debloat code that depends on /proc/self/fd/%d with shared function

I intend to add more Linux workarounds that depend on using these
pathnames, and some of them will be in "syscall" functions that, from
an anti-bloat standpoint, should not depend on the whole snprintf
framework.

src/internal/procfdname.c

@@ -0,0 +1,13 @@
+void __procfdname(char *buf, unsigned fd)
+{
+	unsigned i, j;
+	for (i=0; (buf[i] = "/proc/self/fd/"[i]); i++);
+	if (!fd) {
+		buf[i] = '0';
+		buf[i+1] = 0;
+		return;
+	}
+	for (j=fd; j; j/=10, i++);
+	buf[i] = 0;
+	for (; fd; fd/=10) buf[--i] = '0' + fd%10;
+}

src/misc/realpath.c

@@ -6,6 +6,8 @@
 #include <errno.h>
 #include <unistd.h>
 
+void __procfdname(char *, unsigned);
+
 char *realpath(const char *restrict filename, char *restrict resolved)
 {
 	int fd;
@@ -21,7 +23,7 @@ char *realpath(const char *restrict filename, char *restrict resolved)
 
 	fd = open(filename, O_RDONLY|O_NONBLOCK|O_CLOEXEC);
 	if (fd < 0) return 0;
-	snprintf(buf, sizeof buf, "/proc/self/fd/%d", fd);
+	__procfdname(buf, fd);
 
 	if (!resolved) {
 		alloc = 1;

src/process/fexecve.c

@@ -2,11 +2,12 @@
 #include <stdio.h>
 #include <errno.h>
 
+void __procfdname(char *, unsigned);
+
 int fexecve(int fd, char *const argv[], char *const envp[])
 {
-	static const char proc[] = "/proc/self/fd/%d";
+	char buf[15 + 3*sizeof(int)];
-	char buf[sizeof proc + 3*sizeof(int)];
+	__procfdname(buf, fd);
-	snprintf(buf, sizeof buf, proc, fd);
 	execve(buf, argv, envp);
 	if (errno == ENOENT) errno = EBADF;
 	return -1;

src/stat/fchmodat.c

@@ -4,6 +4,8 @@
 #include <stdio.h>
 #include "syscall.h"
 
+void __procfdname(char *, unsigned);
+
 int fchmodat(int fd, const char *path, mode_t mode, int flag)
 {
 	if (!flag) return syscall(SYS_fchmodat, fd, path, mode, flag);
@@ -26,7 +28,7 @@ int fchmodat(int fd, const char *path, mode_t mode, int flag)
 		return __syscall_ret(fd2);
 	}
 
-	snprintf(proc, sizeof proc, "/proc/self/fd/%d", fd2);
+	__procfdname(proc, fd2);
 	if (!(ret = __syscall(SYS_stat, proc, &st)) && !S_ISLNK(st.st_mode))
 		ret = __syscall(SYS_chmod, proc, mode);
 

src/unistd/ttyname_r.c

@@ -3,6 +3,8 @@
 #include <stdio.h>
 #include <string.h>
 
+void __procfdname(char *, unsigned);
+
 int ttyname_r(int fd, char *name, size_t size)
 {
 	char procname[sizeof "/proc/self/fd/" + 3*sizeof(int) + 2];
@@ -10,7 +12,7 @@ int ttyname_r(int fd, char *name, size_t size)
 
 	if (!isatty(fd)) return ENOTTY;
 
-	snprintf(procname, sizeof procname, "/proc/self/fd/%d", fd);
+	__procfdname(procname, fd);
 	l = readlink(procname, name, size);
 
 	if (l < 0) return errno;