Andrew's git - gitweb.git/blob - Documentation/technical/api-trace.txt

   1trace API
   2=========
   3
   4The trace API can be used to print debug messages to stderr or a file. Trace
   5code is inactive unless explicitly enabled by setting `GIT_TRACE*` environment
   6variables.
   7
   8The trace implementation automatically adds `timestamp file:line ... \n` to
   9all trace messages. E.g.:
  10
  11------------
  1223:59:59.123456 git.c:312               trace: built-in: git 'foo'
  1300:00:00.000001 builtin/foo.c:99        foo: some message
  14------------
  15
  16Data Structures
  17---------------
  18
  19`struct trace_key`::
  20
  21        Defines a trace key (or category). The default (for API functions that
  22        don't take a key) is `GIT_TRACE`.
  23+
  24E.g. to define a trace key controlled by environment variable `GIT_TRACE_FOO`:
  25+
  26------------
  27static struct trace_key trace_foo = TRACE_KEY_INIT(FOO);
  28
  29static void trace_print_foo(const char *message)
  30{
  31        trace_printf_key(&trace_foo, "%s", message);
  32}
  33------------
  34+
  35Note: don't use `const` as the trace implementation stores internal state in
  36the `trace_key` structure.
  37
  38Functions
  39---------
  40
  41`int trace_want(struct trace_key *key)`::
  42
  43        Checks whether the trace key is enabled. Used to prevent expensive
  44        string formatting before calling one of the printing APIs.
  45
  46`void trace_disable(struct trace_key *key)`::
  47
  48        Disables tracing for the specified key, even if the environment
  49        variable was set.
  50
  51`void trace_printf(const char *format, ...)`::
  52`void trace_printf_key(struct trace_key *key, const char *format, ...)`::
  53
  54        Prints a formatted message, similar to printf.
  55
  56`void trace_argv_printf(const char **argv, const char *format, ...)``::
  57
  58        Prints a formatted message, followed by a quoted list of arguments.
  59
  60`void trace_strbuf(struct trace_key *key, const struct strbuf *data)`::
  61
  62        Prints the strbuf, without additional formatting (i.e. doesn't
  63        choke on `%` or even `\0`).
  64
  65`uint64_t getnanotime(void)`::
  66
  67        Returns nanoseconds since the epoch (01/01/1970), typically used
  68        for performance measurements.
  69+
  70Currently there are high precision timer implementations for Linux (using
  71`clock_gettime(CLOCK_MONOTONIC)`) and Windows (`QueryPerformanceCounter`).
  72Other platforms use `gettimeofday` as time source.
  73
  74`void trace_performance(uint64_t nanos, const char *format, ...)`::
  75`void trace_performance_since(uint64_t start, const char *format, ...)`::
  76
  77        Prints the elapsed time (in nanoseconds), or elapsed time since
  78        `start`, followed by a formatted message. Enabled via environment
  79        variable `GIT_TRACE_PERFORMANCE`. Used for manual profiling, e.g.:
  80+
  81------------
  82uint64_t start = getnanotime();
  83/* code section to measure */
  84trace_performance_since(start, "foobar");
  85------------
  86+
  87------------
  88uint64_t t = 0;
  89for (;;) {
  90        /* ignore */
  91        t -= getnanotime();
  92        /* code section to measure */
  93        t += getnanotime();
  94        /* ignore */
  95}
  96trace_performance(t, "frotz");
  97------------
  98
  99Bugs & Caveats
 100--------------
 101
 102GIT_TRACE_* environment variables can be used to tell Git to show
 103trace output to its standard error stream. Git can often spawn a pager
 104internally to run its subcommand and send its standard output and
 105standard error to it.
 106
 107Because GIT_TRACE_PERFORMANCE trace is generated only at the very end
 108of the program with atexit(), which happens after the pager exits, it
 109would not work well if you send its log to the standard error output
 110and let Git spawn the pager at the same time.
 111
 112As a work around, you can for example use '--no-pager', or set
 113GIT_TRACE_PERFORMANCE to another file descriptor which is redirected
 114to stderr, or set GIT_TRACE_PERFORMANCE to a file specified by its
 115absolute path.
 116
 117For example instead of the following command which by default may not
 118print any performance information:
 119
 120------------
 121GIT_TRACE_PERFORMANCE=2 git log -1
 122------------
 123
 124you may want to use:
 125
 126------------
 127GIT_TRACE_PERFORMANCE=2 git --no-pager log -1
 128------------
 129
 130or:
 131
 132------------
 133GIT_TRACE_PERFORMANCE=3 3>&2 git log -1
 134------------
 135
 136or:
 137
 138------------
 139GIT_TRACE_PERFORMANCE=/path/to/log/file git log -1
 140------------