diff --git a/api/pdfrenderer.cpp b/api/pdfrenderer.cpp
index c388bb5af6..e97d9dd6a1 100644
--- a/api/pdfrenderer.cpp
+++ b/api/pdfrenderer.cpp
@@ -670,7 +670,7 @@ bool TessPDFRenderer::BeginDocumentHandler() {
   long int size = ftell(fp);
   fseek(fp, 0, SEEK_SET);
   const std::unique_ptr</*non-const*/ char[]> buffer(new char[size]);
-  if (fread(buffer.get(), 1, size, fp) != size) {
+  if (fread(buffer.get(), 1, size, fp) != static_cast<unsigned long>(size)) {
     fclose(fp);
     return false;
   }
@@ -945,7 +945,7 @@ bool TessPDFRenderer::EndDocumentHandler() {
   if (n >= sizeof(buf)) return false;
   AppendString(buf);
   size_t pages_objsize  = strlen(buf);
-  for (size_t i = 0; i < pages_.size(); i++) {
+  for (size_t i = 0; i < pages_.unsigned_size(); i++) {
     n = snprintf(buf, sizeof(buf),
                  "%ld 0 R ", pages_[i]);
     if (n >= sizeof(buf)) return false;
diff --git a/ccmain/docqual.cpp b/ccmain/docqual.cpp
index 4706fb3b26..ad7f228053 100644
--- a/ccmain/docqual.cpp
+++ b/ccmain/docqual.cpp
@@ -513,7 +513,7 @@ BOOL8 Tesseract::terrible_word_crunch(WERD_RES *word,
 
   if ((word->best_choice->unichar_string().length () == 0) ||
     (strspn (word->best_choice->unichar_string().string(), " ") ==
-    word->best_choice->unichar_string().length ()))
+    word->best_choice->unichar_string().unsigned_size ()))
     crunch_mode = 1;
   else {
     adjusted_len = word->reject_map.length ();
diff --git a/ccutil/genericvector.h b/ccutil/genericvector.h
index 39a245d665..c0a5d9585d 100644
--- a/ccutil/genericvector.h
+++ b/ccutil/genericvector.h
@@ -72,6 +72,12 @@ class GenericVector {
   int size() const {
     return size_used_;
   }
+  // Workaround to avoid g++ -Wsign-compare warnings.
+  unsigned int unsigned_size() const {
+    static_assert(sizeof(size_used_) <= sizeof(unsigned int), "");
+    assert(0 <= size_used_);
+    return static_cast<unsigned int>(size_used_);
+  }
   int size_reserved() const {
     return size_reserved_;
   }
@@ -880,7 +886,7 @@ bool GenericVector<T>::write(
     }
     delete cb;
   } else {
-    if (fwrite(data_, sizeof(T), size_used_, f) != size_used_) return false;
+    if (fwrite(data_, sizeof(T), size_used_, f) != unsigned_size()) return false;
   }
   return true;
 }
@@ -912,7 +918,7 @@ bool GenericVector<T>::read(
 template <typename T>
 bool GenericVector<T>::Serialize(FILE* fp) const {
   if (fwrite(&size_used_, sizeof(size_used_), 1, fp) != 1) return false;
-  if (fwrite(data_, sizeof(*data_), size_used_, fp) != size_used_) return false;
+  if (fwrite(data_, sizeof(*data_), size_used_, fp) != unsigned_size()) return false;
   return true;
 }
 template <typename T>
@@ -933,7 +939,7 @@ bool GenericVector<T>::DeSerialize(bool swap, FILE* fp) {
   if (swap) Reverse32(&reserved);
   reserve(reserved);
   size_used_ = reserved;
-  if (fread(data_, sizeof(T), size_used_, fp) != size_used_) return false;
+  if (fread(data_, sizeof(T), size_used_, fp) != unsigned_size()) return false;
   if (swap) {
     for (int i = 0; i < size_used_; ++i)
       ReverseN(&data_[i], sizeof(data_[i]));
@@ -982,7 +988,7 @@ bool GenericVector<T>::SerializeClasses(tesseract::TFile* fp) const {
 // If swap is true, assumes a big/little-endian swap is needed.
 template <typename T>
 bool GenericVector<T>::DeSerializeClasses(bool swap, FILE* fp) {
-  uinT32 reserved;
+  inT32 reserved;
   if (fread(&reserved, sizeof(reserved), 1, fp) != 1) return false;
   if (swap) Reverse32(&reserved);
   T empty;
@@ -994,7 +1000,7 @@ bool GenericVector<T>::DeSerializeClasses(bool swap, FILE* fp) {
 }
 template <typename T>
 bool GenericVector<T>::DeSerializeClasses(tesseract::TFile* fp) {
-  uinT32 reserved;
+  inT32 reserved;
   if (fp->FReadEndian(&reserved, sizeof(reserved), 1) != 1) return false;
   T empty;
   init_to_size(reserved, empty);
@@ -1005,7 +1011,7 @@ bool GenericVector<T>::DeSerializeClasses(tesseract::TFile* fp) {
 }
 template <typename T>
 bool GenericVector<T>::SkipDeSerializeClasses(tesseract::TFile* fp) {
-  uinT32 reserved;
+  inT32 reserved;
   if (fp->FReadEndian(&reserved, sizeof(reserved), 1) != 1) return false;
   for (int i = 0; i < reserved; ++i) {
     if (!T::SkipDeSerialize(fp)) return false;
diff --git a/ccutil/strngs.h b/ccutil/strngs.h
index 5ca9c4fc8c..17169e21ae 100644
--- a/ccutil/strngs.h
+++ b/ccutil/strngs.h
@@ -20,6 +20,7 @@
 #ifndef           STRNGS_H
 #define           STRNGS_H
 
