diff --git a/.cirrus.yml b/.cirrus.yml
index 66e2c436..06edba50 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -84,6 +84,7 @@ ubuntu_task:
 
   environment:
     CC: clang
+    CFLAGS: -g -O2 -Werror -Wall -Wformat -Wformat=2
     LC_ALL: C
     CIRRUS_CLONE_DEPTH: 1
     HTSDIR: ./htslib
diff --git a/Makefile b/Makefile
index 122762ad..5235c895 100644
--- a/Makefile
+++ b/Makefile
@@ -251,15 +251,15 @@ vcfconvert.o: vcfconvert.c $(htslib_faidx_h) $(htslib_vcf_h) $(htslib_bgzf_h) $(
 vcffilter.o: vcffilter.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(bcftools_h) $(filter_h) rbuf.h regidx.h
 vcfgtcheck.o: vcfgtcheck.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_kbitset_h) $(htslib_hts_os_h) $(htslib_bgzf_h) $(bcftools_h) extsort.h filter.h
 vcfindex.o: vcfindex.c $(htslib_vcf_h) $(htslib_tbx_h) $(htslib_kstring_h) $(htslib_bgzf_h) $(bcftools_h)
-vcfisec.o: vcfisec.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_hts_os_h) $(bcftools_h) $(filter_h)
+vcfisec.o: vcfisec.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_hts_os_h) $(htslib_hts_defs_h) $(bcftools_h) $(filter_h)
 vcfmerge.o: vcfmerge.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_faidx_h) $(htslib_kbitset_h) $(htslib_hts_endian_h) $(bcftools_h) regidx.h vcmp.h $(htslib_khash_h) $(htslib_kbitset_h)
 vcfnorm.o: vcfnorm.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_faidx_h) $(htslib_khash_str2int_h) $(bcftools_h) rbuf.h abuf.h gff.h regidx.h
 vcfquery.o: vcfquery.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_khash_str2int_h) $(htslib_vcfutils_h) $(bcftools_h) $(filter_h) $(convert_h) $(smpl_ilist_h)
 vcfroh.o: vcfroh.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_kstring_h) $(htslib_kseq_h) $(htslib_bgzf_h) $(bcftools_h) HMM.h $(smpl_ilist_h) $(filter_h)
-vcfcnv.o: vcfcnv.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_kstring_h) $(htslib_kfunc_h) $(htslib_khash_str2int_h) $(bcftools_h) HMM.h rbuf.h
+vcfcnv.o: vcfcnv.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_kstring_h) $(htslib_kfunc_h) $(htslib_khash_str2int_h) $(htslib_hts_defs_h) $(bcftools_h) HMM.h rbuf.h
 vcfhead.o: vcfhead.c $(htslib_kstring_h) $(htslib_vcf_h) $(bcftools_h)
-vcfsom.o: vcfsom.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_hts_os_h) $(bcftools_h)
-vcfsort.o: vcfsort.c $(htslib_vcf_h) $(htslib_kstring_h) $(htslib_hts_os_h) $(htslib_bgzf_h) kheap.h $(bcftools_h)
+vcfsom.o: vcfsom.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_hts_os_h) $(htslib_hts_defs_h) $(bcftools_h)
+vcfsort.o: vcfsort.c $(htslib_vcf_h) $(htslib_kstring_h) $(htslib_hts_os_h) $(htslib_hts_defs_h) $(htslib_bgzf_h) kheap.h $(bcftools_h)
 vcfstats.o: vcfstats.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_faidx_h) $(bcftools_h) $(filter_h) bin.h dist.h
 vcfview.o: vcfview.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(bcftools_h) $(filter_h) $(htslib_khash_str2int_h) $(htslib_kbitset_h)
 reheader.o: reheader.c $(htslib_vcf_h) $(htslib_bgzf_h) $(htslib_tbx_h) $(htslib_kseq_h) $(htslib_thread_pool_h) $(htslib_faidx_h) $(htslib_khash_str2int_h) $(bcftools_h) $(khash_str2str_h)
@@ -276,7 +276,7 @@ mcall.o: mcall.c $(htslib_kfunc_h) $(htslib_khash_str2int_h) $(call_h) $(prob1_h
 prob1.o: prob1.c $(prob1_h)
 vcmp.o: vcmp.c $(htslib_hts_h) $(htslib_vcf_h) vcmp.h
 ploidy.o: ploidy.c $(htslib_khash_str2int_h) $(htslib_kseq_h) $(htslib_hts_h) $(bcftools_h) $(ploidy_h)
-polysomy.o: polysomy.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(bcftools_h) peakfit.h
+polysomy.o: polysomy.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_hts_defs_h) $(bcftools_h) peakfit.h
 peakfit.o: peakfit.c peakfit.h $(htslib_hts_h) $(htslib_kstring_h)
 bin.o: bin.c $(bcftools_h) bin.h
 dist.o: dist.c dist.h
@@ -326,7 +326,7 @@ test/test-rbuf.o: test/test-rbuf.c rbuf.h
 test/test-rbuf: test/test-rbuf.o
 	$(CC) $(LDFLAGS) -o $@ $^ $(ALL_LIBS)
 
