C:永远不会返回语句

Question

我正在实现一个程序，它将消息隐藏在 ppm 文件中，然后检索它们。我需要做的最后一件事是，如果打开文件出现问题，则“返回 2”。如果我输入一个无法打开的文件(文件不存在或扩展名错误等)，那么它将在 stderr 上显示错误消息，但由于某种原因它不会执行下一行，“return 2 ”!

然后它只是返回 0 而不是返回 2。

int load_ppm_image(const char *input_file_name, unsigned char **data, int *n,
               int *width, int *height, int *max_color)
{
    char line[80];
    char c;

    FILE * image_file;
    //image_file = fopen(input_file_name, "rb");

    if (fopen(input_file_name, "rb") == NULL)  // checks to see if file cant be opened
    {
        fprintf(stderr, "The input image file could not be opened\n");
        return 2;  // why isn't this being returned???
    }
    else
    {
        image_file = fopen(input_file_name, "rb");
    }

    fscanf(image_file, "%s", line);

    fscanf(image_file, "%d %d", width, height);

    *n = (3 * (*width) * (*height));

    fscanf(image_file, "%d%c", max_color, &c);
    *data = (unsigned char *)malloc((*n)*sizeof(unsigned char));
    size_t m = fread(*data, sizeof(unsigned char), *n, image_file);

    assert(m == *n);
    fclose(image_file);
    return 0;
}

int hide_message(const char *input_file_name, const char *message, const char *output_file_name)
{
    unsigned char * data;
    int n;
    int width;
    int height;
    int max_color;

    n = 3 * width * height;
    int code = load_ppm_image(input_file_name, &data, &n, &width, &height, &max_color);

    if (code)
    {
        // return the appropriate error message if the image doesn't load correctly
        return code;
    }

    int len_message;
    int count = 0;
    unsigned char letter;

    // get the length of the message to be hidden
    len_message = (int)strlen(message);

    for(int j = 0; j < len_message; j++)
    {

        letter = message[j];
        int mask = 0x80;

        // loop through each byte
        for(int k = 0; k < 8; k++)
        {
            if((letter & mask) == 0)
            {
                //set right most bit to 0
                data[count] = 0xfe & data[count];
            }
            else
            {
                //set right most bit to 1
                data[count] = 0x01 | data[count];
            }
            // shift the mask
            mask = mask>>1 ;
            count++;
        }
    }
    // create the null character at the end of the message (00000000)
    for(int b = 0; b < 8; b++){
        data[count] = 0xfe & data[count];
        count++;
    }

    // write a new image file with the message hidden in it
    int code2 = write_ppm_image(output_file_name, data, n, width, height, max_color);

    if (code2)
    {
        // return the appropriate error message if the image doesn't load correctly
        return code2;
    }

    return 0;
}

<小时/>

编辑:

int main(int argc, const char * argv[])
{
    if (argc < 2 || argv[1][0] != '-')
    {
        // the user didn't enter a switch
        fprintf(stderr, "Usage: no switch was entered.\n");
        return 1;
    }

    else if ((strcmp(argv[1], "-e") == 0) && (argc == 5))
    {
        // hides a message in an output file
        hide_message(argv[2], argv[3], argv[4]);
    }

    else if ((strcmp(argv[1], "-d") == 0) && (argc == 3))
    {
        // retrieves a hidden message in a given file
        retrieve_message(argv[2]);
    }

    else
    {
        // all other errors prompt a general usage error
        fprintf(stderr, "Usage: error");
        return 1;
    }
}

Answer 1

程序结束，退出代码:0

C 程序的退出代码是显式 exit 函数调用中 main 的返回值或 status 值。在您的情况下，load_ppm_image 返回一个值到其调用函数 hide_message，而该函数又将相同的值返回到其调用函数 main。但是，对于调用 hide_message 的情况，main 不会显式返回任何内容。 C 标准指定，如果 main 到达函数末尾而没有显式返回，则隐式返回 0。因此，在您的情况下，退出代码为 0。

要获得您想要的行为，请修改您的 main 代码以返回 hide_message 返回值。

else if ((strcmp(argv[1], "-e") == 0) && (argc == 5))
{
    // hides a message in an output file
    return (hide_message(argv[2], argv[3], argv[4]));
}