builtin: make 5 C functions trusted, tweak signatures (#8730)

vlib/builtin/cfns.c.v

@@ -7,6 +7,7 @@ fn C.memcmp(byteptr, byteptr, int) int
 
 fn C.memmove(byteptr, byteptr, int) voidptr
 
+[trusted]
 fn C.calloc(int, int) byteptr
 
 fn C.malloc(int) byteptr
@@ -15,6 +16,7 @@ fn C.realloc(a byteptr, b int) byteptr
 
 fn C.free(ptr voidptr)
 
+[trusted]
 fn C.exit(code int)
 
 fn C.qsort(base voidptr, items size_t, item_size size_t, cb qsort_callback_func)
@@ -25,7 +27,8 @@ fn C.strlen(s charptr) int
 
 fn C.sscanf(byteptr, byteptr, ...byteptr) int
 
-fn C.isdigit(s byteptr) bool
+[trusted]
+fn C.isdigit(c int) bool
 
 // stdio.h
 fn C.popen(c charptr, t charptr) voidptr
@@ -70,7 +73,7 @@ fn C.pclose() int
 // process execution, os.process:
 fn C.getpid() int
 
-fn C.system() int
+fn C.system(cmd charptr) int
 
 fn C.posix_spawn(child_pid &int, path charptr, file_actions voidptr, attrp voidptr, argv &charptr, envp &charptr) int
 
@@ -78,6 +81,7 @@ fn C.posix_spawnp(child_pid &int, exefile charptr, file_actions voidptr, attrp v
 
 fn C.execve(cmd_path charptr, args voidptr, envs voidptr) int
 
+[trusted]
 fn C.fork() int
 
 fn C.wait(status &int) int
@@ -112,7 +116,7 @@ fn C.memset() int
 
 fn C.sigemptyset() int
 
-fn C.getcwd() int
+fn C.getcwd(buf charptr, size size_t) charptr
 
 fn C.signal(signal int, handlercb voidptr) voidptr
 
@@ -131,7 +135,13 @@ fn C.closedir() int
 
 fn C.mkdir() int
 
-fn C.srand() int
+// C.rand returns a pseudorandom integer from 0 (inclusive) to C.RAND_MAX (exclusive)
+[trusted]
+fn C.rand() int
+
+// C.srand seeds the internal PRNG with the given value.
+[trusted]
+fn C.srand(seed uint)
 
 fn C.atof() int
 

vlib/rand/sys/system_rng.c.v

@@ -9,10 +9,11 @@ import rand.constants
 
 // Implementation note:
 // ====================
-// C.rand() is okay to use within its defined range of C.RAND_MAX.
+// C.rand returns a pseudorandom integer from 0 (inclusive) to C.RAND_MAX (exclusive)
+// C.rand() is okay to use within its defined range.
 // (See: https://web.archive.org/web/20180801210127/http://eternallyconfuzzled.com/arts/jsw_art_rand.aspx)
 // The problem is, this value varies with the libc implementation. On windows,
-// for example, RAND_MAX is usually a measly 32767, whereas on (newer) linux it's generaly
+// for example, RAND_MAX is usually a measly 32767, whereas on (newer) linux it's generally
 // 2147483647. The repetition period also varies wildly. In order to provide more entropy
 // without altering the underlying algorithm too much, this implementation simply
 // requests for more random bits until the necessary width for the integers is achieved.
@@ -29,11 +30,6 @@ fn calculate_iterations_for(bits int) int {
 	return base + extra
 }
 
-// C.rand returns a pseudorandom integer from 0 (inclusive) to C.RAND_MAX (exclusive)
-fn C.rand() int
-
-// C.srand seeds the internal PRNG with the given int seed.
-// fn C.srand(seed int)
 // SysRNG is the PRNG provided by default in the libc implementiation that V uses.
 pub struct SysRNG {
 mut:
@@ -47,7 +43,7 @@ pub fn (mut r SysRNG) seed(seed_data []u32) {
 		exit(1)
 	}
 	r.seed = seed_data[0]
-	unsafe { C.srand(int(r.seed)) }
+	C.srand(r.seed)
 }
 
 // r.default_rand() exposes the default behavior of the system's RNG