kernel+libc: Implement read()

apps/app.c

@@ -21,6 +21,12 @@ int main()
         return 1;
     }
 
+    char buffer[512];
+    ssize_t nread = read(fd, buffer, sizeof(buffer));
+    buffer[nread] = 0;
+
+    printf("/etc/motd says: %s", buffer);
+
     close(fd);
 
     time_t now = time(NULL);

kernel/CMakeLists.txt

@@ -27,6 +27,7 @@ set(SOURCES
     src/sys/allocate_memory.cpp
     src/sys/usleep.cpp
     src/sys/open.cpp
+    src/sys/file.cpp
     src/fs/VFS.cpp
     src/fs/tmpfs/FileSystem.cpp
     src/InitRD.cpp

kernel/src/sys/file.cpp

@@ -0,0 +1,28 @@
+#include "Log.h"
+#include "fs/VFS.h"
+#include "memory/MemoryManager.h"
+#include "sys/Syscall.h"
+#include "thread/Scheduler.h"
+
+Result<u64> sys_read(Registers*, SyscallArgs args)
+{
+    int fd = (int)args[0];
+    u8* buf = (u8*)args[1];
+    usize size = (usize)args[2];
+
+    if (!MemoryManager::validate_user_write(buf, size)) return err(EFAULT);
+
+    if (fd < 0 || fd >= FD_MAX) return err(EBADF);
+
+    Thread* current = Scheduler::current();
+
+    Option<FileDescriptor>& descriptor = current->fd_table->fds[fd];
+
+    if (!descriptor.has_value()) return err(EBADF);
+
+    usize nread = TRY(descriptor->inode->read(buf, descriptor->offset, size));
+
+    descriptor->offset += nread;
+
+    return nread;
+}

kernel/src/sys/open.cpp

@@ -34,12 +34,13 @@ Result<u64> sys_close(Registers*, SyscallArgs args)
 
     Thread* current = Scheduler::current();
 
-    if (!current->fd_table->fds[fd].has_value()) return err(EBADF);
+    Option<FileDescriptor>& descriptor = current->fd_table->fds[fd];
 
-    kinfoln("close: closing file descriptor %d (was referencing inode %zu)", fd,
-            current->fd_table->fds[fd]->inode->inode_number());
+    if (!descriptor.has_value()) return err(EBADF);
 
-    current->fd_table->fds[fd] = {};
+    kinfoln("close: closing file descriptor %d (was referencing inode %zu)", fd, descriptor->inode->inode_number());
+
+    descriptor = {};
 
     return 0;
 }

libc/include/sys/types.h

@@ -9,6 +9,7 @@
 #include <bits/fixed-size-types.h>
 
 typedef int pid_t;
+typedef __i64_t ssize_t;
 typedef __i64_t time_t;
 typedef __u16_t mode_t;
 typedef __u64_t useconds_t;

libc/include/unistd.h

@@ -33,6 +33,9 @@ extern "C"
     /* Closes a file descriptor. */
     int close(int fd);
 
+    /* Reads bytes from a file descriptor. */
+    ssize_t read(int fd, void* buf, size_t size);
+
 #ifdef __cplusplus
 }
 #endif

libc/src/unistd.cpp

@@ -43,4 +43,10 @@ extern "C"
         long rc = syscall(SYS_close, fd);
         __errno_return(rc, int);
     }
+
+    ssize_t read(int fd, void* buf, size_t size)
+    {
+        long rc = syscall(SYS_read, fd, buf, size);
+        __errno_return(rc, ssize_t);
+    }
 }

libluna/include/luna/Syscall.h

@@ -1,7 +1,8 @@
 #pragma once
 
 #define enumerate_syscalls(_e)                                                                                         \
-    _e(exit) _e(console_write) _e(clock_gettime) _e(allocate_memory) _e(deallocate_memory) _e(usleep) _e(open) _e(close)
+    _e(exit) _e(console_write) _e(clock_gettime) _e(allocate_memory) _e(deallocate_memory) _e(usleep) _e(open)         \
+        _e(close) _e(read)
 
 enum Syscalls
 {