4dda133385
Fix a typo in an example in the trace API documentation. * ep/trace-doc-sample-fix: api-trace.txt: fix typo
141 lines
3.8 KiB
Plaintext
141 lines
3.8 KiB
Plaintext
trace API
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=========
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The trace API can be used to print debug messages to stderr or a file. Trace
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code is inactive unless explicitly enabled by setting `GIT_TRACE*` environment
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variables.
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The trace implementation automatically adds `timestamp file:line ... \n` to
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all trace messages. E.g.:
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------------
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23:59:59.123456 git.c:312 trace: built-in: git 'foo'
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00:00:00.000001 builtin/foo.c:99 foo: some message
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------------
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Data Structures
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---------------
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`struct trace_key`::
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Defines a trace key (or category). The default (for API functions that
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don't take a key) is `GIT_TRACE`.
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+
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E.g. to define a trace key controlled by environment variable `GIT_TRACE_FOO`:
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+
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------------
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static struct trace_key trace_foo = TRACE_KEY_INIT(FOO);
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static void trace_print_foo(const char *message)
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{
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trace_printf_key(&trace_foo, "%s", message);
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}
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------------
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+
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Note: don't use `const` as the trace implementation stores internal state in
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the `trace_key` structure.
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Functions
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---------
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`int trace_want(struct trace_key *key)`::
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Checks whether the trace key is enabled. Used to prevent expensive
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string formatting before calling one of the printing APIs.
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`void trace_disable(struct trace_key *key)`::
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Disables tracing for the specified key, even if the environment
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variable was set.
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`void trace_printf(const char *format, ...)`::
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`void trace_printf_key(struct trace_key *key, const char *format, ...)`::
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Prints a formatted message, similar to printf.
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`void trace_argv_printf(const char **argv, const char *format, ...)``::
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Prints a formatted message, followed by a quoted list of arguments.
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`void trace_strbuf(struct trace_key *key, const struct strbuf *data)`::
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Prints the strbuf, without additional formatting (i.e. doesn't
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choke on `%` or even `\0`).
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`uint64_t getnanotime(void)`::
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Returns nanoseconds since the epoch (01/01/1970), typically used
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for performance measurements.
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+
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Currently there are high precision timer implementations for Linux (using
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`clock_gettime(CLOCK_MONOTONIC)`) and Windows (`QueryPerformanceCounter`).
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Other platforms use `gettimeofday` as time source.
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`void trace_performance(uint64_t nanos, const char *format, ...)`::
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`void trace_performance_since(uint64_t start, const char *format, ...)`::
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Prints the elapsed time (in nanoseconds), or elapsed time since
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`start`, followed by a formatted message. Enabled via environment
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variable `GIT_TRACE_PERFORMANCE`. Used for manual profiling, e.g.:
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+
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------------
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uint64_t start = getnanotime();
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/* code section to measure */
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trace_performance_since(start, "foobar");
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------------
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+
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------------
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uint64_t t = 0;
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for (;;) {
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/* ignore */
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t -= getnanotime();
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/* code section to measure */
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t += getnanotime();
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/* ignore */
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}
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trace_performance(t, "frotz");
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------------
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Bugs & Caveats
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--------------
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GIT_TRACE_* environment variables can be used to tell Git to show
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trace output to its standard error stream. Git can often spawn a pager
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internally to run its subcommand and send its standard output and
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standard error to it.
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Because GIT_TRACE_PERFORMANCE trace is generated only at the very end
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of the program with atexit(), which happens after the pager exits, it
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would not work well if you send its log to the standard error output
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and let Git spawn the pager at the same time.
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As a work around, you can for example use '--no-pager', or set
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GIT_TRACE_PERFORMANCE to another file descriptor which is redirected
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to stderr, or set GIT_TRACE_PERFORMANCE to a file specified by its
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absolute path.
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For example instead of the following command which by default may not
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print any performance information:
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------------
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GIT_TRACE_PERFORMANCE=2 git log -1
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------------
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you may want to use:
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------------
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GIT_TRACE_PERFORMANCE=2 git --no-pager log -1
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------------
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or:
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------------
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GIT_TRACE_PERFORMANCE=3 3>&2 git log -1
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------------
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or:
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------------
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GIT_TRACE_PERFORMANCE=/path/to/log/file git log -1
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------------
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