LIBTRACEFS(3)

NAME
tracefs_instance_file_open, tracefs_instance_file_write, tracefs_instance_file_write_number, tracefs_instance_file_append, tracefs_instance_file_clear, tracefs_instance_file_read, tracefs_instance_file_read_number - Work with files in tracing instances.

SYNOPSIS

#include <tracefs.h>
int tracefs_instance_file_open(struct tracefs_instance *instance, const char *file, int mode);
int tracefs_instance_file_write(struct tracefs_instance *instance, const char *file, const char *str);
int tracefs_instance_file_write_number(struct tracefs_instance *instance, const char *file, size_t val);
int tracefs_instance_file_append(struct tracefs_instance *instance, const char *file, const char *str);
int tracefs_instance_file_clear(struct tracefs_instance *instance, const char *file);
char *tracefs_instance_file_read(struct tracefs_instance *instance, const char *file, int *psize);
int tracefs_instance_file_read_number(struct tracefs_instance *instance, const char *file, long long int *res);

DESCRIPTION

This set of APIs can be used to work with trace files in all trace instances. Each of these APIs take an instance argument, that can be NULL to act on the top level instance. Otherwise, it acts on an instance created with tracefs_insance_create(3)

The tracefs_instance_file_open() function opens trace file from given instance and returns a file descriptor to it. The file access mode can be specified, see open(3) for more details. If -1 is passed as mode, default O_RDWR is used.

The tracefs_instance_file_write() function writes a string str in a file from the given instance, without the terminating NULL character. When opening the file, this function tries to truncates the size of the file to zero, which clears all previously existing settings.

The tracefs_instance_file_write_number() function converts val into a string and then writes it to the given file. This is a helper function that does the number conversion to string and then calls tracefs_instance_file_write().

The tracefs_instance_file_append() function writes a string str in a file from the given instance, without the terminating NULL character. This function is similar to tracefs_instance_file_write(), but the existing content of the is not cleared. Thus the new settings are appended to the existing ones (if any).

The tracefs_instance_file_clear() function tries to truncates the size of the file to zero, which clears all previously existing settings. If the file has content that does not get cleared in this way, this will not have any effect.

The tracefs_instance_file_read() function reads the content of a file from the given instance.

The tracefs_instance_file_read_number() function reads the content of a file from the given instance and converts it to a long long integer, which is stored in res.

RETURN VALUE

The tracefs_instance_file_open() function returns a file descriptor to the opened file. It must be closed with close(3). In case of an error, -1 is returned.

The tracefs_instance_file_write() function returns the number of written bytes, or -1 in case of an error.

The tracefs_instance_file_write_number() function returns 0 on success and -1 on error.

The tracefs_instance_file_append() function returns the number of written bytes, or -1 in case of an error.

The tracefs_instance_file_clear() function returns 0 on success, or -1 in case of an error.

The tracefs_instance_file_read() function returns a pointer to a NULL terminated string, read from the file, or NULL in case of an error. The returned string must be freed with free().

The tracefs_instance_file_read_number() function returns 0 if a valid integer is read from the file and stored in res or -1 in case of an error.

EXAMPLE

    #include <tracefs.h>
    struct tracefs_instance *inst = tracefs_instance_create("foo");
            if (!inst) {
                    /* Error creating a new trace instance */
                    ...
            }
            if (tracefs_file_exists(inst,"trace_clock")) {
                    /* The instance foo supports trace clock */
                    char *path, *clock;
                    int size;
                    path =  = tracefs_instance_get_file(inst, "trace_clock")
                    if (!path) {
                            /* Error getting the path to trace_clock file in instance foo */
                            ...
                    }
                    ...
                    tracefs_put_tracing_file(path);
                    clock = tracefs_instance_file_read(inst, "trace_clock", &size);
                    if (!clock) {
                            /* Failed to read trace_clock file in instance foo */
                            ...
                    }
                    ...
                    free(clock);
                    if (tracefs_instance_file_write(inst, "trace_clock", "global") != strlen("global")) {
                            /* Failed to set gloabl trace clock in instance foo */
                            ...
                    }
            } else {
                    /* The instance foo does not support trace clock */
            }
            if (tracefs_dir_exists(inst,"options")) {
                    /* The instance foo supports trace options */
                    char *path = tracefs_instance_get_file(inst, "options");
                    if (!path) {
                            /* Error getting the path to options directory in instance foo */
                            ...
                    }
                    tracefs_put_tracing_file(path);
            } else {
                    /* The instance foo does not support trace options */
            }
            ...
            if (tracefs_instance_is_new(inst))
                    tracefs_instance_destroy(inst);
            else
                    tracefs_instance_free(inst);
            ...
            long long int res;
            if (tracefs_instance_file_read_number(NULL, "tracing_on", &res) == 0) {
                    if (res == 0) {
                            /* tracing is disabled in the top instance */
                    } else if (res == 1) {
                            /* tracing is enabled in the top instance */
                    } else {
                            /* Unknown tracing state of the top instance */
                    }
            } else {
                    /* Failed to read integer from tracing_on file */
            }
            ...
            int fd;
            fd = tracefs_instance_file_open(NULL, "tracing_on", O_WRONLY);
            if (fd >= 0) {
                    /* Got file descriptor to the tracing_on file from the top instance for writing */
                    ...
                    close(fd);
            }

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.

SEE ALSO

libtracefs(3), libtraceevent(3), trace-cmd(1)

AUTHOR

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

REPORTING BUGS

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

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) 2020 VMware, 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.  tz.stoyanov@gmail.com
    mailto:tz.stoyanov@gmail.com

3.  linux-trace-devel@vger.kernel.org
    mailto:linux-trace-devel@vger.kernel.org