Writing my own Cat function in C

#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; }

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.

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