record - Man Page

Fast event logging in circular buffers


#include <recorder/recorder.h>

ringidx_t record(recorder_name recname, const char *format, ...);
ringidx_t record_fast(recorder_name recname, const char *format, ...);


The record() function stores an event in the recorder identified by recname. The event record includes a time stamp provided by the gettimeofday(3) function (or a similar absolute time function), as well as the format string and up to 12 additional arguments. The format string should follow the printf(3) conventions, with the exceptions and additions indicated below.

The record_fast() reuses the time stamp of the previous record() function for the same recorder, making it approximately twice as fast on some modern systems. The cost difference should be in the order of the cost of a call like gettimeofday(3)

Event recording code is designed to remain active in your program at all times. Unlike typical printf(3) messages, events recorded by record() are not generally intended to be seen immediately, but rather to be shown laster, for example in case of crash. See recorder_dump(3). Formatting only happens when the message is actually printed.

Events can also be traced on a per-recorder basis. In that case, the record() and record_fast() functions will immediately format their output. See recorder_set_trace(3)

Automatic end of line

Event records are always printed on an individual line. It is not necessary to add a \n character at the end of the format string. If one is present, it will have no effect.

    record(my_recorder, "Hello");
    record(my_recorder, "World0);

The use of \n at other positions in the string is possible and has the usual effect.

Formatting character strings: %s and %+s

Since formatting an event may happen at an unknown time after its recording, the %s and S formats are treated like %p unless formatting happens as part of tracing. If you are passing a character string that is known to be constant, or at least still valid when it will be formatted, then you can use the %+s format instead.

    const char *name[2] = { "false", "true" };
    record(my_recorder, "Value %d is %+s", value, name[!!value]);

Extended format characters

It is possible to configure additional format characters. See recorder_configure_type. When this function is used, format strings may deviate notably from the printf(3) standard, or may conflict with other extensions to that standard.

Return Value

The return value is the index of the event that was recorded. This value is rarely useful and can generally be ignored.


The program below records its input arguments, and crashes if passed crash as the first command-line argument.

#include <recorder/recorder.h>
#include <string.h>

RECORDER(program_args, 32, "Program command-line arguments");
int main(int argc, char **argv)
    int a;
    recorder_dump_on_common_signals(0, 0);
    for (a = 0; a < argc; a++)
        record(program_args, "Argument %d is %+s", a, argv[a]);

    if (argc >= 2 && strcmp(argv[1], "crash") == 0)
        char *ptr = NULL;
        strcpy(ptr, argv[1]);

When a crash occurs, previously recorded events are printed out on the console.

The program below is an instrumented version of the classical recursive Fibonacci computation. It uses several recorders corresponding to different types of events, and activates warnings and errors in a way that can be configured by setting an environment variable.

#include <recorder/recorder.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

RECORDER(fib_main,    32, "Loops in fib function");
RECORDER(fib_loops,   32, "Loops in fib function");
RECORDER(fib_warning, 32, "Warnings in fib function");
RECORDER(fib_error,   32, "Errors in fib function");

int fib(int n)
    if (n <= 1) {
        if (n < 0)
            record(fib_error, "fib is undefined for negative value %d", n);
        return n;
    record(fib_loops, "Computing fib(%d)", n);
    int result = fib(n-1) + fib(n-2);
    record(fib_loops, "Computed fib(%d) = %d", n, result);
    return result;

int main(int argc, char **argv)
    int a;
    recorder_dump_on_common_signals(0, 0);
    recorder_trace_set(".*_warning=35 .*_error");
    for (a = 1; a < argc; a++) {
        int n = atoi(argv[a]);
        if (n >= RECORDER_TRACE(fib_warning))
            record(fib_warning, "Computing for %d may take a while", n);
        printf("fib(%d) = %d0, n, fib(n));
        if (n >= RECORDER_TRACE(fib_warning))
            record(fib_warning, "Computation for %d finally completed", n);

This program will produce an output similar to the following:

% fib 1 2 3 4 10 20 30 35 10 40 -1
fib(1) = 1
fib(2) = 1
fib(3) = 2
fib(4) = 3
fib(10) = 55
fib(20) = 6765
fib(30) = 832040
[2714667 0.177725] fib_warning: Computing for 35 may take a while
fib(35) = 9227465
[32575370 1.859156] fib_warning: Computation for 35 finally completed
fib(10) = 55
[32575547 1.859171] fib_warning: Computing for 40 may take a while
fib(40) = 102334155
[363735828 20.527882] fib_warning: Computation for 40 finally completed
[363735829 20.527887] fib_error: fib is undefined for negative value -1
fib(-1) = -1

The first column in trace outputs is the number of events that were recorded. THe second column is the time in seconds since the program started.

The same program can also be run with additional tracing or warnings, for example:

% FIB_TRACES="recorder_location fib_loops fib_warning=3" /tmp/fib 3 4
/tmp/fib.c:33:[82 0.000496] fib_warning: Computing for 3 may take a while
/tmp/fib.c:18:[83 0.000561] fib_loops: Computing fib(3)
/tmp/fib.c:18:[84 0.000570] fib_loops: Computing fib(2)
/tmp/fib.c:20:[85 0.000575] fib_loops: Computed fib(2) = 1
/tmp/fib.c:20:[86 0.000581] fib_loops: Computed fib(3) = 2
fib(3) = 2
/tmp/fib.c:36:[87 0.000590] fib_warning: Computation for 3 finally completed
/tmp/fib.c:33:[88 0.000596] fib_warning: Computing for 4 may take a while
/tmp/fib.c:18:[89 0.000601] fib_loops: Computing fib(4)
/tmp/fib.c:18:[90 0.000607] fib_loops: Computing fib(3)
/tmp/fib.c:18:[91 0.000612] fib_loops: Computing fib(2)
/tmp/fib.c:20:[92 0.000619] fib_loops: Computed fib(2) = 1
/tmp/fib.c:20:[93 0.000625] fib_loops: Computed fib(3) = 2
/tmp/fib.c:18:[94 0.000664] fib_loops: Computing fib(2)
/tmp/fib.c:20:[95 0.000707] fib_loops: Computed fib(2) = 1
/tmp/fib.c:20:[96 0.000724] fib_loops: Computed fib(4) = 3
fib(4) = 3
/tmp/fib.c:36:[97 0.000741] fib_warning: Computation for 4 finally completed


There is a limit to the number of arguments that can be passed to record() and record_fast(). As of version 1.0, the maximum number is 12. If that number is exceeded, the compilation error message is not friendly.

Bugs should be reported using

See Also

recorder_trace_set(3) RECORDER_TRACE(3)
recorder_dump(3), recorder_dump_for(3),
recorder_configure_output(3), recorder_configure_show(3)
recorder_configure_format(3), recorder_configure_type(3)

Additional documentation and tutorials can be found at


Written by Christophe de Dinechin

Referenced By

RECORDER(3), recorder_configure_output(3), recorder_configure_type(3), recorder_scope(1), recorder_trace_set(3).

The man page record_fast(3) is an alias of record(3).

2019-03-09 1.0 Recorder Library