Here are defined some of the errors from the external projects we interact with, and of ours.

IPerf

enum {
    IENONE = 0,             // No error
    /* Parameter errors */
    IESERVCLIENT = 1,       // Iperf cannot be both server and client
    IENOROLE = 2,           // Iperf must either be a client (-c) or server (-s)
    IESERVERONLY = 3,       // This option is server only
    IECLIENTONLY = 4,       // This option is client only
    IEDURATION = 5,         // test duration too long. Maximum value = %dMAX_TIME
    IENUMSTREAMS = 6,       // Number of parallel streams too large. Maximum value = %dMAX_STREAMS
    IEBLOCKSIZE = 7,        // Block size too large. Maximum value = %dMAX_BLOCKSIZE
    IEBUFSIZE = 8,          // Socket buffer size too large. Maximum value = %dMAX_TCP_BUFFER
    IEINTERVAL = 9,         // Invalid report interval (min = %gMIN_INTERVAL, max = %gMAX_INTERVAL seconds)
    IEMSS = 10,             // MSS too large. Maximum value = %dMAX_MSS
    IENOSENDFILE = 11,      // This OS does not support sendfile
    IEOMIT = 12,            // Bogus value for --omit
    IEUNIMP = 13,           // Not implemented yet
    IEFILE = 14,            // -F file couldn't be opened
    IEBURST = 15,           // Invalid burst count. Maximum value = %dMAX_BURST
    IEENDCONDITIONS = 16,   // Only one test end condition (-t, -n, -k) may be specified
    IELOGFILE = 17,	    // Can't open log file
    IENOSCTP = 18,	    // No SCTP support available
    IEBIND = 19,	    // UNUSED:  Local port specified with no local bind option
    IEUDPBLOCKSIZE = 20,    // Block size invalid
    IEBADTOS = 21,	    // Bad TOS value
    IESETCLIENTAUTH = 22,   // Bad configuration of client authentication
    IESETSERVERAUTH = 23,   // Bad configuration of server authentication
    IEBADFORMAT = 24,	    // Bad format argument to -f
    IEREVERSEBIDIR = 25,    // Iperf cannot be both reverse and bidirectional
    IEBADPORT = 26,	    // Bad port number
    IETOTALRATE = 27,       // Total required bandwidth is larger than server's limit
    IETOTALINTERVAL = 28,   // Invalid time interval for calculating average data rate
    IESKEWTHRESHOLD = 29,   // Invalid value specified as skew threshold
    IEIDLETIMEOUT = 30,     // Invalid value specified as idle state timeout
    IERCVTIMEOUT = 31,      // Illegal message receive timeout
    IERVRSONLYRCVTIMEOUT = 32,  // Client receive timeout is valid only in reverse mode
    /* Test errors */
    IENEWTEST = 100,        // Unable to create a new test (check perror)
    IEINITTEST = 101,       // Test initialization failed (check perror)
    IELISTEN = 102,         // Unable to listen for connections (check perror)
    IECONNECT = 103,        // Unable to connect to server (check herror/perror) [from netdial]
    IEACCEPT = 104,         // Unable to accept connection from client (check herror/perror)
    IESENDCOOKIE = 105,     // Unable to send cookie to server (check perror)
    IERECVCOOKIE = 106,     // Unable to receive cookie from client (check perror)
    IECTRLWRITE = 107,      // Unable to write to the control socket (check perror)
    IECTRLREAD = 108,       // Unable to read from the control socket (check perror)
    IECTRLCLOSE = 109,      // Control socket has closed unexpectedly
    IEMESSAGE = 110,        // Received an unknown message
    IESENDMESSAGE = 111,    // Unable to send control message to client/server (check perror)
    IERECVMESSAGE = 112,    // Unable to receive control message from client/server (check perror)
    IESENDPARAMS = 113,     // Unable to send parameters to server (check perror)
    IERECVPARAMS = 114,     // Unable to receive parameters from client (check perror)
    IEPACKAGERESULTS = 115, // Unable to package results (check perror)
    IESENDRESULTS = 116,    // Unable to send results to client/server (check perror)
    IERECVRESULTS = 117,    // Unable to receive results from client/server (check perror)
    IESELECT = 118,         // Select failed (check perror)
    IECLIENTTERM = 119,     // The client has terminated
    IESERVERTERM = 120,     // The server has terminated
    IEACCESSDENIED = 121,   // The server is busy running a test. Try again later.
    IESETNODELAY = 122,     // Unable to set TCP/SCTP NODELAY (check perror)
    IESETMSS = 123,         // Unable to set TCP/SCTP MSS (check perror)
    IESETBUF = 124,         // Unable to set socket buffer size (check perror)
    IESETTOS = 125,         // Unable to set IP TOS (check perror)
    IESETCOS = 126,         // Unable to set IPv6 traffic class (check perror)
    IESETFLOW = 127,        // Unable to set IPv6 flow label
    IEREUSEADDR = 128,      // Unable to set reuse address on socket (check perror)
    IENONBLOCKING = 129,    // Unable to set socket to non-blocking (check perror)
    IESETWINDOWSIZE = 130,  // Unable to set socket window size (check perror)
    IEPROTOCOL = 131,       // Protocol does not exist
    IEAFFINITY = 132,       // Unable to set CPU affinity (check perror)
    IEDAEMON = 133,	    // Unable to become a daemon process
    IESETCONGESTION = 134,  // Unable to set TCP_CONGESTION
    IEPIDFILE = 135,	    // Unable to write PID file
    IEV6ONLY = 136,  	    // Unable to set/unset IPV6_V6ONLY (check perror)
    IESETSCTPDISABLEFRAG = 137, // Unable to set SCTP Fragmentation (check perror)
    IESETSCTPNSTREAM= 138,  //  Unable to set SCTP number of streams (check perror)
    IESETSCTPBINDX= 139,    // Unable to process sctp_bindx() parameters
    IESETPACING= 140,       // Unable to set socket pacing rate
    IESETBUF2= 141,	    // Socket buffer size incorrect (written value != read value)
    IEAUTHTEST = 142,       // Test authorization failed
    IEBINDDEV = 143,        // Unable to bind-to-device (check perror, maybe permissions?)
    IENOMSG = 144,          // No message was received for NO_MSG_RCVD_TIMEOUT time period
    IESETDONTFRAGMENT = 145,   // Unable to set IP Do-Not-Fragment
    IEBINDDEVNOSUPPORT = 146,  // `ip%%dev` is not supported as system does not support bind to device
    IEHOSTDEV = 147,        // host device name (ip%%<dev>) is supported (and required) only for IPv6 link-local address
    /* Stream errors */
    IECREATESTREAM = 200,   // Unable to create a new stream (check herror/perror)
    IEINITSTREAM = 201,     // Unable to initialize stream (check herror/perror)
    IESTREAMLISTEN = 202,   // Unable to start stream listener (check perror)
    IESTREAMCONNECT = 203,  // Unable to connect stream (check herror/perror)
    IESTREAMACCEPT = 204,   // Unable to accepte stream connection (check perror)
    IESTREAMWRITE = 205,    // Unable to write to stream socket (check perror)
    IESTREAMREAD = 206,     // Unable to read from stream (check perror)
    IESTREAMCLOSE = 207,    // Stream has closed unexpectedly
    IESTREAMID = 208,       // Stream has invalid ID
    /* Timer errors */
    IENEWTIMER = 300,       // Unable to create new timer (check perror)
    IEUPDATETIMER = 301,    // Unable to update timer (check perror)
};