-test/test-regidx.o: test/test-regidx.c $(htslib_kstring_h) $(htslib_hts_os_h) regidx.h
+test/test-regidx.o: test/test-regidx.c $(htslib_kstring_h) $(htslib_hts_os_h) $(htslib_hts_defs_h) regidx.h
 
 test/test-regidx: test/test-regidx.o regidx.o | $(HTSLIB)
 	$(CC) $(ALL_LDFLAGS) -o $@ $^ $(HTSLIB_LIB) -lpthread $(ALL_LIBS)
diff --git a/plugins/fill-tags.c b/plugins/fill-tags.c
index b9f9b690..0fcf8a47 100644
--- a/plugins/fill-tags.c
+++ b/plugins/fill-tags.c
@@ -342,12 +342,17 @@ int ftf_filter_expr(args_t *args, bcf1_t *rec, pop_t *pop, ftf_t *ftf)
         error("Error occurred while updating %s at %s:%"PRId64"\n", args->str.s,bcf_seqname(args->in_hdr,rec),(int64_t) rec->pos+1);
     return 0;
 }
-void hdr_append(args_t *args, char *fmt)
-{
-    int i;
-    for (i=0; i<args->npop; i++)
-        bcf_hdr_printf(args->out_hdr, fmt, args->pop[i].suffix,*args->pop[i].name ? " in " : "",args->pop[i].name);
-}
+
+// This is implemented as a macro so the compiler can properly validate the
+// printf format string.
+#define hdr_append(args, fmt) \
+do { \
+    int i; \
+    for (i=0; i<args->npop; i++) \
+        bcf_hdr_printf(args->out_hdr, fmt, args->pop[i].suffix,*args->pop[i].name ? " in " : "",args->pop[i].name); \
+} while (0)
+
+
 int parse_func_pop(args_t *args, pop_t *pop, char *tag_expr, char *expr)
 {
     pop->nftf++;
diff --git a/plugins/scatter.c b/plugins/scatter.c
index 55785f88..35135806 100644
--- a/plugins/scatter.c
+++ b/plugins/scatter.c
@@ -28,6 +28,7 @@
 #include <unistd.h>
 #include <ctype.h>
 #include <htslib/synced_bcf_reader.h>
+#include <htslib/hts_defs.h>
 #include "bcftools.h"
 #include "htslib/khash_str2int.h"
 #include "regidx.h"
@@ -108,7 +109,7 @@ static const char *usage_text(void)
         "\n";
 }
 
-void mkdir_p(const char *fmt, ...);
+void mkdir_p(const char *fmt, ...) HTS_FORMAT(HTS_PRINTF_FMT, 1, 2);
 
