Add more information about thread safety until we provide a thread-safe API.

README.markdown

@@ -94,7 +94,7 @@ Sodium also provides helper functions to generate random numbers,
 leveraging `/dev/urandom` or `/dev/random` on *nix and the cryptographic
 service provider on Windows. The interface is similar to
 `arc4random(3)`. It is `fork(2)`-safe but not thread-safe. This holds
-true for `crypto_sign_keypair()` as well.
+true for `crypto_sign_keypair()` and `crypto_box_keypair()` as well.
 
     uint32_t randombytes_random(void);
 

src/libsodium/include/sodium/crypto_box.h

@@ -1,6 +1,13 @@
 #ifndef crypto_box_H
 #define crypto_box_H
 
+/*
+ * THREAD SAFETY: crypto_box_keypair() is thread-safe,
+ * provided that you called sodium_init() once before using any
+ * other libsodium function.
+ * Other functions are always thread-safe.
+ */
+
 #include <stdlib.h>
 
 #include "crypto_box_curve25519xsalsa20poly1305.h"

src/libsodium/include/sodium/crypto_sign.h

@@ -1,6 +1,13 @@
 #ifndef crypto_sign_H
 #define crypto_sign_H
 
+/*
+ * THREAD SAFETY: crypto_sign_keypair() is thread-safe,
+ * provided that you called sodium_init() once before using any
+ * other libsodium function.
+ * Other functions, including crypto_sign_seed_keypair() are always thread-safe.
+ */
+
 #include <stdlib.h>
 
 #include "crypto_sign_ed25519.h"

src/libsodium/include/sodium/randombytes_salsa20_random.h

@@ -2,6 +2,12 @@
 #ifndef randombytes_salsa20_random_H
 #define randombytes_salsa20_random_H
 
+/*
+ * THREAD SAFETY: randombytes_salsa20_random*() functions are
+ * fork()-safe but not thread-safe.
+ * Always wrap them in a mutex if you need thread safety.
+ */
+
 #include <stdint.h>
 #include <stdlib.h>
 

src/libsodium/include/sodium/randombytes_sysrandom.h

@@ -2,6 +2,12 @@
 #ifndef randombytes_sysrandom_H
 #define randombytes_sysrandom_H
 
+/*
+ * THREAD SAFETY: randombytes_sysrandom() functions are thread-safe,
+ * provided that you called sodium_init() once before using any
+ * other libsodium function.
+ */
+
 #include <stdint.h>
 #include <stdlib.h>