--- name: "tracefs_event_file_read" section: "3" platform: "linux" package: "libtracefs-doc" version: "1.8.1-1" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/tracefs_event_get_file" --- # tracefs\_event\_file\_read(3) Work with trace event files. Part of [tracefs\_event\_get\_file(3)](/tracefs_event_get_file/3.md). ## NAME tracefs\_event\_get\_file, tracefs\_event\_file\_read, tracefs\_event\_file\_write, tracefs\_event\_file\_append, tracefs\_event\_file\_clear, tracefs\_event\_file\_exists - Work with trace event files. ## SYNOPSIS ``` #include char *tracefs_event_get_file(struct tracefs_instance *instance, const char *system, const char *event, const char *file); char *tracefs_event_file_read(struct tracefs_instance *instance, const char *system, const char *event, const char *file, int *psize); int tracefs_event_file_write(struct tracefs_instance *instance, const char *system, const char *event, const char *file, const char *str); int tracefs_event_file_append(struct tracefs_instance *instance, const char *system, const char *event, const char *file, const char *str); int tracefs_event_file_clear(struct tracefs_instance *instance, const char *system, const char *event, const char *file); bool tracefs_event_file_exists(struct tracefs_instance *instance, const char *system, const char *event, const char *file); ``` ## DESCRIPTION These are functions for accessing tracefs event specific files. These functions act similar to the tracefs instance file functions but are easier to get to if the system and events are known before hand. The **tracefs\_event\_get\_file()** returns the full path of the *file* for the given *system* and *event* that is within the given *instance*. If *instance* is NULL, then the file for the *event* for the top level instance is returned. Note, there is no check to see if the file actually exists or even if the system and event exist. It only creates the path name for such an event if it did exist. This acts similar to the [**tracefs\_instance\_get\_file**(3)](/tracefs_instance_get_file/3.md), but is to be used to get to event files if the *system* and *event* are already known. The **tracefs\_event\_file\_read()** reads the content for the *event* *file* for the given *instance* or the top level instance if *instance* is NULL. The content of the file is returned and *psize* is set to the amount of data that was read. The returned content must be freed with [**free**(3)](/free/3.md). This acts similar to the [**tracefs\_instance\_file\_read**(3)](/tracefs_instance_file_read/3.md), but is to be used to read the event file if the *system* and *event* are already known. The **tracefs\_event\_file\_write()** writes *str* to the *event* *file*. It will truncate anything that is already in that file. This acts similar to the [**tracefs\_instance\_file\_write**(3)](/tracefs_instance_file_write/3.md), but is to be used to read the event file if the *system* and *event* are already known. The **tracefs\_event\_file\_append()** appends *str* to the *event* *file*. It will not clear out the file as it writes *sting*. This acts similar to the [**tracefs\_instance\_file\_append**(3)](/tracefs_instance_file_append/3.md), but is to be used to read the event file if the *system* and *event* are already known. The **tracefs\_event\_file\_clear()** clears the content of the *event* *file*. This acts similar to the [**tracefs\_instance\_file\_clear**(3)](/tracefs_instance_file_clear/3.md), but is to be used to read the event file if the *system* and *event* are already known. The **tracefs\_event\_file\_exists()** returns true if the *event* *file* exists, and false otherwise. This acts similar to the **tracefs\_instance\_file\_exists**(3), but is to be used to read the event file if the *system* and *event* are already known. ## RETURN VALUE **tracefs\_event\_get\_file()** returns the path of the given *system*/*event* *file* on success and NULL on error. The return value must be freed with [**tracefs\_put\_tracing\_file**(3)](/tracefs_put_tracing_file/3.md). **tracefs\_event\_file\_read()** reads the content of the *system*/*event* *file* or NULL on error. The return pointer must be freed with [**free**(3)](/free/3.md). **tracefs\_event\_file\_write()** and **tracefs\_event\_file\_append()** returns the number of bytes written to the *system*/*event* *file* or negative on error. **tracefs\_event\_file\_clear()** returns zero on success and -1 on error. **tracefs\_event\_file\_exists()** returns true if the *system*/*event* *file* exists for the given *instance* (or top level if *instance* is NULL) or false otherwise. ## EXAMPLE ``` #include #include #include #include int main(int argc, char **argv) { char *system; char *event; char *file; char *cmd = NULL; char *buf; char *str; char ch = 'r'; int size; if (argc < 4) { printf("usage: %s sytem event file [(-a|-w) write | -c]\n" " reads the system/event file or writes if [write is supplied]\n", argv[0]); exit(0); } system = argv[1]; event = argv[2]; file = argv[3]; if (argc > 4) cmd = argv[4]; if (!tracefs_event_file_exists(NULL, system, event, file)) { fprintf(stderr, "File %s/%s/%s does not exist\n", system, event, file); exit(-1); } if (cmd) { if (cmd[0] != '-') ch = cmd[0]; else ch = cmd[1]; if (!ch) ch = 'c'; } switch (ch) { case 'r': buf = tracefs_event_file_read(NULL, system, event, file, &size); if (buf) printf("%s", buf); else fprintf(stderr, "Failed to read %s/%s/%s\n", system, event, file); free(buf); break; case 'w': case 'a': if (argc < 6) { fprintf(stderr, "%s command requires something to write\n", ch == 'w' ? "write" : "append"); exit(-1); } if (ch == 'w') size = tracefs_event_file_write(NULL, system, event, file, argv[5]); else size = tracefs_event_file_append(NULL, system, event, file, argv[5]); if (size < 0) { fprintf(stderr, "Failed to write '%s' to %s/%s/%s\n", argv[5], system, event, file); exit(-1); } break; case 'c': if (tracefs_event_file_clear(NULL, system, event, file) < 0) { fprintf(stderr, "Failed to clear %s/%s/%s\n", system, event, file); exit(-1); } break; default: fprintf(stderr, "Unknown command '%c'\n", ch); exit(-1); } exit(0); } ``` ## 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 ``` ## REPORTING BUGS Report bugs to \<**linux-trace-devel@vger.kernel.org**\[2\]\> ## 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) 2022 Google, 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\. linux-trace-devel@vger.kernel.org mailto:linux-trace-devel@vger.kernel.org