Writing my own Cat function in C

Question

Hi i don't know how to simulate my own Cat function in C, i know how it works when no arguments are set and i already get it, but my problem is when i tried to open a file and then print itself...

my code until now:

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

int main(int argc, char* argv[])
{  
    char *a1 = (char*) malloc (sizeof(char));
    int sz, fd,cont=0, cont1=0;
    char *b1 = (char*) malloc (sizeof(char));
    //char *a2 = (char*) malloc (sizeof(char));
    char * a2;
    char *b2 = (char*) malloc (sizeof(char));

    // NO PARAMETERS
    while (argc == 1){      
        sz=read(0, a1, 1);
        b1[cont]=a1[0];

        if(b1[cont]=='\n'){
            write(1,b1,cont);
            write(1,"\n",1);
            b1=NULL;            
        }

        cont=cont+1;
        b1=(char*) realloc(b1, sizeof(char)*cont);
      }

    // 1 PARAMETER (FILE)   /*Here is the problem*/
    if (argc > 1){

        fd=open(argv[1],O_RDONLY);
        a2=fgetc(fd);

        while (a2 != EOF){
            b2[cont1]=a2;
            cont1=cont1+1;
            b2=(char*) realloc (b2, sizeof(char)*cont1+1);
            a2=fgetc(fd);
        }

        write(1,b2,cont);
        b2=NULL;
        close(fd);  
    }

    return 0;
}

What am i supposed to do ?

Answer 1

If you are using open() and close(), you cannot use fgetc(). You need to use fopen() and fclose() to be able to use fgetc().

Either way, you need a function which can be called with either the standard input (spelled 0 or stdin) or with the file that was opened (fd or fp are conventional names for 'file descriptor' and 'file pointer'). You can specify the output stream too. Hence, for example, the interfaces:

int cat_fd(int ifd, int ofd);
int cat_fp(FILE *ifp, FILE *ofp);

Your main program then calls your chosen function with the standard input and standard output or with the opened file and standard output.

---

Additionally, you have:

char *a1 = (char*) malloc (sizeof(char));

Ignoring the cast, this is an expensive way of writing:

char a1[1];

---

Your loops are reading a single character at a time. This is OK with the file streams from <stdio.h>, but is bad for performance if you're using file descriptors. You should be reading block of, say, 4096 characters at a time.

int cat_fd(int ifd, int ofd)
{
    char buffer[4096];
    ssize_t nbytes;
    while ((nbytes = read(ifd, buffer, sizeof(buffer))) > 0)
    {
        if (write(ofd, buffer, nbytes) != nbytes)
            return -1;
    }
    return (nbytes < 0) ? -1 : 0;
}

You don't need the dynamic memory allocation; it is only confusing you and wasting time in the program. The code in the main() function then looks more like:

if (argc == 1)
{
    if (cat_fd(0, 1) != 0)
        fprintf(stderr, "failed to copy standard input\n");
}
else
{
    for (int i = 1; i < argc; i++)
    {
        int fd = open(argv[i], O_RDONLY);
        if (fd < 0)
            fprintf(stderr, "failed to open %s for reading\n", argv[i]);
        else
        {
            if (cat_fd(fd, 1) != 0)
                fprintf(stderr, "failed to copy %d to standard output\n", argv[i]);
            close(fd);
        }
    }
}

Rewriting to use cat_fp() is an exercise for the reader. You might find Tried and true simple file copying code in C relevant.