获取jstring / char的长度*？

Question

This is my JNI C code:

JNIEXPORT jint JNICALL Java_org_avuna_httpd_util_CLibJNI_bind(JNIEnv * this, jclass cls, jint sockfd, jint family, jstring path, jint len) {
    struct sockaddr_un sun;
    sun.sun_family = family;
    const char *npath = (*this)->GetStringUTFChars(this, path, 0);
    sun.sun_path = npath;
    return bind(sockfd, sun, sizeof(&npath));
}

I just guessed unsuccessfully ( sizeof(&npath) ) how to get the length. I'm not well versed in C, but I figure there is a way to get a length out of a jstring.

Google yields nothing, what basic thing am I missing?

Answer 1

Since the type of the variable is const char * it suggests that it's a nul terminated string, hence

size_t length = strlen(npath);

should be enough.

Although, your call to bind() is wrong, you shold pass the size of the address structure, for which

return bind(sockfd, sun, sizeof(sun));

should be correct.

Answer 2

First, you have a memory leak. The char* pointer returned by GetStringUTFChars() must be freed using ReleaseStringUTFChars() , which you are not doing.

Second, bind() expects you to specify the byte size of the sockaddr_un struct, not the character length of npath . Also, the second parameter of bind() expects a sockaddr* pointer as input.

Third, sockaddr_un::sun_path is a char[] array, you cannot assign a char* pointer to it. You have to copy the content that is being pointed at.

Try this:

JNIEXPORT jint JNICALL Java_org_avuna_httpd_util_CLibJNI_bind(JNIEnv * this, jclass cls, jint sockfd, jint family, jstring path, jint len)
{
    struct sockaddr_un sun;
    memset(&sun, 0, sizeof(sun));
    sun.sun_family = family;
    const char *npath = (*this)->GetStringUTFChars(this, path, 0);
    strncpy(sun.sun_path, npath, UNIX_PATH_MAX);
    (*this)->ReleaseStringUTFChars(this, path, npath);
    return bind(sockfd, (struct sockaddr *) &sun, sizeof(sun));
}

With that said, sockaddr_un always uses AF_UNIX for its family, so it does not make sense to pass a family as an input value. If you need to pass in different family values, you need to use a different sockaddr_... struct for each family , eg:

JNIEXPORT jint JNICALL Java_org_avuna_httpd_util_CLibJNI_bind(JNIEnv * this, jclass cls, jint sockfd, jint family, jstring path, jint len)
{
    switch (family)
    {
        case AF_UNIX:
        {
            struct sockaddr_un sun;
            memset(&sun, 0, sizeof(sun));
            sun.sun_family = AF_UNIX;
            const char *npath = (*this)->GetStringUTFChars(this, path, 0);
            strncpy(sun.sun_path, npath, UNIX_PATH_MAX);
            (*this)->ReleaseStringUTFChars(this, path, npath);
            return bind(sockfd, (struct sockaddr *) &sun, sizeof(sun));
        }

        case AF_INET:
        {
            struct sockaddr_in sin;
            memset(&sin, 0, sizeof(sin));
            sin.sin_family = AF_INET;
            const char *npath = (*this)->GetStringUTFChars(this, path, 0);
            inet_pton(AF_INET, npath, &sin.sin_addr);
            sin.sin_port = 0;
            (*this)->ReleaseStringUTFChars(this, path, npath);
            return bind(sockfd, (struct sockaddr *) &sin, sizeof(sin));
        }

        case AF_INET6:
        {
            struct sockaddr_in6 sin;
            memset(&sin, 0, sizeof(sin));
            sin.sin6_family = AF_INET6;
            const char *npath = (*this)->GetStringUTFChars(this, path, 0);
            inet_pton(AF_INET6, npath, &sin.sin6_addr);
            sin.sin6_port = 0;
            (*this)->ReleaseStringUTFChars(this, path, npath);
            return bind(sockfd, (struct sockaddr *) &sin, sizeof(sin));
        }

        // and so on ...
    }

    return -1;
}

Answer 3

Regarding the length of the jstring, what is needed is GetStringUTFLength.

https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html

This should be more efficient than calling strlen on the null terminated char* as I don't think it will need to search for the final null.