C Using char* in fscanf causing error Segmentation fault: 11 - c

Question

I am new to C and I came across an issue when using fscanf to read all strings from a .txt file.
The code is as follow:
#include <stdlib.h>
#include <stdio.h>
int main() {
FILE *spIn;
char *numIn;
spIn = fopen("data.txt", "r");
if (spIn == NULL) {
printf("Can't Open This File \n");
}
while ((fscanf(spIn, "%s", numIn)) == 1) {
printf("%s\n", numIn);
};
fclose(spIn);
return 1;
}
This throws an error: Segmentation fault: 11.
The original data on txt file is:
1 2 345 rrtts46
dfddcd gh 21
789 kl
a mix of ints, strings, white space and newline characters.

Answer 1

At least 4 candidate undefined behaviors (UB) that could lead to a fault of some kind.
Code fails to pass to fscanf(spIn,"%s",numIn) an initialized pointer.
Code calls fscanf() even if fopen() fails.
Code calls fclose() even if fopen() fails.
No width limit in fscanf(spIn,"%s",numIn)), worse than gets().
Text files really do not have strings ('\0' terminated data) nor int, they have lines (various characters with a '\n' termination).
To read a line in and save as a string, use fgets(). Do not use fscanf() to read lines of data.
#include <stdlib.h>
#include <stdio.h>
int main() {
FILE *spIn = fopen("data.txt", "r");
if (spIn == NULL) {
printf("Can't Open This File \n");
} else {
char buf[100];
while (fgets(buf, sizeof buf, spIn)) {
printf("%s", buf);
}
fclose(spIn);
}
}

Answer 2

char* numIn is a pointer, and it is uninitalized, you can't really store anything in it, you need to either allocate memory for it or make it point to some valid memory location:
#include<stdlib.h> // for malloc
char* numIn = malloc(100); // space for 99 char + null terminator byte
//...
while ((fscanf(spIn, "%99s", numIn)) == 1)
{
printf("%s\n",numIn);
};
Or:
char str[100];
char *numIn = str;
Which in this small code makes little sense, you should probably make numIn a fixed size array to begin with:
char numIn[100];
Note that that you should use a width specifier in *scanf to avoid buffer overflow. This still has a problem though, it will read word by word, instead of line by line.
Looking at your input file, using fgets seems like a better option, it can read complete lines, including spaces:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main()
{
FILE *spIn;
char numIn[100];
spIn = fopen("data.txt", "r");
if (spIn != NULL)
{
while ((fgets(numIn, sizeof numIn, spIn)))
{
numIn[strcspn(numIn, "\n")] = '\0'; // removing \n
printf("%s\n", numIn);
}
fclose(spIn);
}
else
{
perror("Can't Open This File");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
Since fgets also parses the \n character, I'm removing it with strcspn.
Though you do verify the return value of fopen the execution continues even if it fails to open, I also addressed that issue.