C中的奇怪int输出

Question

I am using OpenCV and I need to convert Iplimage->ID to char[ ] so that I can send it using TCP and then reconvert it to int on the server.

Here is the Iplimage header:

typedef struct _IplImage
{
    int  nSize;
    int  ID;  //<--- ID of type INT
    int  nChannels;
    int  alphaChannel;
    int  depth;
    char colorModel[4];
    char channelSeq[4];
    int  dataOrder;
    int  origin;
    int  align;
    int  width;
    int  height;
    struct _IplROI *roi;
    struct _IplImage *maskROI;
    void  *imageId;
    struct _IplTileInfo *tileInfo;
    int  imageSize;
    char *imageData;
    int  widthStep;
    int  BorderMode[4];
    int  BorderConst[4];
    char *imageDataOrigin;
}
IplImage;

this is my code:

char IDbuffer[10];
snprintf(IDbuffer,10,"%e",frame->ID);//where frame is of type IplImage*
printf("frame->ID= %a\n",IDbuffer);

and this what I got printed:

frame->ID= 0x0.0000000037d0cp-1022

even trying

printf("frame->ID= %a\n",frame->ID);

give me the same output.

Is this an integer format ?? and if yes how could I convert the char * of this format to an int??

Thanks in advance.

Answer 1

Use the %d format specifier since frame->ID is an integer:

snprintf(IDbuffer,10,"%d",frame->ID);

And then use the %s format specifier to print the buffer:

printf("frame->ID= %s\n",IDbuffer);

There's more information about the format specifiers in printf man page.

Answer 2

%e format specifier requires an argument of type double , while you are passing an int instead. The resultant behavior is undefined. That's all there is to it.

By using %e you are making a promise to snprintf . You promise that you will supply a double argument in the corresponding position. Later you break that promise by supplying an int instead of double , which leads to a meaningless result. You are essentially lying to snprintf about the type of frame->ID .

%a also requires a double argument. Why do you insist on using double format specifiers with a int argument? How do you expect this to work?

Either supply an argument of correct type (which is double ), or use the proper format specifier (which is %d for int ). Either this or that. Mixing things up like you do will not achieve anything.

Answer 3

You need cast ID to double before printing it:

snprintf(IDbuffer,10,"%e", (double)frame->ID);

Alternatively, you can print it as integer:

snprintf(IDbuffer,10,"%d", frame->ID)

Check this for more information on snprintf