How to use fgets() in 2d-arrays (multiple dimension arrays)? - c

Question

#include <stdio.h>
#include <stdlib.h>
void *salloc(int x){
char **pointer;
int i;
pointer = malloc(sizeof(char)*x);
if(pointer == NULL){
exit(-1);
}
for(i=0; i<x; i++){
pointer[i] = malloc(sizeof(char) * 20);
if(pointer[i] == NULL){
exit(-1);
}
}
return pointer;
}
void Input(int value, char **array){
for(i = 0; i < value; i++){
printf("%d ----\n", i);
fgets(array[i], 20, stdin);
printf("%d ----\n", i);
}
}
int main(int argc, char *argv[]){
char **array;
int value = 2;
array = salloc(value);
Input(value, array);
return 0;
}
The general idea, can be that I miss some syntax.
So I want to read in a string with spaces. If I run this for the value 2, it will print:
0 ----
0 ----
1 ----
"some string"
and it crashes after I press enter.
If I do this with value 1:
it immediately crashes.
However if I replace fgets() with:
scanf("%s", array[i]);
it works (except for the spaces).
So how does fgets() work in 2d-arrays?
Because I get it to work in 1d-arrays. And for some reason I can print 1d-arrays from row 2 when the array only has 2 rows, so it should only be able to print from rows 0 and 1 right?

Answer 1

Here is a demonstrative program that shows how fgets can be used with a 2D array.
#include <stdio.h>
#define N 5
#define M 10
int main( void )
{
char lines[N][M];
size_t n = 0;
while( n < N && fgets( lines[n], sizeof( *lines ), stdin ) != NULL ) ++n;
for ( size_t i = 0; i < n; i++ ) puts( lines[i] );
return 0;
}
If to enter for example
One
Two
Three
Four
Five
then the program output will be the same
One
Two
Three
Four
Five

Answer 2

When you do
pointer = malloc(sizeof(char)*x);
you only allocate x characters (i.e. bytes), not pointers to characters. Change to
pointer = malloc(sizeof(char*)*x);
Without the change, you might go out of bounds and have undefined behavior. And this is exactly what happens in your code, you allocate only two bytes to store two pointers, and a single pointer is either four or eight bytes, so you don't allocate enough memory for even a single pointer.
Undefined behavior is a common cause of crashes, but sometimes it might also seem to work.

Answer 3

Note also when taking input with fgets (or any of the line-oriented input functions), fgets will read up to, and include, the '\n' at the end of each line read. You should perform 2 additional tests/operations. (1) you should test that the last character read by fgets is in fact the '\n' character. If it is not, that will indicate your input was truncated by fgets at the length you specified in the second parameter to fgets, and additional character remain unread for that line. Without this check and some way to handle lines that exceed the specified width, you next call to fgets will read the remaining characters for the current line as your next line of input.
(2) you should remove the newline included by fgets to prevent your strings from containing embedded '\n' characters at the end. (if you are simply parsing numbers from the line and not storing it as a string, this can be handled in several different manners). But, for the general case, you use strlen to locate the end of the string, and then overwrite the '\n' with a nul-terminating character.
In addition to the above, it may make more sense to allocate memory for each individual pointer in array only after a line of data has been read to prevent over-allocating space in your code. Since you are specifying that the allocation size for each string will be 20, a simple character buffer of that size can be used to take input, and after confirming input, you can allocate storage for that line in array. A short example of your function with the checks included and with allocation included in input would be:
#define MAXC 20 /* max chars per read */
...
void Input (int value, char **array)
{
int i = 0;
size_t len = 0;
char buf[MAXC] = {0};
while (i < value && fgets (buf, MAXC, stdin)) {
printf ("%d ----\n", i);
len = strlen (buf); /* get length */
if (len + 1 == MAXC && buf[len-1] != '\n') /* validate read */
fprintf (stderr, "warning: chars exceed MAXC, line[%d]\n", i);
else
buf[--len] = 0; /* strip '\n' */
printf ("%d ----\n", i);
array[i++] = strdup (buf); /* allocate/copy */
}
}
Lastly, why choose void as the function type? Why not return the number of values read into array if values are successfully read into array, or 0 otherwise. This will at least allow some indication of success or failure of your read and provide a way of returning the number of lines allocated back to the calling function.
An example of your code incorporating the adjusted allocation, necessary checks, and useful return types would be:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXC 20
void *salloc (int x)
{
char **pointer;
pointer = malloc (sizeof *pointer * x);
if (pointer == NULL)
exit(-1);
return pointer;
}
int input (int value, char **array)
{
int i = 0;
size_t len = 0;
char buf[MAXC] = {0};
while (i < value && fgets (buf, MAXC, stdin)) {
printf ("%d ----\n", i);
len = strlen (buf); /* get length */
if (len + 1 == MAXC && buf[len-1] != '\n') /* validate read */
fprintf (stderr, "warning: chars exceed MAXC, line[%d]\n", i);
else
buf[--len] = 0; /* strip '\n' */
printf ("%d ----\n", i);
array[i++] = strdup (buf); /* allocate/copy */
}
return i;
}
int main (int argc, char *argv[]) {
int i, nlines, value = argc > 1 ? atoi (argv[1]) : 2;
char **array;
array = salloc (value);
if (!(nlines = input (value, array))) /* validate input */
return 1;
for (i = 0; i < nlines; i++) /* print input */
printf (" array[%2d] : %s\n", i, array[i]);
for (i = 0; i < nlines; i++) /* free memory */
free (array[i]);
free (array);
return 0;
}
Test Input File
The following is a test input file where line 1 (the 2nd line) exceedS 20 characters:
$ cat dat/captnjack.txt
This is a tale
Of Captain Jack Sparrow
A Pirate So Brave
On the Seven Seas.
Example Output
$ ./bin/readarray 10 <../dat/captnjack.txt
0 ----
0 ----
1 ----
warning: chars exceed MAXC, line[1]
1 ----
2 ----
2 ----
3 ----
3 ----
array[ 0] : This is a tale
array[ 1] : Of Captain Jack Spa
array[ 2] : rrow
array[ 3] : A Pirate So Brave
array[ 4] : On the Seven Seas.
Don't forget to use a memory error check program like valgrind to validate your use of the memory you allocate and to insure you have freed it when it is no longer needed. Let me know if you have any additional questions.