Mild const-correctness improvements.

Only touch a few things, the main focus is to
improve code readability.

Reference:
https://git.suckless.org/st/commit/4536f46cfff50c66a115755def0155d8e246b02f.html

Author: sighook

st.c

@@ -198,18 +198,18 @@ static void tputc(Rune);
 static void treset(void);
 static void tscrollup(int, int);
 static void tscrolldown(int, int);
-static void tsetattr(int *, int);
-static void tsetchar(Rune, Glyph *, int, int);
+static void tsetattr(const int *, int);
+static void tsetchar(Rune, const Glyph *, int, int);
 static void tsetdirt(int, int);
 static void tsetscroll(int, int);
 static void tswapscreen(void);
-static void tsetmode(int, int, int *, int);
+static void tsetmode(int, int, const int *, int);
 static int twrite(const char *, int, int);
 static void tfulldirt(void);
 static void tcontrolcode(uchar );
 static void tdectest(char );
 static void tdefutf8(char);
-static int32_t tdefcolor(int *, int *, int);
+static int32_t tdefcolor(const int *, int *, int);
 static void tdeftran(char);
 static void tstrsequence(uchar);
 static void tsetcolor(int, int, int, uint32_t, uint32_t);
@@ -241,10 +241,10 @@ static int iofd = 1;
 static int cmdfd;
 static pid_t pid;
 
-static uchar utfbyte[UTF_SIZ + 1] = {    0x80,    0,  0xC0,   0xE0,     0xF0};
-static uchar utfmask[UTF_SIZ + 1] = {    0xC0, 0x80,  0xE0,   0xF0,     0xF8};
-static Rune  utfmin[UTF_SIZ + 1]  = {       0,    0,  0x80,  0x800,  0x10000};
-static Rune  utfmax[UTF_SIZ + 1]  = {0x10FFFF, 0x7F, 0x7FF, 0xFFFF, 0x10FFFF};
+static const uchar utfbyte[UTF_SIZ + 1] = {    0x80,    0,  0xC0,   0xE0,     0xF0};
+static const uchar utfmask[UTF_SIZ + 1] = {    0xC0, 0x80,  0xE0,   0xF0,     0xF8};
+static const Rune  utfmin[UTF_SIZ + 1]  = {       0,    0,  0x80,  0x800,  0x10000};
+static const Rune  utfmax[UTF_SIZ + 1]  = {0x10FFFF, 0x7F, 0x7FF, 0xFFFF, 0x10FFFF};
 
 int buffCols;
 
@@ -301,12 +301,14 @@ xrealloc(void *p, size_t len)
 }
 
 char *
-xstrdup(char *s)
+xstrdup(const char *s)
 {
-	if ((s = strdup(s)) == NULL)
+	char *p;
+
+	if ((p = strdup(s)) == NULL)
 		die("strdup: %s\n", strerror(errno));
 
-	return s;
+	return p;
 }
 
 size_t
@@ -823,7 +825,7 @@ stty(char **args)
 }
 
 int
-ttynew(char *line, char *cmd, char *out, char **args)
+ttynew(const char *line, char *cmd, const char *out, char **args)
 {
 	int m, s;
 
@@ -1273,9 +1275,9 @@ tmoveto(int x, int y)
 }
 
 void
-tsetchar(Rune u, Glyph *attr, int x, int y)
+tsetchar(Rune u, const Glyph *attr, int x, int y)
 {
-	static char *vt100_0[62] = { /* 0x41 - 0x7e */
+	static const char *vt100_0[62] = { /* 0x41 - 0x7e */
 		"↑", "↓", "→", "←", "█", "▚", "☃",       /* A - G */
 		0, 0, 0, 0, 0, 0, 0, 0,                  /* H - O */
 		0, 0, 0, 0, 0, 0, 0, 0,                  /* P - W */
@@ -1387,7 +1389,7 @@ tdeleteline(int n)
 }
 
 int32_t
-tdefcolor(int *attr, int *npar, int l)
+tdefcolor(const int *attr, int *npar, int l)
 {
 	int32_t idx = -1;
 	uint r, g, b;
@@ -1439,7 +1441,7 @@ tdefcolor(int *attr, int *npar, int l)
 }
 
 void
-tsetattr(int *attr, int l)
+tsetattr(const int *attr, int l)
 {
 	int i;
 	int32_t idx;
@@ -1557,9 +1559,10 @@ tsetscroll(int t, int b)
 }
 
 void
-tsetmode(int priv, int set, int *args, int narg)
+tsetmode(int priv, int set, const int *args, int narg)
 {
-	int alt, *lim;
+	int alt;
+	const int *lim;
 
 	for (lim = args + narg; args < lim; ++args) {
 		if (priv) {
@@ -2109,7 +2112,7 @@ void
 tdumpline(int n)
 {
 	char buf[UTF_SIZ];
-	Glyph *bp, *end;
+	const Glyph *bp, *end;
 
 	bp = &term.line[n][0];
 	end = &bp[MIN(tlinelen(n), term.col) - 1];

st.h

@@ -103,7 +103,7 @@ void tresize(int, int);
 void tmoveto(int x, int y);
 void tsetdirtattr(int);
 void ttyhangup(void);
-int ttynew(char *, char *, char *, char **);
+int ttynew(const char *, char *, const char *, char **);
 size_t ttyread(void);
 void ttyresize(int, int);
 void ttywrite(const char *, size_t, int);
@@ -121,7 +121,7 @@ size_t utf8encode(Rune, char *);
 
 void *xmalloc(size_t);
 void *xrealloc(void *, size_t);
-char *xstrdup(char *);
+char *xstrdup(const char *);
 
 /*
  * config.h globals

x.c

@@ -176,7 +176,7 @@ static void xresize(int, int);
 static void xhints(void);
 static int xloadcolor(int, const char *, Color *);
 static int xloadfont(Font *, FcPattern *);
-static void xloadfonts(char *, double);
+static void xloadfonts(const char *, double);
 static void xunloadfont(Font *);
 static void xunloadfonts(void);
 static void xsetenv(void);
@@ -988,15 +988,15 @@ xloadfont(Font *f, FcPattern *pattern)
 }
 
 void
-xloadfonts(char *fontstr, double fontsize)
+xloadfonts(const char *fontstr, double fontsize)
 {
 	FcPattern *pattern;
 	double fontval;
 
 	if (fontstr[0] == '-')
 		pattern = XftXlfdParse(fontstr, False, False);
 	else
-		pattern = FcNameParse((FcChar8 *)fontstr);
+		pattern = FcNameParse((const FcChar8 *)fontstr);
 
 	if (!pattern)
 		die("can't open font %s\n", fontstr);