Update file_descriptor.read_bytes do not alloc.

- `read_bytes` takes in Span[Byte] buffer as opposed to allocating
  to avoid n+1 alloc. issues etc.

- Remove c_str_ptr type from ffi module since it is not a convention set
  by C lang., it will potentially get added back in at a later date
  after discussions and under its own PR

- Doc string updates to libC funcs.

Co-authored-by: Connor Gray <accounts@connorgray.com>
Signed-off-by: Hristo (Izo) G. <53634432+izo0x90@users.noreply.github.com>

stdlib/src/builtin/file_descriptor.mojo

@@ -26,7 +26,7 @@ f.close()
 from sys import os_is_macos, os_is_linux
 from sys._amdgpu import printf_append_string_n, printf_begin
 from sys.ffi import c_ssize_t, external_call
-from sys.info import is_amd_gpu, is_nvidia_gpu
+from sys.info import is_amd_gpu, is_gpu, is_nvidia_gpu
 
 from builtin.io import _printf
 from builtin.os import abort
@@ -89,33 +89,35 @@ struct FileDescriptor(Writer):
             )
 
     @always_inline
-    fn read_bytes(mut self, size: UInt) raises -> List[Byte]:
+    fn read_bytes(mut self, mut buffer: Span[Byte, _]) raises -> UInt:
         """
         Read a number of bytes from the file.
 
         Args:
-            size: Number of bytes to read.
+            buffer: Span[Byte] of length n where to store read bytes. n = number of bytes to read.
 
         Returns:
-          A list of bytes.
+            Actual number of bytes read.
         """
 
-        @parameter
-        if is_nvidia_gpu():
-            constrained[False, "Nvidia GPU read bytes not implemented"]()
-            return abort[List[Byte]]()
-        elif is_amd_gpu():
-            constrained[False, "AMD GPU read bytes not implemented"]()
-            return abort[List[Byte]]()
-        elif os_is_macos() or os_is_linux():
-            var ptr = UnsafePointer[Byte].alloc(size)
+        constrained[
+            not is_gpu(), "`read_bytes()` is not yet implemented for GPUs."
+        ]()
 
-            read = external_call["read", c_ssize_t](self.value, ptr, size)
-
-            return List[Byte](ptr=ptr, length=read, capacity=read)
+        @parameter
+        if os_is_macos() or os_is_linux():
+            read = external_call["read", c_ssize_t](
+                self.value, buffer.unsafe_ptr(), len(buffer)
+            )
+            if read < 0:
+                raise Error("Failed to read bytes.")
+            return read
         else:
-            constrained[False, "Unknown platform read bytes not implemented"]()
-            return abort[List[Byte]]()
+            constrained[
+                False,
+                "`read_bytes()` is not yet implemented for unknown platform.",
+            ]()
+            return abort[UInt]()
 
     fn write[*Ts: Writable](mut self, *args: *Ts):
         """Write a sequence of Writable arguments to the provided Writer.

stdlib/src/sys/_libc.mojo

@@ -18,7 +18,7 @@ functionality in the rest of the Mojo standard library.
 """
 
 from sys import os_is_windows
-from sys.ffi import OpaquePointer, c_char, c_int, c_size_t, c_str_ptr
+from sys.ffi import OpaquePointer, c_char, c_int, c_size_t
 
 from memory import UnsafePointer
 
@@ -107,19 +107,23 @@ fn dup(oldfd: c_int) -> c_int:
 
 
 @always_inline
-fn execvp(file: UnsafePointer[c_char], argv: UnsafePointer[c_str_ptr]) -> c_int:
+fn execvp(
+    file: UnsafePointer[c_char], argv: UnsafePointer[UnsafePointer[c_char]]
+) -> c_int:
     """[`execvp`](https://pubs.opengroup.org/onlinepubs/9799919799/functions/exec.html)
     — execute a file.
-    
+
     Args:
-        argv: The c_str_ptr array must be terminated with a NULL pointer.
+        file: NULL terminated UnsafePointer[c_char] (C string), containing path to executable.
+        argv: The UnsafePointer[c_char] array must be terminated with a NULL pointer.
     """
     return external_call["execvp", c_int](file, argv)
 
 
 @always_inline
 fn vfork() -> c_int:
-    """[`vfork()`](https://pubs.opengroup.org/onlinepubs/009696799/functions/vfork.html)."""
+    """[`vfork()`](https://pubs.opengroup.org/onlinepubs/009696799/functions/vfork.html).
+    """
     return external_call["vfork", c_int]()
 
 
@@ -142,7 +146,8 @@ fn kill(pid: c_int, sig: c_int) -> c_int:
 
 @always_inline
 fn pipe(fildes: UnsafePointer[c_int]) -> c_int:
-    """[`pipe()`](https://pubs.opengroup.org/onlinepubs/9799919799/functions/pipe.html) — create an interprocess channel."""
+    """[`pipe()`](https://pubs.opengroup.org/onlinepubs/9799919799/functions/pipe.html) — create an interprocess channel.
+    """
     return external_call["pipe", c_int](fildes)
 
 

stdlib/src/sys/ffi.mojo

@@ -71,9 +71,6 @@ alias c_float = Float32
 alias c_double = Float64
 """C `double` type."""
 
-alias c_str_ptr = UnsafePointer[c_char]
-"""C `*char` type"""
-
 alias OpaquePointer = UnsafePointer[NoneType]
 """An opaque pointer, equivalent to the C `void*` type."""