Fix various warnings

- unused variable ‘walk’ [-Wunused-variable]
- implicit declaration of built-in function ‘free’
[-Wimplicit-function-declaration]
- initialization discards ‘const’ qualifier from pointer target type
[-Wdiscarded-qualifiers]
- variable 'ram_used' is used uninitialized whenever 'if' condition is
false [-Wsometimes-uninitialized]. This is actually easily reproducible
by specifying `memory_used_method = "XXX"`.
- comparison of integers of different signs: 'int' and 'unsigned long'
[-Wsign-compare] (for `exponent`)

include/i3status.h

@@ -206,9 +206,9 @@ char *trim(const char *s);
 /* src/format_placeholders.c */
 typedef struct {
     /* The placeholder to be replaced, e.g., "%title". */
-    char *name;
+    const char *name;
     /* The value this placeholder should be replaced with. */
-    char *value;
+    const char *value;
 } placeholder_t;
 char *format_placeholders(const char *format, placeholder_t *placeholders, int num);
 

src/print_mem.c

@@ -23,7 +23,7 @@ static const char *const iec_symbols[] = {"B", "KiB", "MiB", "GiB", "TiB"};
  */
 static int print_bytes_human(char *outwalk, unsigned long bytes, const char *unit, const int decimals) {
     double base = bytes;
-    int exponent = 0;
+    size_t exponent = 0;
     while (base >= BINARY_BASE && exponent < MAX_EXPONENT) {
         if (strcasecmp(unit, iec_symbols[exponent]) == 0) {
             break;
@@ -86,7 +86,6 @@ void print_memory(yajl_gen json_gen, char *buffer, const char *format, const cha
 
 #if defined(linux)
     const char *selected_format = format;
-    const char *walk;
     const char *output_color = NULL;
 
     int unread_fields = 6;
@@ -140,6 +139,8 @@ void print_memory(yajl_gen json_gen, char *buffer, const char *format, const cha
         ram_used = ram_total - ram_available;
     } else if (BEGINS_WITH(memory_used_method, "classical")) {
         ram_used = ram_total - ram_free - ram_buffers - ram_cached;
+    } else {
+        die("Unexpected value: memory_used_method = %s", memory_used_method);
     }
 
     if (threshold_degraded) {

src/print_path_exists.c

@@ -1,6 +1,7 @@
 // vim:ts=4:sw=4:expandtab
 #include <config.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <yajl/yajl_gen.h>
 #include <yajl/yajl_version.h>