Trying to read first line of file using fgets

Question

I'm trying to write a program that will read the first line of a txt file and output it. I can't get the fgets to display in my fprintf statement when running the program.

#include "example.h"

// function: main

int main(int argc, const char** argv) {

  //declare filepointer
  FILE *fp;

  //declare variables
  char c[10];
  long size;

  //open file
  fp = fopen(argv[1], "r");

  //first output read dimension of vector
  fgets (c, 10, fp);
  fprintf(stdout, "the dimension of the vector is %s\n", c);
  fclose(fp);
  return 0;
}

The text file has the following contents:

4
45.0
62.0
55.6
8.2

But my output is just reading as: the dimension of the vector is

Answer 1

You always need to be careful of what the user is going to enter as input in the program. To do this, as mentioned in the comment, we should always check if the fopen() , fgets() , and the arguments are correctly passed before taking them into use.

---

Let's try the following code:

#include <stdio.h>

#define EXIT_FAILURE 1
#define MAX_LEN      16

int main(int argc, char *argv[]) {
    FILE *fp = NULL;
    char c[MAX_LEN];

    if (argc != 2) {
        fprintf(stdout, "Usage: %s <file>\n", argv[0]);
        return EXIT_FAILURE;
    }

    fp = fopen(argv[1], "r");

    if (fp == NULL) {
        perror(argv[1]);

        fclose(fp); // Cleanup
        return EXIT_FAILURE;
    }

    if (fgets(c, sizeof(c), fp) == NULL) {
        fprintf(stderr, "File read error.\n");
        fclose(fp); // Cleanup
        return EXIT_FAILURE;
    }

    fclose(fp);
    fprintf(stdout, "The dimension of the vector is %s\n", c);

    return 0;
}

In this case, I've got those same values as yours in the data.txt file.

The program outputs:

rohanbari@genesis:~/stack$ ./a.out data.txt
The dimension of the vector is 4