customization of putchar function

That's ANSI 'C' for you!

In all fairness, ANSI C has nothing to do with this. What we're seeing here is a direct consequence of a design decision made by Keil: they decided to support printf(), but not fprintf() and fopen(), which would be the brutally obvious way a job like this would be done in actual ANSI C. I.e. the code at hand should be writable like this:

FILE *lcdout = fopen("mylcd_pseudofile", "w");
fprintf(lcdout, "this goes to the screen");

FILE *serialout = fopen("serial_pseudofile", "w");

printf("this goes to default output device");

But in Keil, you can't really do it this way, because neither fopen() nor fprintf() exist.

You could implement them yourself, though, and with the plan of keeping the main body of the code as close to ANSI C as possible, that may even be the best idea available.

That's ANSI 'C' for you!

In all fairness, ANSI C has nothing to do with this. What we're seeing here is a direct consequence of a design decision made by Keil: they decided to support printf(), but not fprintf() and fopen(), which would be the brutally obvious way a job like this would be done in actual ANSI C. I.e. the code at hand should be writable like this:

FILE *lcdout = fopen("mylcd_pseudofile", "w");
fprintf(lcdout, "this goes to the screen");

FILE *serialout = fopen("serial_pseudofile", "w");

printf("this goes to default output device");

But in Keil, you can't really do it this way, because neither fopen() nor fprintf() exist.

You could implement them yourself, though, and with the plan of keeping the main body of the code as close to ANSI C as possible, that may even be the best idea available.