--- name: "tracefs_instance_file_open" section: "3" platform: "linux" package: "libtracefs" version: "1.8.4-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/tracefs_instance_file_open" --- # tracefs\_instance\_file\_open(3) Work with files in tracing instances. ## 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 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 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)](/libtracefs/3.md), [**libtraceevent**(3)](/libtraceevent/3.md), [**trace-cmd**(1)](/trace-cmd/1.md) ## AUTHOR ``` Steven Rostedt Tzvetomir Stoyanov ``` ## 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