基本C矩阵分段内存分配

Question

I tried to run this program through the terminal and this error showed up. "Segmentation Fault: 11"

I would like to know why . What this program does is, it reads a .ppm file and it saves it's information in a matrix variable of type Pixel, so, a PPM file is basically composed by: the first line is going to be "P3" by default, second line the size of the matrix, and the third line the highest value possible for a Pixel attribute, the other lines will have 3 integers of maximum value of 255, so for each member of the matrix there will be a pixel R, G, B. what I tried to do in the function save_image, first recognize if we are dealing with a ppm file(checking if there is a P3 in the first line), then read the number of rows and columns for the matrix, then it creates a new matrix using the malloc function, then it will save the data in the lines of the file to the .r and .g and .b of the variable myImg. I am very new to debugging/programming so I'm sorry if this isn't enough information but I tried my best.

#include <stdio.h>
#include <stdlib.h>
typedef struct{
    int r;
    int g;
    int b;
}Pixel;

void save_image(FILE* img, Pixel ** newImg) {
    int i;
    int j;
    int fcount;
    int scount;
    int count;
    int dcc;
    char init[3];
    fscanf(img,"%s",init);
    if(init[0]=='P' && init[1]=='3'){
        printf("worked!\n");
        fscanf(img,"%d %d",&j,&i);
        fscanf(img, "%d",&dcc);
        *newImg = (Pixel*)malloc(sizeof(Pixel) * i);
        for ( count = 0; count < i ; ++count)
        {
            newImg[count] = (Pixel*)malloc(sizeof(Pixel) * j);
        }
        for (fcount = 0; fcount <= i ; ++fcount)
        {
         for (scount = 0; scount <= j; ++scount)
         {
            fscanf(img,"%i %i %i",&newImg[i][j].r,&newImg[i][j].g,&newImg[i][j].b);
         }
        }
    }
    else 
        printf("Type of file not recognized\n");

    fclose(img);
}


int main(int argc, char const *argv[])
{
    FILE* image;
    Pixel myImg;
    Pixel** newImg;
    **newImg = myImg;
    image = fopen(argv[1],"r");
    save_image(image,newImg);
    return 0;
}

Answer 1

The program fails because the initial malloc for newImg[] is malloc'ing some multiple of the size of Pixel rather than size of pointer to Pixel combined with problems with the passing of the pointer to the newImg as a parameter to the save_image() function. See my comment about where the variable newImg should be defined and the desirable modification to the declaration of the save_image() function

Given the was the posted code is written, it seems to be expecting the 'plain' .ppm file format

and the posted code is failing to allow for any embedded comments within the file

given this description of the format of a .ppm file:

The format definition is as follows. You can use the libnetpbm C subroutine library to read and interpret the format conveniently and accurately.

A PPM file consists of a sequence of one or more PPM images. There are no data, delimiters, or padding before, after, or between images.

Each PPM image consists of the following:

A "magic number" for identifying the file type. A ppm image's magic number is the two characters "P6".
Whitespace (blanks, TABs, CRs, LFs).
A width, formatted as ASCII characters in decimal.
Whitespace.
A height, again in ASCII decimal.
Whitespace.
The maximum color value (Maxval), again in ASCII decimal. Must be less than 65536 and more than zero.
A single whitespace character (usually a newline).
A raster of Height rows, in order from top to bottom. Each row consists of Width pixels, in order from left to right. Each pixel is a triplet of red, green, and blue samples, in that order. Each sample is represented in pure binary by either 1 or 2 bytes. If the Maxval is less than 256, it is 1 byte. Otherwise, it is 2 bytes. The most significant byte is first.

A row of an image is horizontal. A column is vertical. The pixels in the image are square and contiguous.

In the raster, the sample values are "nonlinear." They are proportional to the intensity of the ITU-R Recommendation BT.709 red, green, and blue in the pixel, adjusted by the BT.709 gamma transfer function. (That transfer function specifies a gamma number of 2.2 and has a linear section for small intensities). A value of Maxval for all three samples represents CIE D65 white and the most intense color in the color universe of which the image is part (the color universe is all the colors in all images to which this image might be compared).

ITU-R Recommendation BT.709 is a renaming of the former CCIR Recommendation 709. When CCIR was absorbed into its parent organization, the ITU, ca. 2000, the standard was renamed. This document once referred to the standard as CIE Rec. 709, but it isn't clear now that CIE ever sponsored such a standard.

Note that another popular color space is the newer sRGB. A common variation on PPM is to substitute this color space for the one specified.

Note that a common variation on the PPM format is to have the sample values be "linear," i.e. as specified above except without the gamma adjustment. pnmgamma takes such a PPM variant as input and produces a true PPM as output. 

Strings starting with "#" may be comments, the same as with PBM.

Note that you can use pamdepth to convert between a the format with 1 byte per sample and the one with 2 bytes per sample.

All characters referred to herein are encoded in ASCII. "newline" refers to the character known in ASCII as Line Feed or LF. A "white space" character is space, CR, LF, TAB, VT, or FF (Ie what the ANSI standard C isspace() function calls white space). Plain PPM

There is actually another version of the PPM format that is fairly rare: "plain" PPM format. The format above, which generally considered the normal one, is known as the "raw" PPM format. See pbm for some commentary on how plain and raw formats relate to one another and how to use them.

The difference in the plain format is:

There is exactly one image in a file.
The magic number is P3 instead of P6.
Each sample in the raster is represented as an ASCII decimal number (of arbitrary size).
Each sample in the raster has white space before and after it. There must be at least one character of white space between any two samples, but there is no maximum. There is no particular separation of one pixel from another -- just the required separation between the blue sample of one pixel from the red sample of the next pixel.
No line should be longer than 70 characters. 

Here is an example of a small image in this format.

 P3
 # feep.ppm
 4 4
15
 0  0  0    0  0  0    0  0  0   15  0 15
 0  0  0    0 15  7    0  0  0    0  0  0
 0  0  0    0  0  0    0 15  7    0  0  0
15  0 15    0  0  0    0  0  0    0  0  0

There is a newline character at the end of each of these lines.

Programs that read this format should be as lenient as possible, accepting anything that looks remotely like a PPM image.