Lecture 112 - Implementing FSTAT in user land

Author: nibblebits

PeachOS64Bit/programs/blank/blank.c

@@ -8,11 +8,11 @@ int main(int argc, char** argv)
     int fd = fopen("@:/blank.elf", "r");
     if (fd > 0)
     {
-
-        char buf[512] = {0};
+        struct file_stat file_stat = {0};
+       
         printf("File blank.elf opened\n");
-        fread(buf, 1, sizeof(buf), fd);
-        
+        fstat(fd, &file_stat);
+        printf("File size: %i\n", file_stat.filesize);
         fclose(fd);
     }
     else

PeachOS64Bit/programs/blank/blank.elf

@@ -21,4 +21,9 @@ int fread(void* buffer, size_t size, size_t count, long fd)
 int fseek(int fd, int offset, int whence)
 {
     return (int) peachos_fseek(fd, offset, whence);
+}
+
+int fstat(int fd, struct file_stat* file_stat_out)
+{
+    return (int) peachos_fstat(fd, file_stat_out);
 }

PeachOS64Bit/programs/blank/build/blank.o

@@ -4,6 +4,14 @@
 #include <stdint.h>
 #include <stddef.h>
 
+typedef unsigned int FILE_STAT_FLAGS;
+struct file_stat
+{
+    FILE_STAT_FLAGS flags;
+    uint32_t filesize;
+};
+
+int fstat(int fd, struct file_stat* file_stat_out);
 int fopen(const char* filename, const char* mode);
 void fclose(int fd);
 int fread(void* buffer, size_t size, size_t count, long fd);

PeachOS64Bit/programs/shell/shell.elf

@@ -15,6 +15,7 @@ global peachos_fopen:function
 global peachos_fclose:function
 global peachos_fread:function
 global peachos_fseek:function
+global peachos_fstat:function
 
 ; void print(const char* filename)
 print:
@@ -121,4 +122,13 @@ peachos_fseek:
     push qword rdi ; fd
     int 0x80       ; invokes the kernel
     add rsp, 24    ; restores the stack
-    ret            ; return
+    ret            ; return
+
+; long peachos_fstat(long fd, struct file_stat* file_stat_out)
+peachos_fstat:
+    mov rax, 14    ; Command 14 fstat
+    push qword rsi  ; file_stat_out
+    push qword rdi  ; fd
+    int 0x80        ; call kernel
+    add rsp, 16     ; restore stack
+    ret

PeachOS64Bit/programs/stdlib/build/file.o

@@ -16,6 +16,8 @@ struct process_arguments
     char** argv;
 };
 
+// Forward declare file stat.
+struct file_stat;
 
 void print(const char* filename);
 int peachos_getkey();
@@ -36,4 +38,5 @@ int peachos_fopen(const char* filename, const char* mode);
 void peachos_fclose(size_t fd);
 long peachos_fread(void* buffer, size_t size, size_t count, long fd);
 long peachos_fseek(long fd, long offset, long whence);
+long peachos_fstat(long fd, struct file_stat* file_stat_out);
 #endif