termux-usb: add option to pass file descriptor as env var

And remove termux-callback shell script, we can implement that
function in termux-api.c.

Author: Grimler91

Makefile

@@ -6,10 +6,9 @@ termux-api: termux-api.c
 install: termux-api
 	mkdir -p $(PREFIX)/bin/ $(PREFIX)/libexec/
 	install termux-api $(PREFIX)/libexec/
-	install termux-callback $(PREFIX)/libexec/
 	cd scripts; for i in *; do \
-	  sed -e "s|@TERMUX_PREFIX@|$(PREFIX)|g" $$i > $(PREFIX)/bin/$$i; \
-	  chmod 700 $(PREFIX)/bin/$$i; \
+		sed -e "s|@TERMUX_PREFIX@|$(PREFIX)|g" $$i > $(PREFIX)/bin/$$i; \
+		chmod 700 $(PREFIX)/bin/$$i; \
 	done
 
 .PHONY: install

scripts/termux-usb

@@ -9,7 +9,10 @@ show_usage () {
     echo "  -l           list available devices"
     echo "  -r           show permission request dialog if necessary"
     echo "  -e command   execute the specified command with a file descriptor"
-    echo "                 referring to the device as its argument"
+    echo "                 referring to the device as an argument (unless -E"
+    echo "                 argument is given)"
+    echo "  -E           transfer file descriptor as env var instead of as"
+    echo "                 command line argument"
     exit 0
 }
 
@@ -23,6 +26,7 @@ do
         l) ACTION="list"; ((MASK |= 1));;
         r) PARAMS="$PARAMS --ez request true"; ((MASK |= 2));;
         e) ACTION="open"; export TERMUX_CALLBACK="$OPTARG"; ((MASK |= 2));;
+        E) export TERMUX_EXPORT_FD=true;;
         ?) echo "$SCRIPTNAME: illegal option -$OPTARG"; exit 1;
     esac
 done

termux-api.c

@@ -69,9 +69,27 @@ _Noreturn void exec_am_broadcast(int argc, char** argv, char* input_address_stri
 _Noreturn void exec_callback(int fd)
 {
     char *fds;
-    if(asprintf(&fds, "%d", fd) == -1) { perror("asprintf"); }
-    execl(PREFIX "/libexec/termux-callback", "termux-callback", fds, NULL);
-    perror("execl(\"" PREFIX "/libexec/termux-callback\")");
+    char *callback_fun = getenv("TERMUX_CALLBACK");
+    if (callback_fun == NULL) {
+        perror("TERMUX_CALLBACK is not set");
+        exit(1);
+    }
+
+    if (asprintf(&fds, "%d", fd) == -1)
+        perror("asprintf");
+
+    char errmsg[256];
+    char *export_to_env = getenv("TERMUX_EXPORT_FD");
+    if (strncmp(export_to_env, "true", 4) == 0) {
+        if (setenv("TERMUX_USB_FD", fds, true) == -1)
+            perror("setenv");
+        execl(callback_fun, callback_fun, NULL);
+        sprintf(errmsg, "execl(\"%s\", %s)", callback_fun, fds);
+    } else {
+        execl(callback_fun, callback_fun, fds, NULL);
+        sprintf(errmsg, "execl(\"%s\")", callback_fun);
+    }
+    perror(errmsg);
     exit(1);
 }