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");