A message from C

The library needs a short shelf message. Start at main: that is where this C program runs. The braces hold its statements, and each statement ends with a semicolon. #include <stdio.h> makes the declaration of printf available. Inside a printed string, \n starts a new line.

int main(void) takes no arguments and returns an integer exit status; return 0; reports success. printf sends the quoted string to standard output. Run this complete program. Then give the shelf a different message and run it again:

#include <stdio.h>

int main(void) {
    printf("Library open\n");
    return 0;
}

The quoted text is data, not a C instruction. Two printf calls appear below; watch their order:

#include <stdio.h>

int main(void) {
    printf("Shelf A\n");
    printf("Shelf B\n");
    return 0;
}

Remove the first \n for a run. Does Shelf B still begin on its own line? Put it back afterward.

Running a C file

Save the whole example as main.c if you want to try it locally. With GCC, run gcc -std=c11 -Wall -Wextra main.c -o main, then ./main (on Windows, run the resulting executable instead). The compiler checks the source first and produces an executable only if compilation succeeds. The site's Run button does both steps when its C runner is available.

Remove the semicolon after printf once and read the compiler error. A comment such as /* shelf label */ is ignored; a missing semicolon is not. Restore it before moving on.