CODEWITHSUNDEEP
cvariadic-functions
Noz.i.kor
Noz.i.kor

Reputation: 3747

Variadic function: Extract arguments and append string

I have a function log_info (copied from printf's implementation) that accepts variable no. of arguments and passes it to vprintf:

int log_info(const char *format, ...) {
  va_list arg;
  int done;

  va_start (arg, format);
  done = vfprintf (stdout, format, arg);
  va_end (arg);

  return done;
}

I want to prepend and append strings to these arguments. So if a user calls the above function this way:

log_info("at iteration %d, float value is %f", i, f);

instead of printing

at iteration 4, float value is 102.34

I want to print

[INFO] [at iteration 4, float value is 102.34] [timestamp: xxxx]

I could do it in 3 separate steps

fprintf(stdout, "[INFO] [");
vprintf(stdout, format, arg);
fprintf(stdout, "] [timestamp:%f]", ts);

But the program is multi-threaded, and hence I want all data to be written in a single call to vprintf (vprintf being thread-safe).

The other option is to lock a mutex and then write it in 3 steps as shown above, but if appending strings to the args is not too complex, I would like to try that.

Edit: Performance overhead due to use of mutex is not really an issue, but I dont want to use one unless necessary.

Thanks in advance

Upvotes: 1

Views: 1849

Answers (2)

jared_schmitz
jared_schmitz

Reputation: 593

You want to add new fields to the output, so just construct a new format string.

#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>

int loginfo(const char *format, ...) {
    int ret;
    int len;
    va_list ap;
    char *new_fmt;
    char *timestamp;
    const char fmt_template[] = "[INFO] [%s] [timestamp: %s]";

    /* Grab the timestamp now, since if we call it twice, it may change in length */
    timestamp = get_time_string();

    /* Calculate length for the augmented format string and allocate. */
    len = snprintf(NULL, 0, fmt_template, format, timestamp);
    new_fmt = malloc(len + 1);

    /* Construct the new format string */
    snprintf(new_fmt, len + 1, fmt_template, format, timestamp);

    /* Print as before, using new format string */
    va_start (ap, format);
    ret = vfprintf (stdout, new_fmt, ap);
    va_end (ap);

    free(new_fmt);
    return ret;
}

Upvotes: 5

Greg Prisament
Greg Prisament

Reputation: 2216

One approach is to use vsnprintf to write the original message to a (dynamically allocated) string. Then use fprintf to output the metadata plus original message:

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
...

static const char * get_timestamp_str()
{
    ...
}

static void Log(const char *format, ...)
{
    va_list arg;
    int len;
    char * orig_msg;

    /* Compute length of original message */
    va_start(arg, format);
    len = vsnprintf(NULL, 0, format, arg);
    va_end(arg);

    /* Allocate space for original message */
    orig_msg = (char *)calloc(len+1, sizeof(char));

    /* Write original message to string */
    va_start(arg, format);
    vsnprintf(orig_msg, len+1, format, arg);
    va_end(arg);

    /* Write metadata plus original message to stderr */
    fprintf(stderr, "[INFO] [timestamp %s] %s\n", get_timestamp_str(), orig_msg);
    free(orig_msg);
}

Upvotes: 3

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