Replace setlocale with thread-safe uselocale for locale handling

The previous implementation used setlocale(LC_ALL/LC_NUMERIC, "C") which
affects the entire process and is not thread-safe. This could cause issues
in multi-threaded environments where different threads might need different
locale settings.

Replace setlocale calls with the POSIX.1-2008 thread-safe alternatives:
- Use newlocale() to create a C locale for numeric formatting
- Use uselocale() to set the locale for the current thread only
- Properly restore the previous locale and free resources

Changes:
- libvmaf/src/svm.cpp: Update svm_save_model() locale handling
- libvmaf/src/read_json_model.c: Update vmaf_read_json_model() locale handling
- libvmaf/src/output.c: Update all output functions (vmaf_write_output_xml,
  vmaf_write_output_json, vmaf_write_output_csv, vmaf_write_output_sub) to
  use thread-safe locale handling with LC_ALL_MASK

This ensures consistent numeric formatting (decimal point vs comma) when
reading/writing model files and generating output in all formats (XML, JSON,
CSV, subtitle) while being thread-safe and not affecting other parts of the
application.

Author: diegonieto

libvmaf/src/output.c

@@ -19,6 +19,7 @@
 #include <errno.h>
 #include <math.h>
 #include <stdio.h>
+#include <locale.h>
 
 #include "feature/alias.h"
 #include "feature/feature_collector.h"
@@ -72,6 +73,12 @@ int vmaf_write_output_xml(VmafContext *vmaf, VmafFeatureCollector *fc,
     if (!fc) return -EINVAL;
     if (!outfile) return -EINVAL;
 
+    locale_t c_locale = newlocale(LC_ALL_MASK, "C", NULL);
+    locale_t old_locale = (locale_t)0;
+    if (c_locale != (locale_t)0) {
+        old_locale = uselocale(c_locale);
+    }
+
     fprintf(outfile, "<VMAF version=\"%s\">\n", vmaf_version());
     fprintf(outfile, "  <params qualityWidth=\"%d\" qualityHeight=\"%d\" />\n",
             width, height);
@@ -154,13 +161,24 @@ int vmaf_write_output_xml(VmafContext *vmaf, VmafFeatureCollector *fc,
 
     fprintf(outfile, "</VMAF>\n");
 
+    if (c_locale != (locale_t)0) {
+        uselocale(old_locale);
+        freelocale(c_locale);
+    }
+
     return 0;
 }
 
 int vmaf_write_output_json(VmafContext *vmaf, VmafFeatureCollector *fc,
                            FILE *outfile, unsigned subsample, double fps,
                            unsigned pic_cnt)
 {
+    locale_t c_locale = newlocale(LC_ALL_MASK, "C", NULL);
+    locale_t old_locale = (locale_t)0;
+    if (c_locale != (locale_t)0) {
+        old_locale = uselocale(c_locale);
+    }
+
     int leading_zeros_count;
     fprintf(outfile, "{\n");
     fprintf(outfile, "  \"version\": \"%s\",\n", vmaf_version());
@@ -299,12 +317,23 @@ int vmaf_write_output_json(VmafContext *vmaf, VmafFeatureCollector *fc,
     fprintf(outfile, "\n  }\n");
     fprintf(outfile, "}\n");
 
+    if (c_locale != (locale_t)0) {
+        uselocale(old_locale);
+        freelocale(c_locale);
+    }
+
     return 0;
 }
 
 int vmaf_write_output_csv(VmafFeatureCollector *fc, FILE *outfile,
                            unsigned subsample)
 {
+    locale_t c_locale = newlocale(LC_ALL_MASK, "C", NULL);
+    locale_t old_locale = (locale_t)0;
+    if (c_locale != (locale_t)0) {
+        old_locale = uselocale(c_locale);
+    }
+
     int leading_zeros_count;
     fprintf(outfile, "Frame,");
     for (unsigned i = 0; i < fc->cnt; i++) {
@@ -342,12 +371,23 @@ int vmaf_write_output_csv(VmafFeatureCollector *fc, FILE *outfile,
         fprintf(outfile, "\n");
     }
 
