fgetpos/fsetpos

inc/stdio.h

@@ -8,7 +8,7 @@
 typedef struct FILE FILE;
 
 typedef struct {
-    int64_t   pos;
+    int64_t   offset;
     mbstate_t mbstate;
 } fpos_t;
 

src/_win/stdio.c

@@ -413,6 +413,41 @@ cum:
     return 0;
 }
 
+int fgetpos(FILE *restrict stream, fpos_t *restrict pos) {
+    LONG pos_hi = 0;
+    DWORD pos_lo = SetFilePointer(stream->handle, 0, &pos_hi, FILE_CURRENT);
+    if(pos_lo == INVALID_SET_FILE_POINTER) {
+        return 1;
+    }
+    int64_t offset = ((int64_t)pos_hi << 32) | (int64_t)pos_lo;
+    pos->offset = offset;
+    pos->mbstate = stream->mbstate;
+    return 0;
+}
+
+int fseek(FILE *stream, long int offset, int whence) {
+    return 0;
+}
+
+int fsetpos(FILE *stream, const fpos_t *pos) {
+    LONG pos_hi = pos->offset >> 32;
+    LONG pos_lo = (LONG)(pos->offset & 0xffffffff);
+    DWORD status = SetFilePointer(stream->handle, pos_lo, &pos_hi, FILE_BEGIN);
+    if(status == INVALID_SET_FILE_POINTER) {
+        return 1;
+    }
+    stream->mbstate = pos->mbstate;
+    return 0;
+}
+
+long int ftell(FILE *stream) {
+    return 0;
+}
+
+void rewind(FILE *stream) {
+
+}
+
 int getchar(void) {
     return fgetc(stdin);
 }

test.bin

@@ -0,0 +1 @@
+abdc

test/test_io.c

@@ -1,16 +1,30 @@
 
 #include <stdio.h>
 #include <stdlib.h>
-
+#include <assert.h>
 int main(void)
 {
-    puts("stdout is printed to console");
-    if (freopen("redir.txt", "w", stdout) == NULL)
-    {
-       perror("freopen() failed");
-       return EXIT_FAILURE;
-    }
-    puts("stdout is redirected to a file"); // this is written to redir.txt
-    fclose(stdout);
-    return EXIT_SUCCESS;
+    // prepare a file holding 4 values of type char
+    enum {SIZE = 4};
+    FILE* fp = fopen("test.bin", "wb");
+    assert(fp);
+    fputc('a', fp);
+    fputc('b', fp);
+    fputc('d', fp);
+    fputc('c', fp);
+    fclose(fp);
+ 
+    // demo using fsetpos to return to the beginning of a file
+    fp = fopen("test.bin", "rb");
+    fpos_t pos;
+    int c = fgetc(fp);
+    printf("First value in the file: %c\n", c);
+    fgetpos(fp, &pos);
+    c = fgetc(fp);
+    printf("Second value in the file: %c\n", c);
+    fsetpos(fp,&pos);
+    c = fgetc(fp);
+    printf("Second value in the file again: %c\n", c);
+    fclose(fp);
+    return 0;
 }