从文件读取到字符串数组，C

Question

I'm trying to write a function that read string from a file into an array of strings using fgets(), but I keep getting zsh: segmentation fault at runtime.

Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void read(FILE*, char[][31],int*,int);
void printAll(char[][31],int*);

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


    FILE *in = fopen("strings.dat", "r");
    char strarr[50][31];
    int items=0;

    read(in,strarr,&items,31);
    printAll(strarr,&items);

}

void read(FILE *in,char arr[][31],int *items,int len){
    int i = 0;
    char temp[31];
    while (fgets(temp, len, in) != NULL) {
        strcpy(arr[i], temp);
        i++;
        items++;
    }
}
void printAll(char arr[][31],int *items){
    for(int i = 0;i<*items;i++){
        printf("String %d: %s\n",i+1,arr[i]);
    }
}

How can I fix it?

Answer 1

Your code is almost correct.

Errors:

1. In the read function items++ is wrong, this increments the pointer but you want to increment the variable pointed by the pointer, therefore you need (*items)++ instead.

2. Depending on the platform read will conflict with the posix read function.

---

The program will fail under following conditions:

1. If the file strings.dat does not exist, in will be NULL and doing file operations on a NULL FILE pointer results in undefined behaviour which often manifests in a seg fault. This is probably what happens to you.

2. If the file contains more than 50 lines you will be overflowing strarr which also results in undefined behaviour

3. If lines in the file have more than 31 characters, lines will be truncated and/or split.

---

Bonus:

The temp variable is not necessary, you can read directly into the array:

Instead of this:

 char temp[31];
 while (fgets(temp, len, in) != NULL) {
    strcpy(arr[i], temp);
    ...

You can have this:

 while (fgets(arr[i], len, in) != NULL) {
    ...

---

Final code with corrections:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void read(FILE*, char[][31], int*, int);
void printAll(char[][31], int*);

int main(int argc, char** argv) {
  FILE* in = fopen("strings.dat", "r");
  if (in == NULL)
  {
    printf("file does not exist\n");
    exit(1);
  }
  char strarr[50][31];
  int items = 0;

  read(in, strarr, &items, 31);
  printAll(strarr, &items);

}

void read(FILE* in, char arr[][31], int* items, int len) {
  int i = 0;
  while (fgets(arr[i], len, in) != NULL) {
    i++;
    (*items)++;
  }
}

void printAll(char arr[][31], int* items) {
  for (int i = 0; i < *items; i++) {
    printf("String %d: %s\n", i + 1, arr[i]);
  }
}

---

Alternative version of read

void read(FILE* in, char arr[][31], int* items, int len) {
  int i = 0;
  while (fgets(arr[i++], len, in) != NULL) {
    (*items)++;
  }
}

Even shorter:

void read(FILE* in, char arr[][31], int* items, int len) {
  for (int i = 0; fgets(arr[i], len, in) != NULL; i++)
    (*items)++;
}