diff --git a/.cirrus.yml b/.cirrus.yml
index 9d80ea07d..61f094c2b 100644
--- a/.cirrus.yml
+++ b/.cirrus.yml
@@ -113,7 +113,7 @@ ubuntu_task:
     - environment:
        # Cirrus-CI's clang isn't installed with ubsan, so we do that in gcc
        USE_CONFIG: yes
-       CFLAGS: -std=c99 -pedantic -Wformat -g -Wall -O3
+       CFLAGS: -std=c99 -pedantic -Wformat -Wformat=2 -g -Wall -O3
        USE_LIBDEFLATE: yes
 
   # NB: we could consider building a docker image with these
diff --git a/bgzip.c b/bgzip.c
index d795c80a9..740aef117 100644
--- a/bgzip.c
+++ b/bgzip.c
@@ -48,7 +48,7 @@
 
 static const int WINDOW_SIZE = BGZF_BLOCK_SIZE;
 
-static void error(const char *format, ...)
+static void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) error(const char *format, ...)
 {
     va_list ap;
     va_start(ap, format);
@@ -689,7 +689,7 @@ int main(int argc, char **argv)
                     if ( bgzf_index_load(fp, argv[optind], ".gzi") < 0 )
                         error("Could not load index: %s.gzi\n", argv[optind]);
                 }
-                if ( bgzf_useek(fp, start, SEEK_SET) < 0 ) error("Could not seek to %d-th (uncompressd) byte\n", start);
+                if ( bgzf_useek(fp, start, SEEK_SET) < 0 ) error("Could not seek to %ld-th (uncompressd) byte\n", start);
             }
 
             if (threads > 1)
diff --git a/htsfile.c b/htsfile.c
index 9af4ae31b..25af3f584 100644
--- a/htsfile.c
+++ b/htsfile.c
@@ -46,7 +46,7 @@ int show_headers = 1;
 int verbose = 0;
 int status = EXIT_SUCCESS;  /* Exit status from main */
 
-void error(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) error(const char *format, ...)
 {
     int err = errno;
     va_list args;
diff --git a/htslib/vcf.h b/htslib/vcf.h
index e60911ab5..9a36cab05 100644
--- a/htslib/vcf.h
+++ b/htslib/vcf.h
@@ -596,7 +596,8 @@ set to one of BCF_ERR* codes and must be checked before calling bcf_write().
     int bcf_hdr_append(bcf_hdr_t *h, const char *line);
 
     HTSLIB_EXPORT
-    int bcf_hdr_printf(bcf_hdr_t *h, const char *format, ...);
+    int bcf_hdr_printf(bcf_hdr_t *h, const char *format, ...)
+    HTS_FORMAT(HTS_PRINTF_FMT, 2, 3);
 
     /** VCF version, e.g. VCFv4.2 */
     HTSLIB_EXPORT
diff --git a/test/hfile.c b/test/hfile.c
index 8f06a971f..741cf7a8d 100644
--- a/test/hfile.c
+++ b/test/hfile.c
@@ -35,7 +35,8 @@ DEALINGS IN THE SOFTWARE.  */
 #include "../htslib/hts_defs.h"
 #include "../htslib/kstring.h"
 
-void HTS_NORETURN fail(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) HTS_NORETURN
+fail(const char *format, ...)
 {
     int err = errno;
     va_list args;
diff --git a/test/test-bcf-sr.c b/test/test-bcf-sr.c
index 80daf0423..b4943b5ef 100644
--- a/test/test-bcf-sr.c
+++ b/test/test-bcf-sr.c
@@ -40,7 +40,7 @@
 #include "../htslib/hts.h"
 #include "../htslib/vcf.h"
 
-void error(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) error(const char *format, ...)
 {
     va_list ap;
     va_start(ap, format);
diff --git a/test/test-bcf-translate.c b/test/test-bcf-translate.c
index c2f069e39..263e71eb8 100644
--- a/test/test-bcf-translate.c
+++ b/test/test-bcf-translate.c
@@ -29,7 +29,7 @@
 
 #include "../htslib/vcf.h"
 
-void error(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) error(const char *format, ...)
 {
     va_list ap;
     va_start(ap, format);
diff --git a/test/test-bcf_set_variant_type.c b/test/test-bcf_set_variant_type.c
index 3688609f6..eb12ecde3 100644
--- a/test/test-bcf_set_variant_type.c
+++ b/test/test-bcf_set_variant_type.c
@@ -29,7 +29,7 @@ DEALINGS IN THE SOFTWARE.  */
 #include "../htslib/hts.h"
 #include "../vcf.c"
 
-void error(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) error(const char *format, ...)
 {
     va_list ap;
     va_start(ap, format);
diff --git a/test/test-vcf-api.c b/test/test-vcf-api.c
index ff16fa194..b86b71d99 100644
--- a/test/test-vcf-api.c
+++ b/test/test-vcf-api.c
@@ -33,7 +33,7 @@ DEALINGS IN THE SOFTWARE.  */
 #include "../htslib/kstring.h"
 #include "../htslib/kseq.h"
 
-void error(const char *format, ...)
+void HTS_FORMAT(HTS_PRINTF_FMT, 1, 2) error(const char *format, ...)
 {
     va_list ap;
     va_start(ap, format);