termux-packages/packages/termux-api/termux-api.c

// termux-api.c - helper binary for calling termux api classes
// Usage: termux-api ${API_METHOD} ${ADDITIONAL_FLAGS}
//        This executes
//          am broadcast com.termux.api/.TermuxApiReceiver --es socket_input ${INPUT_SOCKET} 
//                                                        --es socket_output ${OUTPUT_SOCKET}
//                                                        --es ${API_METHOD}
//                                                        ${ADDITIONAL_FLAGS}
//        where ${INPUT_SOCKET} and ${OUTPUT_SOCKET} are addresses to linux abstract namespace sockets,
//        used to pass on stdin to the java implementation and pass back output from java to stdout.
#define _POSIX_SOURCE
#define _GNU_SOURCE
#include <fcntl.h>
#include <pthread.h>
#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/un.h>
#include <time.h>
#include <unistd.h>

// Function which execs "am broadcast ..".
void exec_am_broadcast(int argc, char** argv, char* input_address_string, char* output_address_string)
{
        // Redirect stdout to /dev/null (but leave stderr open):
        close(STDOUT_FILENO);
        open("/dev/null", O_RDONLY);
        // Close stdin:
        close(STDIN_FILENO);

        char const* const file = "/system/bin/am";
        // Avoid the system am binary from linking to wrong libraries:
        unsetenv("LD_LIBRARY_PATH");

	// The user is calculated from the uid in android.os.UserHandle#getUserId(int uid) as "uid / 100000", so we do the same:
	uid_t current_uid = getuid();
	int android_user_id = current_uid / 100000;
	char* android_user_id_string;
	if (asprintf(&android_user_id_string, "%d", android_user_id) == -1) {
		fprintf(stderr, "asprintf() error");
		return;
	}

        int const extra_args = 15; // Including ending NULL.
        char** child_argv = malloc((sizeof(char*)) * (argc + extra_args));

        child_argv[0] = "am";
        child_argv[1] = "broadcast";
        child_argv[2] = "--user";
	child_argv[3] = android_user_id_string;
        child_argv[4] = "-n";
        child_argv[5] = "com.termux.api/.TermuxApiReceiver";
        child_argv[6] = "--es";
        // Input/output are reversed for the java process (our output is its input):
        child_argv[7] = "socket_input";
        child_argv[8] = output_address_string;
        child_argv[9] = "--es";
        child_argv[10] = "socket_output";
        child_argv[11] = input_address_string;
        child_argv[12] = "--es";
        child_argv[13] = "api_method";
        child_argv[14] = argv[1];

        // Copy the remaining arguments -2 for first binary and second api name:
        memcpy(child_argv + extra_args, argv + 2, (argc-1) * sizeof(char*));

        // End with NULL:
        child_argv[argc + extra_args] = NULL;

        execv(file, child_argv);
        perror("execv(\"/system/bin/am\")");
	exit(1);
}

void generate_uuid(char* str) {
        sprintf(str, "%x%x-%x-%x-%x-%x%x%x", 
                        rand(), rand(),                 // Generates a 64-bit Hex number
                        (uint32_t) getpid(),                         // Generates a 32-bit Hex number
                        ((rand() & 0x0fff) | 0x4000),   // Generates a 32-bit Hex number of the form 4xxx (4 indicates the UUID version)
                        rand() % 0x3fff + 0x8000,       // Generates a 32-bit Hex number in the range [0x8000, 0xbfff]
                        rand(), rand(), rand());        // Generates a 96-bit Hex number
}

// Thread function which reads from stdin and writes to socket.
void* transmit_stdin_to_socket(void* arg) {
        int output_server_socket = *((int*) arg);
        struct sockaddr_un remote_addr;
        socklen_t addrlen = sizeof(remote_addr);
        int output_client_socket = accept(output_server_socket, (struct sockaddr*) &remote_addr, &addrlen);

        int len;
        char buffer[1024];
        while (len = read(STDIN_FILENO, &buffer, sizeof(buffer)-1), len > 0) {
                if (write(output_client_socket, buffer, len) < 0) break;
        }
	// Close output socket on end of input:
        close(output_client_socket);
        return NULL;
}

// Main thread function which reads from input socket and writes to stdout.
void transmit_socket_to_stdout(int input_socket_fd) {
        int len;
        char buffer[1024];
        while ((len = read(input_socket_fd, &buffer, sizeof(buffer)-1)) > 0) {
                buffer[len] = 0;
                write(STDOUT_FILENO, buffer, len);
        }
        if (len < 0) perror("read()");
}

int main(int argc, char** argv) {
        // Do not transform children into zombies when they terminate:
        struct sigaction sigchld_action = { .sa_handler = SIG_DFL, .sa_flags = SA_RESTART | SA_NOCLDSTOP | SA_NOCLDWAIT };
        sigaction(SIGCHLD, &sigchld_action, NULL);

        char input_address_string[100];  // This program reads from it.
        char output_address_string[100]; // This program writes to it.

	// Seed the random number generator:
	struct timeval time;
	gettimeofday(&time,NULL);
	srand((time.tv_sec * 1000) + (time.tv_usec / 1000));

        generate_uuid(input_address_string);
        generate_uuid(output_address_string);

        struct sockaddr_un input_address = { .sun_family = AF_UNIX };
        struct sockaddr_un output_address = { .sun_family = AF_UNIX };
        // Leave struct sockaddr_un.sun_path[0] as 0 and use the UUID string as abstract linux namespace:
        strncpy(&input_address.sun_path[1], input_address_string, strlen(input_address_string));
        strncpy(&output_address.sun_path[1], output_address_string, strlen(output_address_string));

        int input_server_socket = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);
        if (input_server_socket == -1) { perror("socket()"); return 1; }
        int output_server_socket = socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0);
        if (output_server_socket == -1) { perror("socket()"); return 1; }

        if (bind(input_server_socket, (struct sockaddr*) &input_address, sizeof(sa_family_t) + strlen(input_address_string) + 1) == -1) {
		perror("bind(input)");
		return 1;
	}
        if (bind(output_server_socket, (struct sockaddr*) &output_address, sizeof(sa_family_t) + strlen(output_address_string) + 1) == -1) {
		perror("bind(output)");
		return 1;
	}

        if (listen(input_server_socket, 1) == -1) { perror("listen()"); return 1; }
        if (listen(output_server_socket, 1) == -1) { perror("listen()"); return 1; }

        pid_t fork_result = fork();
        switch (fork_result) {
                case -1: perror("fork()"); return 1;
                case 0: exec_am_broadcast(argc, argv, input_address_string, output_address_string); return 0;
        }

        struct sockaddr_un remote_addr;
        socklen_t addrlen = sizeof(remote_addr);
        int input_client_socket = accept(input_server_socket, (struct sockaddr*) &remote_addr, &addrlen);

        pthread_t transmit_thread;
        pthread_create(&transmit_thread, NULL, transmit_stdin_to_socket, &output_server_socket);

	transmit_socket_to_stdout(input_client_socket);

        return 0;
}