 // most of this code was inspired by Petr Danecek's code in regidx.c
 #define MAX_COOR_0 REGIDX_MAX   // CSI and hts_itr_query limit, 0-based
diff --git a/plugins/split-vep.c b/plugins/split-vep.c
index f7bae848..a59f6bb1 100644
--- a/plugins/split-vep.c
+++ b/plugins/split-vep.c
@@ -711,15 +711,13 @@ static void parse_column_str(args_t *args)
         ann->idx = j = column[i];
         ann->field = strdup(args->field[j]);
         ann->tag = strdup(args->field[j]);
-        args->kstr.l = 0;
         const char *type = "String";
         if ( ann->type==BCF_HT_REAL ) type = "Float";
         else if ( ann->type==BCF_HT_INT ) type = "Integer";
         else if ( ann->type==BCF_HT_FLAG ) type = "Flag";
         else if ( ann->type==BCF_HT_STR ) type = "String";
         else if ( ann->type==-1 ) type = get_column_type(args, args->field[j], &ann->type);
-        ksprintf(&args->kstr,"##INFO=<ID=%%s,Number=.,Type=%s,Description=\"The %%s field from INFO/%%s\">",type);
-        bcf_hdr_printf(args->hdr_out, args->kstr.s, ann->tag,ann->field,args->vep_tag);
+        bcf_hdr_printf(args->hdr_out, "##INFO=<ID=%s,Number=.,Type=%s,Description=\"The %s field from INFO/%s\">", ann->tag,type,ann->field,args->vep_tag);
         if ( str.l ) kputc(',',&str);
         kputs(ann->tag,&str);
     }
diff --git a/plugins/split.c b/plugins/split.c
index 5e25497a..bac2ad7c 100644
--- a/plugins/split.c
+++ b/plugins/split.c
@@ -143,7 +143,7 @@ static const char *usage_text(void)
         "\n";
 }
 
-void mkdir_p(const char *fmt, ...);
+void mkdir_p(const char *fmt, ...) HTS_FORMAT(HTS_PRINTF_FMT, 1, 2);
 
 static char *create_unique_file_name(args_t *args, const char *template)
 {
diff --git a/polysomy.c b/polysomy.c
index 1a99f98a..809ee113 100644
--- a/polysomy.c
+++ b/polysomy.c
@@ -34,6 +34,7 @@
 #include <gsl/gsl_multifit_nlin.h>
 #include <htslib/vcf.h>
 #include <htslib/synced_bcf_reader.h>
+#include <htslib/hts_defs.h>
 #include "bcftools.h"
 #include "peakfit.h"
 
@@ -62,7 +63,7 @@ typedef struct
 }
 args_t;
 
-FILE *open_file(char **fname, const char *mode, const char *fmt, ...);
+FILE *open_file(char **fname, const char *mode, const char *fmt, ...) HTS_FORMAT(HTS_PRINTF_FMT, 3, 4);
 
 static void init_dist(args_t *args, dist_t *dist, int verbose)
 {
diff --git a/test/test-regidx.c b/test/test-regidx.c
index a12aa129..eed36ab7 100644
--- a/test/test-regidx.c
+++ b/test/test-regidx.c
@@ -33,12 +33,14 @@
 #include <getopt.h>
 #include <htslib/kstring.h>
 #include <htslib/hts_os.h>
+#include <htslib/hts_defs.h>
 #include <time.h>
 #include "regidx.h"
 
 static int verbose = 0;
 
-void debug(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2)
+debug(const char *format, ...)
 {
     if ( verbose<2 ) return;
     va_list ap;
@@ -46,7 +48,8 @@ void debug(const char *format, ...)
     vfprintf(stderr, format, ap);
     va_end(ap);
 }
-void info(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2)
+info(const char *format, ...)
 {
     if ( verbose<1 ) return;
     va_list ap;
@@ -54,7 +57,8 @@ void info(const char *format, ...)
     vfprintf(stderr, format, ap);
     va_end(ap);
 }
-void error(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) HTS_NORETURN
+error(const char *format, ...)
 {
     va_list ap;
     va_start(ap, format);
diff --git a/vcfcnv.c b/vcfcnv.c
index ecad4d62..e970b043 100644
--- a/vcfcnv.c
+++ b/vcfcnv.c
@@ -41,6 +41,7 @@
 #include <htslib/kstring.h>
 #include <htslib/kfunc.h>
 #include <htslib/khash_str2int.h>
+#include <htslib/hts_defs.h>
 #include "bcftools.h"
 #include "HMM.h"
 #include "rbuf.h"
@@ -105,7 +106,7 @@ typedef struct _args_t
 }
 args_t;
 
