which.c: C implementation of UNIX which(1)

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

which.c

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+/* C implementation of UNIX which(1)
+ *
+ * Copyright (c) 2017  Joachim Nilsson <troglobit@gmail.com>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+#include <errno.h>
+#include <stdio.h>		/* snprintf() */
+#include <stdlib.h>		/* realloc(), free() */
+#include <string.h>		/* strtok_r() */
+#include <unistd.h>		/* access() */
+
+/*
+ * Like which(1), returns a malloc'ed path to cmd on success, or NULL
+ */
+char *which(const char *cmd)
+{
+	size_t pathlen = 0;
+	char *ptr, *tok, *env, *path = NULL;
+
+	if (!cmd) {
+		errno = EINVAL;
+		return NULL;
+	}
+
+	if (cmd[0] == '/') {
+		path = strdup(cmd);
+		if (!access(path, X_OK))
+			return path;
+
+		if (path)
+			free(path);
+
+		return NULL;
+	}
+
+	ptr = getenv("PATH");
+	if (!ptr)
+		return NULL;
+
+	env = strdup(ptr);
+	if (!env)
+		return NULL;
+
+	tok = strtok_r(env, ":", &ptr);
+	while (tok) {
+		size_t len = strlen(tok) + strlen(cmd) + 2;
+
+		if (pathlen < len) {
+			path = realloc(path, len);
+			if (!path) {
+				free(env);
+				return NULL;
+			}
+			pathlen = len;
+		}
+
+		snprintf(path, pathlen, "%s/%s", tok, cmd);
+		if (!access(path, X_OK)) {
+			free(env);
+			return path;
+		}
+
+		tok = strtok_r(NULL, ":", &ptr);
+	}
+
+	if (path)
+		free(path);
+	free(env);
+
+	return NULL;
+}
+
+/**
+ * Local Variables:
+ *  indent-tabs-mode: t
+ *  c-file-style: "linux"
+ * End:
+ */