libc: Implement fseek(), ftell() and rewind()

All three use the new syscall seek() (with its lseek() wrapper in unistd.h)!!

apps/src/init.c

@@ -85,6 +85,23 @@ int main()
         return 1;
     }
 
+    if (fseek(config, 0, SEEK_END) < 0)
+    {
+        perror("fseek");
+        return 1;
+    }
+
+    long offset = ftell(config);
+    if (offset < 0)
+    {
+        perror("ftell");
+        return 1;
+    }
+
+    printf("%s is %ld bytes long\n", filename, offset);
+
+    rewind(config);
+
     char buf[4096];
 
     size_t nread = fread(buf, sizeof(buf), 1, config);

kernel/src/fs/FileDescriptor.cpp

@@ -36,7 +36,8 @@ ssize_t Descriptor::write(size_t size, const char* buffer)
 
 int Descriptor::seek(long offset)
 {
-    if (m_node->type != VFS_DEVICE && offset > m_node->length)
+    if (m_node->type != VFS_DEVICE && (uint64_t)offset > m_node->length)
         return -EINVAL; // FIXME: Support seeking beyond the current file's length.
     m_offset = (uint64_t)offset;
+    return 0;
 }

kernel/src/sys/Syscall.cpp

@@ -20,7 +20,7 @@ void Syscall::entry(Context* context)
     case SYS_open: sys_open(context, (const char*)context->rdi, (int)context->rsi); break;
     case SYS_read: sys_read(context, (int)context->rdi, context->rsi, (char*)context->rdx); break;
     case SYS_close: sys_close(context, (int)context->rdi); break;
-    case SYS_seek: sys_seek(context, (int)context->rdi, (long)context->rsi, (int)context->rdx);
+    case SYS_seek: sys_seek(context, (int)context->rdi, (long)context->rsi, (int)context->rdx); break;
     default: context->rax = -ENOSYS; break;
     }
 }

libs/libc/include/stdio.h

@@ -38,8 +38,14 @@ extern "C"
     /* Reads nmemb items of size size from the file stream into buf. */
     size_t fread(void* buf, size_t size, size_t nmemb, FILE* stream);
 
-    int fseek(FILE*, long, int); // Not implemented.
-    long ftell(FILE*);           // Not implemented.
+    /* Moves stream's read/write offset by offset, depending on whence. */
+    int fseek(FILE* stream, long offset, int whence);
+
+    /* Returns the current offset for stream. */
+    long ftell(FILE* stream);
+
+    /* Rewinds stream's offset to start of file. */
+    void rewind(FILE* stream);
 
     /* Writes nmemb items of size size from buf into the file stream. */
     size_t fwrite(const void* buf, size_t size, size_t nmemb, FILE* stream);

libs/libc/include/sys/mman.h

@@ -2,8 +2,7 @@
 #define _SYS_MMAN_H
 
 #include <stddef.h>
-
-typedef unsigned long off_t;
+#include <sys/types.h> // for off_t
 
 /* Address returned by mmap when it fails. */
 #define MAP_FAILED (void*)-1

libs/libc/src/file.cpp

@@ -67,14 +67,24 @@ extern "C"
         stream->f_err = stream->f_eof = 0;
     }
 
-    int fseek(FILE*, long, int)
+    int fseek(FILE* stream, long offset, int whence)
     {
-        NOT_IMPLEMENTED("fseek");
+        long result = lseek(stream->f_fd, offset, whence);
+        if (result < 0) { return -1; }
+        return 0;
     }
 
-    long ftell(FILE*)
+    long ftell(FILE* stream)
     {
-        NOT_IMPLEMENTED("ftell");
+        return lseek(stream->f_fd, 0,
+                     SEEK_CUR); // FIXME: Store the last seeked position in the file struct to avoid redundant syscalls
+                                // maybe? We'd have to update this value in fread() and fwrite() as well...
+    }
+
+    void rewind(FILE* stream)
+    {
+        lseek(stream->f_fd, 0, SEEK_SET);
+        clearerr(stream);
     }
 
     size_t fwrite(const void* buf, size_t size, size_t nmemb, FILE* stream)