+#include          <assert.h>
 #include          <stdio.h>
 #include          <string.h>
 #include          "platform.h"
@@ -66,6 +67,12 @@ class TESS_API STRING
     BOOL8 contains(const char c) const;
     inT32 length() const;
     inT32 size() const { return length(); }
+    // Workaround to avoid g++ -Wsign-compare warnings.
+    uinT32 unsigned_size() const {
+      const inT32 len = length();
+      assert(0 <= len);
+      return static_cast<uinT32>(len);
+    }
     const char *string() const;
     const char *c_str() const;
 
diff --git a/classify/blobclass.cpp b/classify/blobclass.cpp
index 314a71f726..2edb42b84f 100644
--- a/classify/blobclass.cpp
+++ b/classify/blobclass.cpp
@@ -97,7 +97,7 @@ void Classify::LearnBlob(const STRING& fontname, TBLOB* blob,
 bool Classify::WriteTRFile(const STRING& filename) {
   STRING tr_filename = filename + ".tr";
   FILE* fp = Efopen(tr_filename.string(), "wb");
-  int len = tr_file_data_.length();
+  size_t len = tr_file_data_.length();
   bool result =
       fwrite(&tr_file_data_[0], sizeof(tr_file_data_[0]), len, fp) == len;
   fclose(fp);
diff --git a/classify/featdefs.cpp b/classify/featdefs.cpp
index dd31f91d86..52b22edc7a 100644
--- a/classify/featdefs.cpp
+++ b/classify/featdefs.cpp
@@ -139,10 +139,8 @@ void InitFeatureDefs(FEATURE_DEFS_STRUCT *featuredefs) {
  * @note History: Wed May 23 13:52:19 1990, DSJ, Created.
  */
 void FreeCharDescription(CHAR_DESC CharDesc) {
-  int i;
-
   if (CharDesc) {
-    for (i = 0; i < CharDesc->NumFeatureSets; i++)
+    for (size_t i = 0; i < CharDesc->NumFeatureSets; i++)
       FreeFeatureSet (CharDesc->FeatureSets[i]);
     Efree(CharDesc);
   }
@@ -163,12 +161,10 @@ void FreeCharDescription(CHAR_DESC CharDesc) {
  */
 CHAR_DESC NewCharDescription(const FEATURE_DEFS_STRUCT &FeatureDefs) {
   CHAR_DESC CharDesc;
-  int i;
-
   CharDesc = (CHAR_DESC) Emalloc (sizeof (CHAR_DESC_STRUCT));
   CharDesc->NumFeatureSets = FeatureDefs.NumFeatureTypes;
 
-  for (i = 0; i < CharDesc->NumFeatureSets; i++)
+  for (size_t i = 0; i < CharDesc->NumFeatureSets; i++)
     CharDesc->FeatureSets[i] = NULL;
 
   return (CharDesc);
@@ -196,16 +192,15 @@ CHAR_DESC NewCharDescription(const FEATURE_DEFS_STRUCT &FeatureDefs) {
  */
 void WriteCharDescription(const FEATURE_DEFS_STRUCT& FeatureDefs,
                           CHAR_DESC CharDesc, STRING* str) {
-  int Type;
   int NumSetsToWrite = 0;
 
