Cleanup: Interface for Unique Strings

Of course a unique string should not be mutable. Hence function UStrAdd and UStrAdd_ should of course return a const pointer. You should therefore update the header to:

session19/ustr/ustr.h 
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#ifndef UTILS_USTR_H
#define UTILS_USTR_H

#include <stdbool.h>
#include <stddef.h>

struct UStr
{
    size_t len;
    char cstr[];
};

const struct UStr *UStrAdd_(const char *s, bool *added);
const struct UStr *UStrAdd(const char *s);
void UStrPrintPool(void);

#endif // UTILS_USTR_H

Here the corresponding source file and the updated test program:

session19/ustr/ustr.c 
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "ustr.h"

struct Node
{
    struct Node *next;
    struct UStr ustr;
};

static struct Node *node;

const struct UStr *
UStrAdd_(const char *s, bool *added)
{
    size_t len = strlen(s);
    if (added) {
        *added = true;
    }
    for (struct Node *n = node; n; n = n->next) {
        if (len == n->ustr.len && !strcmp(s, n->ustr.cstr)) {
            if (added) {
                *added = false;
            }
            return &n->ustr;
        }
    }

    struct Node *n = malloc(len + 1 + sizeof(size_t) + sizeof(struct Node *));
    if (!n) {
        fprintf(stderr, "makeUStr: out of memory\n");
        abort();
    }

    n->next = node;
    n->ustr.len = len;
    strcpy(n->ustr.cstr, s);
    node = n;
    return &node->ustr;
}

const struct UStr *
UStrAdd(const char *s)
{
    return UStrAdd_(s, 0);
}

void
UStrPrintPool(void)
{
    for (const struct Node *n = node; n; n = n->next) {
        printf("%s\n", n->ustr.cstr);
    }
}
session19/ustr/xtest_ustr.c 
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#include <stdio.h>

#include "ustr.h"

int
main(void)
{
    const struct UStr *kwIf = UStrAdd("if");
    const struct UStr *kwWhile = UStrAdd("while");

    char *line = 0;
    size_t capacity = 0;
    ssize_t len;

    while ((len = getline(&line, &capacity, stdin)) > 0) {
        line[len - 1] = 0;
        bool added = false;
        const struct UStr *ident = UStrAdd_(line, &added);

        if (ident == kwIf) {
            printf("keyword 'if'\n");
        } else if (ident == kwWhile) {
            printf("keyword 'while'\n");
        } else {
            printf("identifier '%s'\n", ident->cstr);
            if (added) {
                printf("this is new\n");
            }
        }
    }

    printf("Pool of UStr:\n");
    UStrPrintPool();
}