CODEWITHSUNDEEP
c
daisy
daisy

Reputation: 23501

Create a printf function with custom prefix

I'm trying to create a printf function that print like this

[INFO] whatever 123

va_args works but I don't know how to add a prefix. At least the following code won't do

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

void myprintf (char *fmt, ...)
{
  va_list argp;
  va_start (argp, fmt); 
  vfprintf (stdout, "[INFO] " fmt, argp); 
  va_end (argp);
}

int main (int argc , char **argv)
{
    myprintf ("arg count is %d\n", argc);
    return 0;
}

Any ideas?

Upvotes: 0

Views: 1531

Answers (1)

user149341
user149341

Reputation: 

"[INFO] " fmt

This code won't work. The "string pasting" behavior you're trying to use here is a preprocessor behavior, not a C operator. It can only be used on string constants -- not variables.

The easiest way of getting the behavior you want here will be to simply call printf twice:

printf("[INFO] ");
va_start(argp, fmt);
vfprintf(stdout, fmt, argp);
va_end(argp);

More difficult approaches which you may want to consider include:

  • Define myprintf() as a macro instead of a function so that it can use string pasting on the format argument.

  • Copy "[INFO] " and fmt into a temporary buffer and use that as a formatting string.

  • Copy "[INFO] " into a temporary buffer, use vsnprintf() to append the output to the buffer, then output that.

Upvotes: 3

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