From 0fb0009283ca38cd45975a85e8c428afce549702 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20Ankarstr=C3=B6m?= <john@ankarstrom.se>
Date: Sun, 11 Jul 2021 12:47:02 +0200
Subject: Chdir to dirname(file)

---
 build.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 50 insertions(+), 8 deletions(-)

diff --git a/build.c b/build.c
index b430fb2..621daa7 100644
--- a/build.c
+++ b/build.c
@@ -17,9 +17,10 @@
 	fprintf(stderr, __VA_ARGS__); } while(0);
 
 #define MAXBUF 1024
-#define MAXCMDS 32	/* Maximum number of commands across all files. */
 #define MAXCMD 1024
+#define MAXCMDS 32	/* Maximum number of commands across all files. */
 #define MAXDEP 1024
+#define MAXPATH 1024
 #define MAXTGT 64
 
 extern char *optarg;
@@ -27,12 +28,13 @@ extern int optind;
 
 void cleandep(char **);
 void cleantgt(char **);
+char *dirname(char *);
 char *nextdep(char **);
 
 int
 main(int argc, char *argv[])
 {
-	char *b, buf[MAXBUF], *cmd[MAXCMDS], *d, *dep, *name, *tgt;
+	char *b, buf[MAXBUF], *cmd[MAXCMDS], *d, *dep, *name, *tgt, wd[MAXPATH];
 	FILE *fp;
 	int c, dflag, fflag, i, icmd, j, s;
 	struct stat sb, ssb;
@@ -48,6 +50,13 @@ main(int argc, char *argv[])
 
 	tgt[0] = dep[0] = 0;
 
+	/*
+	 * The original working directory is saved. (When acting on the
+	 * build information in a file, the program temporarily moves into
+	 * its directory.)
+	 */
+	getcwd(wd, MAXPATH);
+
 	/* Process command-line flags (debug, force). */
 	name = argv[0];
 	dflag = fflag = 0;
@@ -72,8 +81,12 @@ main(int argc, char *argv[])
 
 	/* Process dependencies and commands in each file. */
 	for(i = icmd = 0; i < argc; i++, icmd = 0){
+
+		/* Return to original directory. */
+		chdir(wd);
+
 		if(!(fp = fopen(argv[i], "r")))
-			err(1, "fopen");
+			err(1, "fopen(%s)", argv[i]);
 
 		/* Read line by line, at most twenty. */
 		for(j = 0; j < 20 && fgets(buf, MAXBUF, fp); j++){
@@ -115,21 +128,33 @@ main(int argc, char *argv[])
 		}
 
 		/* Build immediately if forced or no target found. */
-		if(fflag || !*tgt)
+		if(fflag || !*tgt){
+			chdir(dirname(argv[i]));
 			goto build;
+		}
 
 		/* Trim shell meta-characters and whitespace. */
 		cleantgt(&tgt);
 		cleandep(&dep);
 
-		/* Build immediately if source is newer than target. */
 		dd("%s: target '%s'\n", argv[i], tgt);
+
+		/* Get information about source. */
 		if(stat(argv[i], &sb))
-			err(1, argv[i]);
+			err(1, "stat(%s)", argv[i]);
+
+		/*
+		 * Subsequent operations should take place inside the
+		 * directory of the source, as command lines and dependency
+		 * lines may refer to relative paths.
+		 */
+		chdir(dirname(argv[i]));
+
+		/* Build immediately if source is newer than target. */
 		if(stat(tgt, &ssb)){
 			if(errno == ENOENT)
 				goto build;
-			err(1, tgt);
+			err(1, "stat(%s)", tgt);
 		}
 		if(sb.st_mtime > ssb.st_mtime){
 			d("%s: %s is modified, building\n",
@@ -154,7 +179,7 @@ main(int argc, char *argv[])
 						"does not exist\n", argv[i], d);
 					continue;
 				}
-				err(1, d);
+				err(1, "stat(%s)", d);
 			}
 			if(sb.st_mtime > ssb.st_mtime){
 				d("%s: %s is modified, building\n", argv[i], d);
@@ -239,6 +264,23 @@ cleantgt(char **tgt)
 		}
 }
 
+char *
+dirname(char *file)
+{
+	char *dir;
+	int i;
+
+	if(!(dir = malloc(strlen(file)+1)))
+		err(1, "malloc");
+
+	for(i = strlen(file); i >= 0; i--)
+		if(file[i] == '/') break;
+	for(dir[i--] = 0; i >= 0; i--)
+		dir[i] = file[i];
+
+	return dir[0] ? dir : ".";
+}
+
 char *
 nextdep(char **dep)
 {
-- 
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