-  for (Type = 0; Type < CharDesc->NumFeatureSets; Type++)
+  for (size_t Type = 0; Type < CharDesc->NumFeatureSets; Type++)
     if (CharDesc->FeatureSets[Type])
       NumSetsToWrite++;
 
   str->add_str_int(" ", NumSetsToWrite);
   *str += "\n";
-  for (Type = 0; Type < CharDesc->NumFeatureSets; Type++) {
+  for (size_t Type = 0; Type < CharDesc->NumFeatureSets; Type++) {
     if (CharDesc->FeatureSets[Type]) {
       *str += FeatureDefs.FeatureDesc[Type]->ShortName;
       *str += " ";
@@ -220,7 +215,7 @@ bool ValidCharDescription(const FEATURE_DEFS_STRUCT &FeatureDefs,
                           CHAR_DESC CharDesc) {
   bool anything_written = false;
   bool well_formed = true;
-  for (int Type = 0; Type < CharDesc->NumFeatureSets; Type++) {
+  for (size_t Type = 0; Type < CharDesc->NumFeatureSets; Type++) {
     if (CharDesc->FeatureSets[Type]) {
       for (int i = 0; i < CharDesc->FeatureSets[Type]->NumFeatures; i++) {
         FEATURE feat = CharDesc->FeatureSets[Type]->Features[i];
diff --git a/lstm/networkio.cpp b/lstm/networkio.cpp
index 92edff2ab6..dfa46aa4cc 100644
--- a/lstm/networkio.cpp
+++ b/lstm/networkio.cpp
@@ -184,7 +184,7 @@ void NetworkIO::FromPixes(const StaticShape& shape,
   stride_map_.SetStride(h_w_pairs);
   ResizeToMap(int_mode(), stride_map_, shape.depth());
   // Iterate over the images again to copy the data.
-  for (int b = 0; b < pixes.size(); ++b) {
+  for (size_t b = 0; b < pixes.size(); ++b) {
     Pix* pix = const_cast<Pix*>(pixes[b]);
     float black = 0.0f, white = 255.0f;
     if (shape.depth() != 3) ComputeBlackWhite(pix, &black, &white);
diff --git a/lstm/stridemap.cpp b/lstm/stridemap.cpp
index b3e70bca9e..26a2cf6669 100644
--- a/lstm/stridemap.cpp
+++ b/lstm/stridemap.cpp
@@ -43,7 +43,8 @@ bool StrideMap::Index::IsLast(FlexDimensions dimension) const {
 int StrideMap::Index::MaxIndexOfDim(FlexDimensions dim) const {
   int max_index = stride_map_->shape_[dim] - 1;
   if (dim == FD_BATCH) return max_index;
-  int batch = indices_[FD_BATCH];
+  assert(0 <= indices_[FD_BATCH]);
+  const size_t batch = indices_[FD_BATCH];
   if (dim == FD_HEIGHT) {
     if (batch >= stride_map_->heights_.size() ||
         stride_map_->heights_[batch] > max_index)
diff --git a/training/commontraining.cpp b/training/commontraining.cpp
index 0d32daf28b..af483ff917 100644
--- a/training/commontraining.cpp
+++ b/training/commontraining.cpp
@@ -368,7 +368,6 @@ void ReadTrainingSamples(const FEATURE_DEFS_STRUCT& feature_defs,
   LABELEDLIST char_sample;
   FEATURE_SET feature_samples;
   CHAR_DESC char_desc;
-  int   i;
   int feature_type = ShortNameToFeatureType(feature_defs, feature_name);
   // Zero out the font_sample_count for all the classes.
   LIST it = *training_samples;
@@ -404,7 +403,7 @@ void ReadTrainingSamples(const FEATURE_DEFS_STRUCT& feature_defs,
     } else {
       FreeFeatureSet(feature_samples);
     }
-    for (i = 0; i < char_desc->NumFeatureSets; i++) {
+    for (size_t i = 0; i < char_desc->NumFeatureSets; i++) {
       if (feature_type != i)
         FreeFeatureSet(char_desc->FeatureSets[i]);
     }