liblibc: Fix prototype of execve() and friends

These functions do not modify their arguments, so they does not need
mutable pointers. The C prototypes take a constant array of mutable
C-strings, but that's a legacy quirk from before C had const (since
string literals have type `char *`). The Rust prototype had `*mut` in
the wrong place, anyway: to match the C prototype, it should have been
`*const *mut c_char`.

Also fix the one caller of execve in libstd, which now no longer needs
an unsafe cast.

Author: geofft

src/liblibc/lib.rs

@@ -5490,17 +5490,17 @@ pub mod funcs {
                 pub fn dup2(src: c_int, dst: c_int) -> c_int;
                 #[link_name = "_execv"]
                 pub fn execv(prog: *const c_char,
-                             argv: *mut *const c_char) -> intptr_t;
+                             argv: *const *const c_char) -> intptr_t;
                 #[link_name = "_execve"]
-                pub fn execve(prog: *const c_char, argv: *mut *const c_char,
-                              envp: *mut *const c_char)
+                pub fn execve(prog: *const c_char, argv: *const *const c_char,
+                              envp: *const *const c_char)
                               -> c_int;
                 #[link_name = "_execvp"]
                 pub fn execvp(c: *const c_char,
-                              argv: *mut *const c_char) -> c_int;
+                              argv: *const *const c_char) -> c_int;
                 #[link_name = "_execvpe"]
-                pub fn execvpe(c: *const c_char, argv: *mut *const c_char,
-                               envp: *mut *const c_char) -> c_int;
+                pub fn execvpe(c: *const c_char, argv: *const *const c_char,
+                               envp: *const *const c_char) -> c_int;
                 #[link_name = "_getcwd"]
                 pub fn getcwd(buf: *mut c_char, size: size_t) -> *mut c_char;
                 #[link_name = "_getpid"]
@@ -5684,12 +5684,12 @@ pub mod funcs {
                 pub fn dup(fd: c_int) -> c_int;
                 pub fn dup2(src: c_int, dst: c_int) -> c_int;
                 pub fn execv(prog: *const c_char,
-                             argv: *mut *const c_char) -> c_int;
-                pub fn execve(prog: *const c_char, argv: *mut *const c_char,
-                              envp: *mut *const c_char)
+                             argv: *const *const c_char) -> c_int;
+                pub fn execve(prog: *const c_char, argv: *const *const c_char,
+                              envp: *const *const c_char)
                               -> c_int;
                 pub fn execvp(c: *const c_char,
-                              argv: *mut *const c_char) -> c_int;
+                              argv: *const *const c_char) -> c_int;
                 pub fn fork() -> pid_t;
                 pub fn fpathconf(filedes: c_int, name: c_int) -> c_long;
                 pub fn getcwd(buf: *mut c_char, size: size_t) -> *mut c_char;
@@ -5749,12 +5749,12 @@ pub mod funcs {
                 pub fn dup(fd: c_int) -> c_int;
                 pub fn dup2(src: c_int, dst: c_int) -> c_int;
                 pub fn execv(prog: *const c_char,
-                             argv: *mut *const c_char) -> c_int;
-                pub fn execve(prog: *const c_char, argv: *mut *const c_char,
-                              envp: *mut *const c_char)
+                             argv: *const *const c_char) -> c_int;
+                pub fn execve(prog: *const c_char, argv: *const *const c_char,
+                              envp: *const *const c_char)
                               -> c_int;
                 pub fn execvp(c: *const c_char,
-                              argv: *mut *const c_char) -> c_int;
+                              argv: *const *const c_char) -> c_int;
                 pub fn fork() -> pid_t;
                 pub fn getcwd(buf: *mut c_char, size: size_t) -> *mut c_char;
                 pub fn getegid() -> gid_t;

src/libstd/sys/unix/process.rs

@@ -311,7 +311,7 @@ impl Process {
         if !envp.is_null() {
             *sys::os::environ() = envp as *const _;
         }
-        let _ = libc::execvp(*argv, argv as *mut _);
+        let _ = libc::execvp(*argv, argv);
         fail(&mut output)
     }