+    if (c_locale != (locale_t)0) {
+        uselocale(old_locale);
+        freelocale(c_locale);
+    }
+
     return 0;
 }
 
 int vmaf_write_output_sub(VmafFeatureCollector *fc, FILE *outfile,
                           unsigned subsample)
 {
+    locale_t c_locale = newlocale(LC_ALL_MASK, "C", NULL);
+    locale_t old_locale = (locale_t)0;
+    if (c_locale != (locale_t)0) {
+        old_locale = uselocale(c_locale);
+    }
+
     int leading_zeros_count;
     for (unsigned i = 0 ; i < max_capacity(fc); i++) {
         if ((subsample > 1) && (i % subsample))
@@ -381,5 +421,10 @@ int vmaf_write_output_sub(VmafFeatureCollector *fc, FILE *outfile,
         fprintf(outfile, "\n");
     }
 
+    if (c_locale != (locale_t)0) {
+        uselocale(old_locale);
+        freelocale(c_locale);
+    }
+
     return 0;
 }

libvmaf/src/read_json_model.c

@@ -24,6 +24,7 @@
 #include <errno.h>
 #include <stdlib.h>
 #include <string.h>
+#include <locale.h>
 
 #define MAX_FEATURE_COUNT 64 //FIXME
 #define MAX_KNOT_COUNT 10 //FIXME
@@ -432,6 +433,7 @@ static int model_parse(json_stream *s, VmafModel *model,
 static int vmaf_read_json_model(VmafModel **model, VmafModelConfig *cfg,
                                 json_stream *s)
 {
+    int err = -EINVAL;
     VmafModel *const m = *model = malloc(sizeof(*m));
     if (!m) return -ENOMEM;
     memset(m, 0, sizeof(*m));
@@ -449,7 +451,20 @@ static int vmaf_read_json_model(VmafModel **model, VmafModelConfig *cfg,
     if (!m->score_transform.knots.list) return -ENOMEM;
     memset(m->score_transform.knots.list, 0, knots_sz);
 
-    return model_parse(s, m, cfg->flags);
+    locale_t c_locale = newlocale(LC_ALL_MASK, "C", NULL);
+    locale_t old_locale = (locale_t)0;
+    if (c_locale != (locale_t)0) {
+        old_locale = uselocale(c_locale);
+    }
+
+    err = model_parse(s, m, cfg->flags);
+
+    if (c_locale != (locale_t)0) {
+        uselocale(old_locale);
+        freelocale(c_locale);
+    }
+
+    return err;
 }
 
 int vmaf_read_json_model_from_buffer(VmafModel **model, VmafModelConfig *cfg,

libvmaf/src/svm.cpp

@@ -2653,11 +2653,11 @@ int svm_save_model(const char *model_file_name, const svm_model *model)
 	FILE *fp = fopen(model_file_name,"w");
 	if(fp==NULL) return -1;
 
-	char *old_locale = setlocale(LC_ALL, NULL);
-	if (old_locale) {
-		old_locale = strdup(old_locale);
+	locale_t c_locale = newlocale(LC_ALL_MASK, "C", NULL);
+	locale_t old_locale = (locale_t)0;
+	if (c_locale != (locale_t)0) {
+		old_locale = uselocale(c_locale);
 	}
-	setlocale(LC_ALL, "C");
 
 	const svm_parameter& param = model->param;
 
@@ -2738,8 +2738,10 @@ int svm_save_model(const char *model_file_name, const svm_model *model)
 		fprintf(fp, "\n");
 	}
 
-	setlocale(LC_ALL, old_locale);
-	free(old_locale);
+	if (c_locale != (locale_t)0) {
+		uselocale(old_locale);
+		freelocale(c_locale);
+	}
 
 	if (ferror(fp) != 0 || fclose(fp) != 0) return -1;
 	else return 0;