X11 Xlib截屏问题及深入分析四 —— XOpenDisplay函数源码分析（1）

XOpenDisplay函数对应的源码在src/OpenDis.c中。完整源码请参见：

X11 Xlib截屏问题及深入分析二 —— 源码实现1_蓝天居士的博客-CSDN博客

XOpenDisplay函数很长，有500行左右。在此为了便于分析，列出函数名和前边变量定义的部分。代码如下：

/*
 * Connects to a server, creates a Display object and returns a pointer to
 * the newly created Display back to the caller.
 */
Display *
XOpenDisplay (
	register _Xconst char *display)
{
	register Display *dpy;		/* New Display object being created. */
	register int i;
	int j, k;			/* random iterator indexes */
	char *display_name;		/* pointer to display name */
	char *setup = NULL;		/* memory allocated at startup */
	int iscreen;			/* screen number */
	xConnSetupPrefix prefix;	/* prefix information */
	int vendorlen;			/* length of vendor string */
	union {
		xConnSetup *setup;
		char *failure;
		char *vendor;
		xPixmapFormat *sf;
		xWindowRoot *rp;
		xDepth *dp;
		xVisualType *vp;
	} u;				/* proto data returned from server */
	long setuplength;	/* number of bytes in setup message */
	long usedbytes = 0;     /* number of bytes we have processed */
	unsigned long mask;
	long int conn_buf_size;
	char *xlib_buffer_size;
 
……
}

余下的代码分块来进行分析。

第1部分代码

    /*
	 * If the display specifier string supplied as an argument to this
	 * routine is NULL or a pointer to NULL, read the DISPLAY variable.
	 */
	if (display == NULL || *display == '\0') {
		if ((display_name = getenv("DISPLAY")) == NULL) {
			/* Oops! No DISPLAY environment variable - error. */
			return(NULL);
		}
	}
	else {
		/* Display is non-NULL, copy the pointer */
		display_name = (char *)display;
	}

如果XOpenDisplay函数的输入参数display为空或者字符串结束符，则默认从环境变量中获取对应的值，赋给char *display_name；否则直接将display赋值给display_name。笔者环境变量DISPLAY对应的值为：

$ echo $DISPLAY
:0

在例程（同样参见《X11 Xlib截屏问题及深入分析二》）中是如此调用的XOpenDisplay：

Display *display = XOpenDisplay(NULL);

综上，最终得到display_name的值为":0"。

第2部分代码

/*
 * Set the default error handlers.  This allows the global variables to
 * default to NULL for use with shared libraries.
 */
	if (_XErrorFunction == NULL) (void) XSetErrorHandler (NULL);
	if (_XIOErrorFunction == NULL) (void) XSetIOErrorHandler (NULL);

这部分代码比较好理解，设置默认的错误处理函数和IO错误处理函数。

_XErrorFunction和_XIOErrorFunction是函数指针，在include/X11/Xlibint.h中定义：

extern int (*_XIOErrorFunction)(
    Display*	/* dpy */
);
extern int (*_XErrorFunction)(
    Display*		/* dpy */,
    XErrorEvent*	/* error_event */
);

第3部分代码

/*
 * Attempt to allocate a display structure. Return NULL if allocation fails.
 */
	if ((dpy = Xcalloc(1, sizeof(Display))) == NULL) {
		return(NULL);
	}
 
	if ((dpy->display_name = strdup(display_name)) == NULL) {
		OutOfMemory(dpy);
		return(NULL);
	}

这段代码也好理解，动态分配Display对象，赋给register Display *dpy；并且将dpy->display_name的值赋为":0"。

Display结构的定义在include/X11/Xlib.h中：

/*
 * Display datatype maintaining display specific data.
 * The contents of this structure are implementation dependent.
 * A Display should be treated as opaque by application code.
 */
#ifndef XLIB_ILLEGAL_ACCESS
typedef struct _XDisplay Display;
#endif
 
struct _XPrivate;		/* Forward declare before use for C++ */
struct _XrmHashBucketRec;
 
typedef struct
#ifdef XLIB_ILLEGAL_ACCESS
_XDisplay
#endif
{
	XExtData *ext_data;	/* hook for extension to hang data */
	struct _XPrivate *private1;
	int fd;			/* Network socket. */
	int private2;
	int proto_major_version;/* major version of server's X protocol */
	int proto_minor_version;/* minor version of servers X protocol */
	char *vendor;		/* vendor of the server hardware */
        XID private3;
	XID private4;
	XID private5;
	int private6;
	XID (*resource_alloc)(	/* allocator function */
		struct _XDisplay*
	);
	int byte_order;		/* screen byte order, LSBFirst, MSBFirst */
	int bitmap_unit;	/* padding and data requirements */
	int bitmap_pad;		/* padding requirements on bitmaps */
	int bitmap_bit_order;	/* LeastSignificant or MostSignificant */
	int nformats;		/* number of pixmap formats in list */
	ScreenFormat *pixmap_format;	/* pixmap format list */
	int private8;
	int release;		/* release of the server */
	struct _XPrivate *private9, *private10;
	int qlen;		/* Length of input event queue */
	unsigned long last_request_read; /* seq number of last event read */
	unsigned long request;	/* sequence number of last request. */
	XPointer private11;
	XPointer private12;
	XPointer private13;
	XPointer private14;
	unsigned max_request_size; /* maximum number 32 bit words in request*/
	struct _XrmHashBucketRec *db;
	int (*private15)(
		struct _XDisplay*
		);
	char *display_name;	/* "host:display" string used on this connect*/
	int default_screen;	/* default screen for operations */
	int nscreens;		/* number of screens on this server*/
	Screen *screens;	/* pointer to list of screens */
	unsigned long motion_buffer;	/* size of motion buffer */
	unsigned long private16;
	int min_keycode;	/* minimum defined keycode */
	int max_keycode;	/* maximum defined keycode */
	XPointer private17;
	XPointer private18;
	int private19;
	char *xdefaults;	/* contents of defaults from server */
	/* there is more to this structure, but it is private to Xlib */
}

Display结构中有很多成员，后边用到哪个的时候再针对于哪个进行详细说明。

为了便于理解，本文对于XOpenDisplay函数就分析到第3部分代码，接来下的文章继续分析其余部分。