-FILE *open_file(char **fname, const char *mode, const char *fmt, ...);
+FILE *open_file(char **fname, const char *mode, const char *fmt, ...) HTS_FORMAT(HTS_PRINTF_FMT, 3, 4);
 
 static inline void hmm2cn_state(int nstates, int i, int *a, int *b)
 {
diff --git a/vcfisec.c b/vcfisec.c
index 98ed25b6..24a45685 100644
--- a/vcfisec.c
+++ b/vcfisec.c
@@ -34,6 +34,7 @@ THE SOFTWARE.  */
 #include <htslib/synced_bcf_reader.h>
 #include <htslib/vcfutils.h>
 #include <htslib/hts_os.h>
+#include <htslib/hts_defs.h>
 #include "bcftools.h"
 #include "filter.h"
 
@@ -69,19 +70,21 @@ args_t;
  *  mkdir_p() - create new directory for a file $fname
  *  @fname:   the file name to create the directory for, the part after last "/" is ignored
  */
-void mkdir_p(const char *fmt, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2)
+mkdir_p(const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
     int n = vsnprintf(NULL, 0, fmt, ap) + 2;
     va_end(ap);
 
-    char *path = (char*)malloc(n);
+    char *tmp = (char*)malloc(n);
+    if (!tmp) error("Couldn't allocate space for path: %s\n", strerror(errno));
     va_start(ap, fmt);
-    vsnprintf(path, n, fmt, ap);
+    vsnprintf(tmp, n, fmt, ap);
     va_end(ap);
 
-    char *tmp = strdup(path), *p = tmp+1;
+    char *p = tmp+1;
     while (*p)
     {
         while (*p && *p!='/') p++;
@@ -89,12 +92,11 @@ void mkdir_p(const char *fmt, ...)
         char ctmp = *p;
         *p = 0;
         int ret = mkdir(tmp,S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
-        if ( ret!=0 && errno!=EEXIST ) error("Error creating directory %s: %s\n", path,strerror(errno));
+        if ( ret!=0 && errno!=EEXIST ) error("Error creating directory %s: %s\n", tmp,strerror(errno));
         *p = ctmp;
         while ( *p && *p=='/' ) p++;
     }
     free(tmp);
-    free(path);
 }
 
 /**
@@ -105,7 +107,8 @@ void mkdir_p(const char *fmt, ...)
  *
  *  Returns open file descriptor or NULL if mode is NULL.
  */
-FILE *open_file(char **fname, const char *mode, const char *fmt, ...)
+FILE * HTS_FORMAT(HTS_PRINTF_FMT, 3, 4)
+open_file(char **fname, const char *mode, const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
@@ -117,7 +120,7 @@ FILE *open_file(char **fname, const char *mode, const char *fmt, ...)
     vsnprintf(str, n, fmt, ap);
     va_end(ap);
 
-    mkdir_p(str);
+    mkdir_p("%s", str);
     if ( !mode )
     {
         if ( !fname ) error("Uh: expected fname or mode\n");
diff --git a/vcfsom.c b/vcfsom.c
index db01d24f..f7a5dbe6 100644
--- a/vcfsom.c
+++ b/vcfsom.c
@@ -37,6 +37,7 @@ THE SOFTWARE.  */
 #include <htslib/synced_bcf_reader.h>
 #include <htslib/vcfutils.h>
 #include <htslib/hts_os.h>
+#include <htslib/hts_defs.h>
 #include <inttypes.h>
 #include "bcftools.h"
 
