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I would like to write a program in C that gets the file content via stdin and reads it line by line and, for each line, converts it to an array of 8-bit integer values.

I also would like to be able to do the reverse process. After working with my array of 8-bit values, I would like to convert it again to "lines" that would be organized as a new buffer.

So basically, I would like to convert a char * line to an int array[] and back (an int array[] to a char * line) while keeping the consistency, so when I create the file again out of the conversions, the file is valid (and by valid I mean, the conversion from int array[] to char * line generates the same content of the original char * line, while reading each line of the stdin.

My code is currently as follows:

#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *stream;
    char *line = NULL;
    size_t len = 0;
    ssize_t read;

    stream = stdin;
    if (stream == NULL)
        exit(EXIT_FAILURE);

    while ((read = getline(&line, &len, stream)) != -1) {
        char * array = line_to_array(line);
        // here I include the rest of my code
        // where I am going to use the generated array
        // ...
    }

    free(line);
    fclose(stream);
    exit(EXIT_SUCCESS);
}

The line_to_array function would be the one to convert the "line" content to the array of integers. In a second file, I would just do the opposite.

The mechanics of the process would be like this:

The first program (first.c) would receive a file content via stdin. By reading it using getline, I would have each line to convert to an array of integers and send each line to a second program (second.c) that would convert each array to a char * buffer again and the reconstruct the file.

In the terminal, I would run it like this:

./first | ./second

I appreciate any help on this matter.

Thank you.

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  • What is the difference between a line of ASCII text in a buffer and an array of 8-bit values, (other than maybe a terminating char)? Commented Jul 17, 2017 at 18:45
  • @martin-james I need to manipulate the data as numbers since I am going to turn the numbers into the input of some equations. I want to mathematically operate on them. That's why I can't work with ASCII text. Should I cast each char to an int? I just wanted to see if there is a preferred method to do so. Commented Jul 17, 2017 at 19:51
  • It would be helpful if an example of a line of input were provided. Do all lines contain the same number of entries? Are line entries decimal digits? Non-negative? Characters of the Latin Alphabet? etc.... Commented Jul 17, 2017 at 20:14
  • 1
    You should know that C places few restrictions on the encoding used for the execution character set; it need not be ASCII or UTF-8 (likely, though). A char is an arithmetic type, and you can do mathematical operations on these values; no need to cast. You may want to store results in a wider type, such as int, to avoid overflow issues (depending on the nature of the mathematical manipulations). Commented Jul 17, 2017 at 20:37
  • 1
    As far as I can tell, what you propose is fine. In an expression, char values will be converted to int implicitly. It may ease your mind to read about the integer promotions and the usual arithmetic conversions. Commented Jul 17, 2017 at 21:20

1 Answer 1

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I believe you may already know that a name of array is a kind of constant pointer. You could verify the fact from following code:

char hello[] = "hello world!";

for( int idx=0; *(hello + idx) != 0; idx++ )
{
    printf("%c", *(hello + idx));
}
printf("\n");

So, there are no reason to convert character pointer to array. For your information, A char variable is a 8bit data in C, this can contain a integer value which is represent a character: 65 represent 'A' in ASCII code.

Secondly, this link may help you to understand how to convert between c string and std::string.

On second thought, may your input file is UNICODE or UTF-8 encoded file which is using multi-byte character code. In that case, you may not able to use getline() to read the string from the file. If so, please refer this question: Reading unicode characters.

I wish following code assist you to understand char type, array and pointer in C/C++:

std::string hello("Hello world");
const char *ptr = hello.c_str();

for( int idx=0; idx < hello.size(); idx++ )
{
    printf("%3d ", *(ptr + idx));
}
printf("\n");

std::string hello("Hello world");
const char *ptr = hello.c_str();

for( int idx=0; idx < hello.size(); idx++ )
{
    printf("%3d ", ptr[idx]);
}
printf("\n");
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6 Comments

@DavidBowling Thank you for your commenting. I wasn't able to explaining the relationship between pointer and name of array. So, tell 'a kind of lie'. :D
Hi @joohae-kim , thank you for your answer. The only reason I am looking for numbers is that I need to mathematically operate on them. I am creating a way of transform data from files in a mathematical way. So I need to work with numbers. In Ruby, I am able to do that with the method unpack for String and pack for Array. My ruby code does the following: array = line.unpack("C*") and to revert the process I do: line = array.pack("C*") . Basically, I am working with the same data but in different formats. I just wanted to check if there is any recommended way to do this in C.
@DWS, in C/C++, null terminated string is a sequence of char, and char is 8 bit integer. so no need to convert between them.
@DWS, I think you could assume this: const char* array = line.c_str(); may similar to array = line.unpack("C*") and std::string line(array) also similar to line = array.pack("C*"). it's very long time since I was using Ruby , I'm not 100% sure. I wish additional answer and comment help you.
@DWS, can I see a line of input data? I think we are not in the same page.
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