct tracecmd_input *tracecmd_open_head(const char *file, int flags);
int tracecmd_init_data(struct tracecmd_input *handle);
void tracecmd_close(struct tracecmd_input *handle);

This set of APIs can be used to open and close a trace file recorded by trace-cmd(1) and containing tracing information from ftrace, the official Linux kernel tracer. The opened file is represented by a tracecmd_input structure, all other library APIs that work with the file require a pointer to the structure. The APIs for opening a trace file have a flag input parameter, which controls how the file will be opened and parsed. The flag is a combination of these options:

TRACECMD_FL_LOAD_NO_PLUGINS - Do not load any plugins
TRACECMD_FL_LOAD_NO_SYSTEM_PLUGINS - Do not load system wide plugins, load only "local only"

plugins from user's home directory.

The tracecmd_open() function opens a given trace file, parses the metadata headers from the file, allocates and initializes а tracecmd_input handler structure representing the file. It also initializes the handler for reading trace data from the file. The returned handler is ready to be used with tracecmd_read_ APIs.

The tracecmd_open_fd() function does the same as tracecmd_open(), but works with a file descriptor to a trace file, opened for reading.

The tracecmd_open_head() function is the same as tracecmd_open(), but does not initialize the handler for reading trace data. It reads and parses the metadata headers only. The tracecmd_init_data() should be used before using the tracecmd_read_ APIs.

The tracecmd_init_data() function initializes a handle, allocated with tracecmd_open_head(), for reading trace data from the file associated with it. This API must be called before any of the tracecmd_read_ APIs.

The tracecmd_close() function frees a handle, pointer to tracecmd_input structure, previously allocated with tracecmd_open(), tracecmd_open_fd() or tracecmd_open_head() APIs.

The tracecmd_open(), tracecmd_open_fd() and tracecmd_open_head() functions return a pointer to tracecmd_input structure or NULL in case of an error. The returned structure must be free with tracecmd_close(). Note that if tracecmd_open_fd() is used to allocate a tracecmd_input handler, when tracecmd_close() is called to close it, that fd will be closed also.

The tracecmd_init_data() function returns -1 in case of an error or 0 otherwise.

The are two different use patterns for opening and reading trace data from
a trace file, which can be used depending on the use case.
1. Open and initialise the trace file in а single step:
#include <trace-cmd.h>
...
struct tracecmd_input *handle = tracecmd_open("trace.dat");

if (!handle) {
/* Failed to open trace.dat file */
} ...
/* Read tracing data from the file, using the handle */ ...
tracecmd_close(handle); ... int fd;
fd = = open("trace.dat", O_RDONLY);
if (fd < 0) {
/* Failed to open trace file for reading */
}
handle = tracecmd_open_fd(fd);
if (!handle) {
close(fd);
/* Failed to initialise handler for reading the trace file */
} ...
/* Read tracing data from the file, using the handle */ ...
tracecmd_close(handle); ... 2. Open and initialise the trace file in two steps. This allows to perform some processing based on metadata, read from the file, before initialising the trace data for reading. Example for such use case is when opening multiple trace files recorded in a same trace session. In that case timestamps of all trace events must be adjusted based on the information from the file's metadata and before reading the trace data. #include <trace-cmd.h> ... struct tracecmd_input *handle = tracecmd_open_head("trace.dat");
if (!handle) {
/* Failed to open trace.dat file */
} ...
/* do some processing, before initialising the trace data for reading */ ...
if (tracecmd_init_data(handle) < 0) {
/* Failed to initialize hadle for reading the trace data */
} ...
/* Read tracing data from the file, using the handle */ ...
tracecmd_close(handle); ...

trace-cmd.h

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

libtracefs(3), libtraceevent(3), trace-cmd(1) trace-cmd.dat(5)

Steven Rostedt <rostedt@goodmis.org[1]>
Tzvetomir Stoyanov <tz.stoyanov@gmail.com[2]>

Report bugs to <linux-trace-devel@vger.kernel.org[3]>

libtracecmd is Free Software licensed under the GNU LGPL 2.1

https://git.kernel.org/pub/scm/utils/trace-cmd/trace-cmd.git/

Copyright (C) 2020 VMware, 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.
tz.stoyanov@gmail.com
mailto:tz.stoyanov@gmail.com
3.
linux-trace-devel@vger.kernel.org
mailto:linux-trace-devel@vger.kernel.org
11/11/2021 libtracefs