How do I retrieve an error string from WSAGetLastError()?

Question

I'm porting some sockets code from Linux to Windows.

In Linux, I could use strerror() to convert an errno code into a human-readable string.

MSDN documentation shows equivalent strings for each error code returned from WSAGetLastError(), but I don't see anything about how to retrieve those strings. Will strerror() work here too?

How can I retrieve human-readable error strings from Winsock?

score 43 · Answer 1 · edited Oct 17 '16 at 14:40

43

wchar_t *s = NULL;
FormatMessageW(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, 
               NULL, WSAGetLastError(),
               MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
               (LPWSTR)&s, 0, NULL);
fprintf(stderr, "%S\n", s);
LocalFree(s);

edited Oct 17 '16 at 14:40

Stefan Profanter

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answered May 23 '13 at 20:27

mxcl

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4

The `&s` argument must be casted to LPTSTR here because this function uses a hack here with this parameter returning a pointer to string as a string. – Alexey Biryukov Apr 15 '14 at 08:38
1

Only if you also change FormatMessageW to FormatMessage. Otherwise, you may need to cast instead to LPWSTR, depending on the settings in your Visual Studio project. – Camille Goudeseune Sep 20 '14 at 20:40
I edited the answer to include the cast to `LPWSTR`, otherwise the compiler fails – Stefan Profanter Oct 17 '16 at 14:41

score 21 · Accepted Answer · answered Aug 03 '10 at 21:44

21

As the documentation for WSAGetLastError says you can use FormatMessage to obtain a text version of the error message.

You need to set FORMAT_MESSAGE_FROM_SYSTEM in the dwFlags parameter and pass the error code as the dwMessage parameter.

answered Aug 03 '10 at 21:44

CB Bailey

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Thanks--I just discovered that myself. I need to remember to look at the online MSDN (vs. the off-line copy installed on my laptop!). – Drew Hall Aug 03 '10 at 21:49

score 11 · Answer 3 · answered Sep 07 '17 at 20:17

A slightly simpler version of mxcl's answer, which removes the need for malloc/free and the risks implicit therein, and which handles the case where no message text is available (since Microsoft doesn't document what happens then):

int
   err;

char
   msgbuf [256];   // for a message up to 255 bytes.


msgbuf [0] = '\0';    // Microsoft doesn't guarantee this on man page.

err = WSAGetLastError ();

FormatMessage (FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,   // flags
               NULL,                // lpsource
               err,                 // message id
               MAKELANGID (LANG_NEUTRAL, SUBLANG_DEFAULT),    // languageid
               msgbuf,              // output buffer
               sizeof (msgbuf),     // size of msgbuf, bytes
               NULL);               // va_list of arguments

if (! *msgbuf)
   sprintf (msgbuf, "%d", err);  // provide error # if no string available

How do I retrieve an error string from WSAGetLastError()?

3 Answers3

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