Why won't this work ? Dynamic memory beginner program - c

Question

int main()
{
//FILE *out = fopen("keimeno.txt", "w+");
FILE *in = fopen("keimeno.txt", "r");
int fullbufflen=0 , i;
char buffer[100];
fgets(buffer, 100, in);
int bufflen = strlen(buffer);
char *text;
text =calloc(bufflen,sizeof(char));
char *strcat(text, buffer);
// printf("line of \"keimeno.txt\": %s", buffer);
// printf("size of line \"keimeno.txt\": %i\n\n", bufflen);
fullbufflen = bufflen;
while(fgets(buffer, 100, in)!=NULL)
{
// printf("line of \"keimeno.txt\": %s", buffer);
// printf("size of line \"keimeno.txt\": %i\n\n", bufflen);
text =realloc(text,bufflen*sizeof(char));
char *strcat(text, buffer);
fullbufflen = bufflen + fullbufflen ;
}
for (i = 0;i<fullbufflen;i++)
{
printf("%c\n",text[i]);
}
}
I am trying to copy the full text file (keimeno.txt) into a dynamic memory array, with a buffer of 100 character at most everytime. To test it at the end i tried to prinf the results. And i just cant get it to work. Dont know if there is a problem on the printf at the end, or the whole program is just wrong.
Also the dynamic array is supposed to have 0 size at the beggining, so if someone could tell me how to do that too, it would be welcome.
Thanks in advance.

Answer 1

There are a few issues:
char *strcat(text, buffer);
This is not how you call a function. This is a function declaration, and an incorrect one at that since it doesn't define the types of the arguments.
To call strcat, just do this:
strcat(text, buffer);
Next, you're not allocating enough space for your buffers:
text =calloc(bufflen,sizeof(char));
You need to add space for the null bytes that terminates the string:
text =calloc(bufflen + 1,sizeof(char));
Similarly here:
text =realloc(text,bufflen*sizeof(char));
This only reallocates a total of bufflen bytes. It does not add bufflen bytes to what was already allocated, and bufflen is unchanged from when it was first set outside of the while loop. Do this instead:
bufflen = strlen(buffer);
text =realloc(text,bufflen+fullbufflen+1);
This gives you enough space for the current length, the additional buffer, and the null byte.
Finally, make sure you fclose(in) and free(text) at the end to clean up your resources, and be sure to check the return value of fopen to ensure that the file opened successfully, and realloc/calloc to ensure that your allocations worked.
After the above changes, you code should look like this:
int main(void)
{
//FILE *out = fopen("keimeno.txt", "w+");
FILE *in = fopen("keimeno.txt", "r");
if (in == NULL) {
perror("open failed");
exit(1);
}
int fullbufflen=0 , i;
char buffer[100];
fgets(buffer, 100, in);
int bufflen = strlen(buffer);
char *text;
text =calloc(bufflen+1,sizeof(char));
if (text == NULL) {
perror("calloc failed");
exit(1);
}
strcat(text, buffer);
// printf("line of \"keimeno.txt\": %s", buffer);
// printf("size of line \"keimeno.txt\": %i\n\n", bufflen);
fullbufflen = bufflen;
while(fgets(buffer, 100, in)!=NULL)
{
// printf("line of \"keimeno.txt\": %s", buffer);
// printf("size of line \"keimeno.txt\": %i\n\n", bufflen);
bufflen = strlen(buffer);
text =realloc(text,bufflen+fullbufflen+1);
if (text == NULL) {
perror("realloc failed");
exit(1);
}
strcat(text, buffer);
fullbufflen = bufflen + fullbufflen ;
}
fclose(in);
for (i = 0;i<fullbufflen;i++)
{
printf("%c\n",text[i]);
}
free(text);
}

Answer 2

Here is one possible solution:
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#define ERR (-1)
#define GROW 100
static void die ( const char * msg ) { perror(msg); exit(1); }
int main ( void ) {
char * filename = "in.txt";
size_t buffSize = 0;
size_t buffUsed = 0;
ssize_t bytesRead = 0;
char * buffer = NULL;
char * tmp;
int fd;
fd = open(filename, O_RDONLY, 0);
if (fd == ERR) die("open");
do {
buffUsed += (size_t)bytesRead;
if (buffUsed == buffSize) {
tmp = realloc(buffer, buffSize += GROW);
if (tmp == NULL) die("realloc");
buffer = tmp;
}
bytesRead = read(fd, buffer + buffUsed, buffSize - buffUsed);
if (bytesRead == ERR) die("read");
} while (bytesRead > 0);
if (write(STDOUT_FILENO, buffer, buffUsed) == ERR) die("write");
free(buffer);
if (close(fd) == ERR) die("close");
return 0;
}
Like the original, the input filename is hard-coded which sub-optimal...