如何操作字符指针内容？

Question

I am writing a C program that expects a filename as its argument. The program will read the contents of the file, and will output an other file (with a similar name, same extension but with an appended characters at the end of the name).

For example, if I run the binary like this:

./a.out some_file.txt

Then a new file will be created some_file_out.txt .

 New File Created: [some_file_out.txt]

My code below works, but it is too ugly. I am sure there is a better way of doing this. I considered working with std::string and that would simplify things, but I won't be able to use the string value with fopen() since it only accepts char* variables the filename.

What is a better way of manipulating a char* variable?

#include <stdio.h>
#include <string.h>

int main(int argc, char* argv[]) {
    // validate argc count and that argv[1] file exists
    // assume all validation is done. For simplicity's sake.

    char* input_file = argv[1];
    /* do something with the file*/

    char output_file[strlen(argv[1]) + 4];  // reserve a new char[] with the same size
                                            //  as the input file, plus 4 characters for
                                            //  the output filename variation

    // assuming the extension is 3 characters long in all cases
    int dot_location = strlen(argv[1])-4;

    // copy characters from input filename to output filename
    for(int i=0; i<dot_location ; i++) {
        output_file[i] = input_file[i];
    }

    // add the name variation:
    output_file[dot_location  ] = '_';
    output_file[dot_location+1] = 'o';
    output_file[dot_location+2] = 'u';
    output_file[dot_location+3] = 't';

    // add the extension back
    for(int i=0; i<4; i++) {

        int new_index = dot_location + i + 4;
        int old_index = dot_location + i;

        output_file[new_index] = input_file[old_index];
    }

    // make sure the last character is a null value (not required)
    // output_file[dot_location+8] = '\0';

    // do something with the output file...
    printf("New File Created: [%s]\n", output_file);

}

Answer 1

You could use std::string for this usage, just use str.c_str() .

But to answer your question about C coding. I do something like this :

// find extension position
int ext_pos = strrchr(filename, '.') - filename;
// allocate new string
char newname[strlen(filename) + 4 + 1]; // +1 for null byte

strncpy(newname, filename, ext_pos); // copy first part
strncpy(&newname[ext_pos], "_out", 4); // write _out
strcpy(&newname[ext_pos + 4], &filename[ext_pos]); // copy extension and add null byte