string literal combination for colorized readline with string variable

Question

Is it possible to put variable text_info to readline among with colors?
GCC debuger gives me this error: " error: expected ‘)’ before 'text'

#include <readline/readline.h>

#define CYELLOW "\001\e[0;31m\002"
#define RESET   "\001\e[0m\002"

int main(int argc, char **argv)
{
    char *text_info = "its very simple string";
    readline(CYELLOW text_info RESET);
    return 0;
}

I know that the way below works but it's not the case.

int main(int argc, char **argv)
{
    readline(CYELLOW "simple string" RESET);
    return 0;
}

You can concatenate constants, but not variables like that. Plus that should be `char* text_info`. — tadman, Jan 08 '18 at 19:33
You could use `sprintf` to build the string, if the `text_info` part is a variable, not a string literal. — Weather Vane, Jan 08 '18 at 19:33
You could use `char *text_info = CYELLOW "it's a very simple string" RESET; readline(text_info);` — Weather Vane, Jan 08 '18 at 19:42

Benjamin J. · Accepted Answer · 2018-01-08T19:53:37.260

1

The line you posted doesn't work because it can't be combined during build time:

readline(CYELLOW text_info RESET);

As @Weaterh Vane has already metioned in the comment above, the only real solution is to build the string during runtime via sprintf or better snprintf.

 char aBuffer[100];
 snprintf(aBuffer, sizeof(aBuffer), "%s%s%s", CYELLOW, text_info, RESET);

Edit: Please keep in mind, that aBuffer in the above example may be to small (depending on the length of text_info). Depending on your application you can just increase the size of 100 or dynamically allocating the buffer.

edited Jan 08 '18 at 19:53

answered Jan 08 '18 at 19:39

Benjamin J.

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2

Or: `snprintf(aBuffer, sizeof(aBuffer), CYELLOW "%s" RESET, text_info);` – HolyBlackCat Jan 08 '18 at 19:43

score 1 · Answer 2 · answered Jan 08 '18 at 19:41

In the working code, this line

readline(CYELLOW text_info RESET);

is macro-expanded to

readline("\001\e[0;31m\002" "simple string" "\001\e[0m\002");

which in turn, after string literal combination ends up as

readline("\001\e[0;31m\002simple string\001\e[0m\002");

which contains a simple string literal. No operators are involved, but it still ends up as a single parameter to the function.

In contrast to that, this line

readline(CYELLOW text_info RESET);

is macro-expanded to

readline("\001\e[0;31m\002" text_info "\001\e[0m\002");

which is not subject to string literal combination and therefor ends up with three string expressions (i.e. not exclusively literal strings, one being a non-literal string, a pointer to chars) without any operator in between.

Also, there is no simple operator for string concatenation (cases outside of string literal combination).

So in order to give a single string parameter ("string" as in "C-compatible zero-terminated sequence of chars, maybe array), you will have to construct such a string and then use it as single parameter for the function.

As Weather Vane has proposed, sprintf() is probably the tool to do that.

string literal combination for colorized readline with string variable

2 Answers2