I have an coding assignment that must be done in C. I got sidetracked and decided to try to make a format macro i can use for styling text with ansi codes. My code is as follows:
<format.h>

#include <stdarg.h>
#include <string.h>

/**
 * global buffer to store format arguments for prints
 */
char format_buffer[128];

/**
 * Count the number of char* in variadic arguments, auxiliary for FMT macro
 */
#define FARGS(...)  (sizeof((char*[]){__VA_ARGS__})/sizeof(char*))

/**
 * Automatically fills in parameters buffer and argc for the format function. Buffer is a global variable, therefore it is never out of scope.
 */
#define FMT(...) format(format_buffer, FARGS(__VA_ARGS__), __VA_ARGS__)

#define A_START "\x1B["
#define A_END "m"

/**
 * creates a format string based on input parameters.
 * @param argc the number of ansi flags to use
 * @param buffer the buffer to write the format string into. A size of 64 should be enough for most reasonable uses
 * @param ... the ansi flags to use. Examples include RED, BLUE, DASH, etc
 * @return the same pointer buffer, for ease of use in printf functions
 */
char* format(char* buffer, int argc, ...);

// ansi macros for formatting, all of the style ";NN"

<format.c>

#include "format.h"

char format_buffer[128] = {};

char* format(char* buffer, int argc, ...){
    buffer[0] = '\0';

    va_list argv;
    int i;

    // initialize argv to get data from the variable arguments
    va_start(argv, argc);

    strcat(buffer, A_START);
    /* access all the arguments assigned to valist */
    for (i = 0; i < argc; i++) {
        strcat(buffer, va_arg(argv, char*));
    }
    strcat(buffer, A_END);

    /* clean memory reserved for valist */
    va_end(argv);

    return buffer;

}

Using that, I can call the macro as follows, which is what I want:
printf("%sHello!\n", FMT(RED, BOLD)) //prints <Hello!\n> in red and bold

The problem

The problem I have is when I try to use multiple calls in the same print statement:
printf("%sHello, %sWorld!\n", FMT(RED, BOLD), FMT(YELLOW)) //prints <Hello, World!\n> in yellow
I’m positive it’s not working as expected because FMT(...) always returns the same global char*, but I don’t know how could i change it so that:

• I can call the format macro as shown above: printf("%sHello!\n", FMT(RED, BOLD)).
• I can use multiple FMT calls in the same print statement, as in printf("%sHello, %sWorld!\n", FMT(RED, BOLD), FMT(YELLOW)) should print <Hello, > in red and bold and <World!\n> in yellow.

As a final note, I’d rather code the solution instead of using a library or header that already has an implementation of this.

I first tried creating a new char[] inside the format function, but it would be stored in the stack so I assume that’s a much worse outcome.

>Solution :

Simply change the macro to:

#define FMT(...) format((char[128]){}, FARGS(__VA_ARGS__), __VA_ARGS__)

https://godbolt.org/z/KvWWcYfPb