# trace API

The trace API can be used to print debug messages to stderr or a file. Trace code is inactive unless explicitly enabled by setting `GIT_TRACE*` environment variables.

The trace implementation automatically adds `timestamp file:line ... \n` to all trace messages. E.g.:

    23:59:59.123456 git.c:312               trace: built-in: git 'foo'
    00:00:00.000001 builtin/foo.c:99        foo: some message

## Data Structures

`struct trace_key`

  Defines a trace key (or category). The default (for API functions that don’t take a key) is `GIT_TRACE`.

  E.g. to define a trace key controlled by environment variable `GIT_TRACE_FOO`:

      static struct trace_key trace_foo = TRACE_KEY_INIT(FOO);

      static void trace_print_foo(const char *message)
      {
              trace_print_key(&trace_foo, message);
      }

  Note: don’t use `const` as the trace implementation stores internal state in the `trace_key` structure.

## Functions

`int trace_want(struct trace_key *key)`

  Checks whether the trace key is enabled. Used to prevent expensive string formatting before calling one of the printing APIs.

`void trace_disable(struct trace_key *key)`

  Disables tracing for the specified key, even if the environment variable was set.

`void trace_printf(const char *format, ...)`
`void trace_printf_key(struct trace_key *key, const char *format, ...)`

  Prints a formatted message, similar to printf.

`void trace_argv_printf(const char **argv, const char *format, ...)``

  Prints a formatted message, followed by a quoted list of arguments.

`void trace_strbuf(struct trace_key *key, const struct strbuf *data)`

  Prints the strbuf, without additional formatting (i.e. doesn’t choke on `%` or even `\0`).

`uint64_t getnanotime(void)`

  Returns nanoseconds since the epoch (01/01/1970), typically used for performance measurements.

  Currently there are high precision timer implementations for Linux (using `clock_gettime(CLOCK_MONOTONIC)`) and Windows (`QueryPerformanceCounter`). Other platforms use `gettimeofday` as time source.

`void trace_performance(uint64_t nanos, const char *format, ...)`
`void trace_performance_since(uint64_t start, const char *format, ...)`

  Prints the elapsed time (in nanoseconds), or elapsed time since `start`, followed by a formatted message. Enabled via environment variable `GIT_TRACE_PERFORMANCE`. Used for manual profiling, e.g.:

      uint64_t start = getnanotime();
      /* code section to measure */
      trace_performance_since(start, "foobar");

      uint64_t t = 0;
      for (;;) {
              /* ignore */
              t -= getnanotime();
              /* code section to measure */
              t += getnanotime();
              /* ignore */
      }
      trace_performance(t, "frotz");