I've been learning C by using some book and there's some code that I don't understand regarding working with base in C programming. So, here's the code:

/* By default, integers are decimal */
#define GN_ANY_BASE 0100 /* Can use any base - like strtol(3) */
#define GN_BASE_8   0200 /* Value is expressed in octal */
#define GN_BASE_16  0400 /* Value is expressed in hexadecimal */

// rlength: Read length bytes from the file, starting at the current file offset, and display them in text form.
int main(int argc, char *argv[]) {
    fd = open(argv[1], O_RDWR | O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH); /* rw-rw-rw- */
    if (fd == -1)
        errExit("open");
    for (ap = 2; ap < argc; ap  ) {
        switch (argv[ap][0]) {
          case 'r': /* Display bytes at current offset, as text */
          case 'R': /* Display bytes at current offset, in hex */
            len = getLong(&argv[ap][1], GN_ANY_BASE, argv[ap]);
            buf = malloc(len);
            ...
}

And here's the getLong function:

long getLong(const char *arg, int flags, const char *name) {
    return getNum("getLong", arg, flags, name);
}

And here's the getNum function:

static long getNum(const char *fname, const char *arg, int flags, const char *name) {
    long res;
    char *endptr;
    int base;

    if (arg == NULL || *arg == '\n') 
        gnFail(fname, "null or empty string", arg, name);

    base = (flags & GN_ANY_BASE) ? 0 : (flags & GN_BASE_8) ? 8 : (flags & GN_BASE_16) ? 16 : 10;
    res = strtol(arg, &endptr, base);

    return res;
}

This line really confuses me, and what happened after that didn't help either:

base = (flags & GN_ANY_BASE) ? 0 : (flags & GN_BASE_8) ? 8 : (flags & GN_BASE_16) ? 16 : 10;

What did it do? and why do we need this? what is GN_BASE ?

I can run the programs from the terminal by using the command:

./main file.txt r50

And it will give the following output like this:

r50: this is a text inside file.txt

I tried removing as many codes as possible for this example and hope it helps clear the example.

I'm sorry for asking beforehand. Thank you very much.

CodePudding user response：

1. The argument flag can carry other information so we mask what we are interested in with bitwise and-operator (&). Btw, often those mutually exclusive flags are written with a left shift instead of absolute values (1<<6, 1<<7, 1<<8).
2. We map the value provided to something that is more convenient for use internally (0100 to 0, 0200 to 8, and 0400 to 16).
3. Finally, we provide a default value of 10 if none of those 3 base bits were found.