Add mtar_seek_data()

This allows users to seek within a file without accidentally
skipping over the file's boundaries.

2 files changed, 52 insertions, 6 deletions

diff --git a/src/microtar.c b/src/microtar.c
index 5ae7eeb..d304e6d 100644
--- a/src/microtar.c
+++ b/src/microtar.c
@@ -244,9 +244,14 @@ static int ensure_header(mtar_t* tar)
     return MTAR_ESUCCESS;
 }
 
+static unsigned data_beg_pos(const mtar_t* tar)
+{
+    return tar->header_pos + HEADER_LEN;
+}
+
 static unsigned data_end_pos(const mtar_t* tar)
 {
-    return tar->header_pos + HEADER_LEN + tar->header.size;
+    return data_beg_pos(tar) + tar->header.size;
 }
 
 const char* mtar_strerror(int err)
@@ -373,6 +378,45 @@ int mtar_read_data(mtar_t* tar, void* ptr, unsigned size)
     return (int)size;
 }
 
+int mtar_seek_data(mtar_t* tar, int offset, int whence)
+{
+    if(!(tar->state & S_HEADER_VALID))
+        return MTAR_EAPI;
+
+    unsigned data_beg = data_beg_pos(tar);
+    unsigned data_end = data_end_pos(tar);
+    unsigned newpos;
+
+    switch(whence) {
+    case SEEK_SET:
+        if(offset < 0)
+            return MTAR_ESEEKRANGE;
+
+        newpos = data_beg + offset;
+        break;
+
+    case SEEK_CUR:
+        if((offset > 0 && (unsigned) offset > data_end - tar->pos) ||
+           (offset < 0 && (unsigned)-offset > tar->pos - data_beg))
+            return MTAR_ESEEKRANGE;
+
+        newpos = tar->pos + offset;
+        break;
+
+    case SEEK_END:
+        if(offset > 0)
+            return MTAR_ESEEKRANGE;
+
+        newpos = data_end + offset;
+        break;
+
+    default:
+        return MTAR_EAPI;
+    }
+
+    return tseek(tar, newpos);
+}
+
 int mtar_eof_data(mtar_t* tar)
 {
     /* API usage error, but just claim EOF. */
diff --git a/src/microtar.h b/src/microtar.h
index b3ff05f..b6de153 100644
--- a/src/microtar.h
+++ b/src/microtar.h
@@ -38,11 +38,12 @@ enum {
     MTAR_EREADFAIL    = -3,
     MTAR_EWRITEFAIL   = -4,
     MTAR_ESEEKFAIL    = -5,
-    MTAR_EBADCHKSUM   = -6,
-    MTAR_ENULLRECORD  = -7,
-    MTAR_ENOTFOUND    = -8,
-    MTAR_EOVERFLOW    = -9,
-    MTAR_EAPI         = -10,
+    MTAR_ESEEKRANGE   = -6,
+    MTAR_EBADCHKSUM   = -7,
+    MTAR_ENULLRECORD  = -9,
+    MTAR_ENOTFOUND    = -10,
+    MTAR_EOVERFLOW    = -11,
+    MTAR_EAPI         = -12,
     MTAR_ELAST        = MTAR_EAPI,
 };
 
@@ -106,6 +107,7 @@ int mtar_next(mtar_t* tar);
 int mtar_foreach(mtar_t* tar, mtar_foreach_cb cb, void* arg);
 int mtar_find(mtar_t* tar, const char* name);
 int mtar_read_data(mtar_t* tar, void* ptr, unsigned size);
+int mtar_seek_data(mtar_t* tar, int offset, int whence);
 int mtar_eof_data(mtar_t* tar);
 
 #ifdef __cplusplus