New helper function, popenf(), from the Infix project

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

src/popenf.c

@@ -0,0 +1,63 @@
+/* Formatted popen()
+ *
+ * Copyright (c) 2023  Tobias Waldekranz <tobias@waldekranz.com>
+ * Copyright (c) 2023  Joachim Wiberg <troglobit@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <errno.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdlib.h>
+
+/**
+ * Formatted pipe stream to or from a process
+ * @param type A popen() type, "r" or "w", can also include "e" for @c O_CLOEXEC.
+ * @param fmt  Formatted string to be composed into a shell command line.
+ *
+ * This function is an extension to popen(), lessening the burden of
+ * first having to compose the filename from parts in a seprate buffer.
+ *
+ * @returns Upon successful completion, popenf() returns a FILE pointer.
+ * Otherwise, @c NULL is returned and @a errno is set to indicate the
+ * error.
+ */
+FILE *popenf(const char *type, const char *fmt, ...)
+{
+	va_list ap;
+	char *cmd;
+	int len;
+
+	va_start(ap, fmt);
+	len = vsnprintf(NULL, 0, fmt, ap);
+	va_end(ap);
+
+	cmd = alloca(len + 1);
+	if (!cmd) {
+		errno = ENOMEM;
+		return NULL;
+	}
+
+	va_start(ap, fmt);
+	vsnprintf(cmd, len + 1, fmt, ap);
+	va_end(ap);
+
+	return popen(cmd, type);
+}