io: allow slurping files with a given size

diff --git a/io.c b/io.c
index d2b3197..3c0a8eb 100644 (file)
--- a/io.c
+++ b/io.c
@@ -67,6 +67,9 @@ PUBLIC_FN ssize_t write_nointr(int fd, void *buf, size_t count, int *last_res) {
 PUBLIC_CONST JH_SLURP_NO_STAT 1
 PUBLIC_CONST JH_SLURP_REALLOC 2 /* realloc result block if it saves RAM */
 PUBLIC_CONST JH_SLURP_8BYTE_PAD 4 /* pad buffer with eight nullbytes, not one */
+PUBLIC_CONST JH_SLURP_FIXSIZE 8 /* len_out is actually input and specifies
+HEADER                             the expected length. if there is more data,
+HEADER                             that might be ignored silently. */
 PUBLIC_FN void *slurp_fd(int fd, size_t *len_out, int flags) {
   int errno_;
   
@@ -75,6 +78,13 @@ PUBLIC_FN void *slurp_fd(int fd, size_t *len_out, int flags) {
   size_t size_guess = 1023;
   bool trusted_guess = false; /* can we rely on the guess? */
   
+  if (flags&JH_SLURP_FIXSIZE) {
+    size_guess = *len_out;
+    trusted_guess = true;
+    // if we want a fixed size, of course we don't want to stat
+    flags |= JH_SLURP_NO_STAT;
+  }
+  
   int padlen = (flags&JH_SLURP_8BYTE_PAD)?8:1;
   
   // If we can determine the exact size, we don't have to guess. So try
@@ -113,6 +123,12 @@ PUBLIC_FN void *slurp_fd(int fd, size_t *len_out, int flags) {
         char *buf_ = realloc(buf, done+padlen);
         if (buf_) buf = buf_;
       }
+      if (done != size_guess && (flags&JH_SLURP_FIXSIZE)) {
+        free(buf);
+        // not the most correct error message ever, but whatever.
+        errno = EFBIG;
+        return NULL;
+      }
       if (len_out) *len_out = done;
       return buf;
     }