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compact tree allocator routines (inspired by talloc)



Tal is a hierarchical allocator; any pointer allocated by tal can become the parent of another allocation. When you free that parent, the children (and grandchildren, etc) are automatically freed.

This allows you to build complex objects based on their lifetimes, eg:

 struct foo *X = tal(NULL, struct foo);
 X->val = tal(X, int);

and the pointer X->val would be a "child" of the tal context "X"; tal_free(X->val) would free X->val as expected, by tal_free(X) would free X and X->val.

With an overhead of approximately 4 pointers per object (vs. talloc's 12 pointers), it uses dynamic allocation for destructors and child lists, so those operations can fail. It does not support talloc's references or failing destructors.

See Also:


#include <stdio.h>
#include <err.h>
#include <ccan/tal/tal.h>

// A structure containing a popened command.
struct command {
        FILE *f;
        char *command;

// When struct command is freed, we also want to pclose pipe.
static void close_cmd(struct command *cmd)

// This function opens a writable pipe to the given command.
static struct command *open_output_cmd(const tal_t *ctx,
                                       const char *a0, const char *a1)
        struct command *cmd = tal(ctx, struct command);

        if (!cmd)
                return NULL;

        // Note that tal/str has helpers to make this much easier!
        cmd->command = tal_arrz(cmd, char, strlen(a0) + strlen(a1) + 2);
        if (!cmd->command) {
                return NULL;
        strcat(cmd->command, a0);
        strcat(cmd->command, " ");
        strcat(cmd->command, a1);

        cmd->f = popen(cmd->command, "w");
        if (!cmd->f) {
                return NULL;
        tal_add_destructor(cmd, close_cmd);
        return cmd;

int main(int argc, char *argv[])
        struct command *cmd;

        if (argc != 2)
                errx(1, "Usage: %s <command>\n", argv[0]);

        cmd = open_output_cmd(NULL, argv[1], "hello");
        if (!cmd)
                err(1, "Running '%s hello'", argv[1]);
        fprintf(cmd->f, "This is a test\n");
        return 0;