sending and receiving strings Receive C string from socket. Caller must free returned string using zstr_free(). Returns NULL if the context is being terminated or the process was interrupted. Receive a series of strings (until NULL) from multipart data. Each string is allocated and filled with string data; if there are not enough frames, unallocated strings are set to NULL. Returns -1 if the message could not be read, else returns the number of strings filled, zero or more. Free each returned string using zstr_free(). If not enough strings are provided, remaining multipart frames in the message are dropped. Send a C string to a socket, as a frame. The string is sent without trailing null byte; to read this you can use zstr_recv, or a similar method that adds a null terminator on the received string. String may be NULL, which is sent as "". Send a C string to a socket, as zstr_send(), with a MORE flag, so that you can send further strings in the same multi-part message. Send a formatted string to a socket. Note that you should NOT use user-supplied strings in the format (they may contain '%' which will create security holes). Send a formatted string to a socket, as for zstr_sendf(), with a MORE flag, so that you can send further strings in the same multi-part message. Send a series of strings (until NULL) as multipart data Returns 0 if the strings could be sent OK, or -1 on error. Accepts a void pointer and returns a fresh character string. If source is null, returns an empty string. Free a provided string, and nullify the parent pointer. Safe to call on a null pointer.