librrd - RRD library functions


librrd contains most of the functionality in RRDTool. The command line utilities and language bindings are often just wrappers around the code contained in librrd.

This manual page documents the librrd API.

NOTE: This document is a work in progress, and should be considered incomplete as long as this warning persists. For more information about the librrd functions, always consult the source code.


rrd_dump_cb_r(char *filename, int opt_header, rrd_output_callback_t cb, void *user)

In some situations it is necessary to get the output of rrd_dump without writing it to a file or the standard output. In such cases an application can ask rrd_dump_cb_r to call an user-defined function each time there is output to be stored somewhere. This can be used, to e.g. directly feed an XML parser with the dumped output or transfer the resulting string in memory.

The arguments for rrd_dump_cb_r are the same as for rrd_dump_opt_r except that the output filename parameter is replaced by the user-defined callback function and an additional parameter for the callback function that is passed untouched, i.e. to store information about the callback state needed for the user-defined callback to function properly.

Recent versions of rrd_dump_opt_r internally use this callback mechanism to write their output to the file provided by the user.

    size_t rrd_dump_opt_cb_fileout(
        const void *data,
        size_t len,
        void *user)
        return fwrite(data, 1, len, (FILE *)user);

The associated call for rrd_dump_cb_r looks like

    res = rrd_dump_cb_r(filename, opt_header,
        rrd_dump_opt_cb_fileout, (void *)out_file);

where the last parameter specifies the file handle rrd_dump_opt_cb_fileout should write to. There's no specific condition for the callback to detect when it is called for the first time, nor for the last time. If you require this for initialization and cleanup you should do those tasks before and after calling rrd_dump_cr_r respectively.



Generates random numbers just like random(). This further ensures that the random number generator is seeded exactly once per process.

rrd_add_ptr(void ***dest, size_t *dest_size, void *src)

Dynamically resize the array pointed to by dest. dest_size is a pointer to the current size of dest. Upon successful realloc(), the dest_size is incremented by 1 and the src pointer is stored at the end of the new dest. Returns 1 on success, 0 on failure.

    type **arr = NULL;
    type *elem = "whatever";
    size_t arr_size = 0;
    if (!rrd_add_ptr(&arr, &arr_size, elem))
rrd_add_strdup(char ***dest, size_t *dest_size, char *src)

Like rrd_add_ptr, except adds a strdup of the source string.

    char **arr = NULL;
    size_t arr_size = NULL;
    char *str  = "example text";
    if (!rrd_add_strdup(&arr, &arr_size, str))
rrd_free_ptrs(void ***src, size_t *cnt)

Free an array of pointers allocated by rrd_add_ptr or rrd_add_strdup. Also frees the array pointer itself. On return, the source pointer will be NULL and the count will be zero.

    /* created as above */
    rrd_free_ptrs(&arr, &arr_size);
    /* here, arr == NULL && arr_size == 0 */
rrd_mkdir_p(const char *pathname, mode_t mode)

Create the directory named pathname including all of its parent directories (similar to mkdir -p on the command line - see mkdir(1) for more information). The argument mode specifies the permissions to use. It is modified by the process's umask. See mkdir(2) for more details.

The function returns 0 on success, a negative value else. In case of an error, errno is set accordingly. Aside from the errors documented in mkdir(2), the function may fail with the following errors:


pathname is NULL or the empty string.


Insufficient memory was available.

any error returned by stat(2)

In contrast to mkdir(2), the function does not fail if pathname already exists and is a directory.


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