@@ -83,10 +84,9 @@ typedef struct
 args_t;
 
 static void usage(void);
-FILE *open_file(char **fname, const char *mode, const char *fmt, ...);
-void mkdir_p(const char *fmt, ...);
+FILE *open_file(char **fname, const char *mode, const char *fmt, ...) HTS_FORMAT(HTS_PRINTF_FMT, 3, 4);
 
-char *msprintf(const char *fmt, ...)
+char * HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) msprintf(const char *fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);
diff --git a/vcfsort.c b/vcfsort.c
index cedce556..6e21f85b 100644
--- a/vcfsort.c
+++ b/vcfsort.c
@@ -41,6 +41,7 @@
 #include <htslib/vcf.h>
 #include <htslib/kstring.h>
 #include <htslib/hts_os.h>
+#include <htslib/hts_defs.h>
 #include <htslib/bgzf.h>
 #include "kheap.h"
 #include "bcftools.h"
@@ -103,7 +104,8 @@ void clean_files(args_t *args)
     }
     rmdir(args->tmp_dir);
 }
-void clean_files_and_throw(args_t *args, const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 2, 3) HTS_NORETURN
+clean_files_and_throw(args_t *args, const char *format, ...)
 {
     va_list ap;
     va_start(ap, format);
@@ -505,7 +507,7 @@ void sort_blocks(args_t *args)
             bcf_destroy(rec);
             break;
         }
-        if ( rec->errcode ) clean_files_and_throw(args,"Error encountered while parsing the input at %s:%d\n",bcf_seqname(args->hdr,rec),rec->pos+1);
+        if ( rec->errcode ) clean_files_and_throw(args,"Error encountered while parsing the input at %s:%"PRIhts_pos"\n",bcf_seqname(args->hdr,rec),rec->pos+1);
         bcf_unpack(rec, BCF_UN_STR);
         buf_push(args, rec);
     }
@@ -655,12 +657,11 @@ void do_partial_merge(args_t *args)
         to_layer = MERGE_LAYERS - 1;
     }
 
-    char *fname = NULL;
     blk_t tmp = { NULL };
     open_tmp_file(args, &tmp, 1);
     merge_blocks(args, tmp.fh, tmp.fname, 0, args->nblk - to_merge);
     if (hts_close(tmp.fh) != 0)
-        clean_files_and_throw(args, "Close failed: %s\n", fname);
+        clean_files_and_throw(args, "Close failed: %s\n", tmp.fname);
 
     args->nblk -= to_merge;
     assert(args->blk[args->nblk].fh == NULL);
@@ -722,7 +723,6 @@ size_t parse_mem_string(const char *str)
     return mem;
 }
 
-void mkdir_p(const char *fmt, ...);
 static void init(args_t *args)
 {
     size_t i;
diff --git a/vcfstats.c b/vcfstats.c
index 415850a3..38b4caf5 100644
--- a/vcfstats.c
+++ b/vcfstats.c
@@ -1529,7 +1529,7 @@ static void print_stats(args_t *args)
             {
                 if ( usr->vals_ts[j]+usr->vals_tv[j] == 0 ) continue;   // skip empty bins
                 float val = usr->min + (usr->max - usr->min)*j/(usr->nbins-1);
-                const char *fmt = usr->type==BCF_HT_REAL ? "USR:%s/%d\t%d\t%e\t%"PRIu64"\t%"PRIu64"\t%"PRIu64"\n" : "USR:%s/%d\t%d\t%.0f\t%"PRIu64"\t%"PRIu64"\t%"PRIu64"\n";
+                const char * const fmt = usr->type==BCF_HT_REAL ? "USR:%s/%d\t%d\t%e\t%"PRIu64"\t%"PRIu64"\t%"PRIu64"\n" : "USR:%s/%d\t%d\t%.0f\t%"PRIu64"\t%"PRIu64"\t%"PRIu64"\n";
                 printf(fmt,usr->tag,usr->idx,id,val,usr->vals_ts[j]+usr->vals_tv[j],usr->vals_ts[j],usr->vals_tv[j]);
             }
         }