Saving a string from a text file to a struct using fscanf (C)

Question

Sample Text file:

234765 PETER 
867574 SMITH

I'm trying to take the id and string from the text file and save it into a struct. The id is saving fine but the string isn't.

typedef struct student 
{
    int id[DATA_SIZE];
    char *student[DATA_SIZE];
}studentinfo;
studentinfo list;

struct student *create_space(int size)
{
    struct student *tmp = (struct student*)malloc(size*sizeof(struct student));
    return(tmp);
}
struct student * readData(struct student*pointer,studentinfo v)
{
    int count =0;
    int tmpid;
    char str[256];
    FILE* in_file; 
    in_file = fopen("studentlist.txt","r");
    while(fscanf(in_file,"%d",&tmpid)!= EOF && count<DATA_SIZE)
    {
        fscanf(in_file,"%s",v.student[count]);
        //printf("%s\n",str );
        v.id[count]=tmpid;
        count++;
    }
    pointer =&v;
    return pointer;

}
int main()
{
    struct student *data;
    struct student *sdata;
    data = create_space(1);
    sdata = readData(data,list);
    //printf("%s\n",sdata->student[2] );

}

char *student[DATA_SIZE]; --> char student[DATA_SIZE][NAME_SIZE]; — BLUEPIXY
– BLUEPIXY, Commented Feb 6, 2017 at 1:13
pointer =&v;return pointer; : v is local variable. So Can't use this return value. Also pointer don't need as argument of this function. — BLUEPIXY
– BLUEPIXY, Commented Feb 6, 2017 at 1:15
How would I then assign the string value obtained from the text to the struct student[DATA_SIZE][NAME_SIZE]? — nnmast
– nnmast, Commented Feb 6, 2017 at 1:55
If v is a pointer, fscanf(in_file,"%s", v->student[count]);. If v isn't a pointer, fscanf(in_file,"%s", v.student[count]);. — BLUEPIXY
– BLUEPIXY, Commented Feb 6, 2017 at 1:58
i have another question, why can't i declare a variable char str[32]; within the function and do fscanf(in_file,"%s", str);, then v.student[count]=str;. It gives me an error. why is that? — nnmast
– nnmast, Commented Feb 6, 2017 at 2:14

RoadRunner · Accepted Answer · 2017-02-06 03:31:32Z

Their are a couple of issues:

fscanf() reads formatted input, and returns the number of items read.

This line:
```
while(fscanf(in_file,"%d",&tmpid)!= EOF && count<DATA_SIZE)
```
Could be this:
```
while (count < DATA_SIZE && fscanf(in_file, "%d %255s", &list.id[count], str) == 2) {
```
Which verifies that 2 values are being read on each line successfully.
You are not checking if in_file returns NULL. It's safe to do this. This goes the same for malloc().

You need to correctly create space for char *students[DATA_SIZE], as this is an array of char * pointers. Once you allocate space for this via malloc() or strdup(), then you can copy the contents into students.

Here is an example of doing such a thing:

while (count < DATA_SIZE && fscanf(in_file, "%d %255s", &list.id[count], str) == 2) {

    /* allocate space for one student */
    list.student[count] = malloc(strlen(str)+1);
    if (!list.student[count]) {
        printf("Cannot allocate string\n");
        exit(EXIT_FAILURE);
    }

    /* copy it into array */
    strcpy(list.student[count], str);
    count++;
}

Here is an example that you can use to help achieve your desired result:

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

#define DATA_SIZE 256

typedef struct {
    int id[DATA_SIZE];
    char *student[DATA_SIZE];
} studentinfo_t;

int main(void) {
    FILE *in_file;
    studentinfo_t list;
    char str[DATA_SIZE];
    size_t count = 0;

    in_file = fopen("studentlist.txt", "r");
    if (!in_file) {
        fprintf(stderr, "%s\n", "Error reading file");
        exit(EXIT_FAILURE);
    }

    while (count < DATA_SIZE && fscanf(in_file, "%d %255s", &list.id[count], str) == 2) {
        list.student[count] = malloc(strlen(str)+1);
        if (!list.student[count]) {
            printf("Cannot allocate string\n");
            exit(EXIT_FAILURE);
        }
        strcpy(list.student[count], str);
        count++;
    }

    for (size_t i = 0; i < count; i++) {
        printf("%d %s\n", list.id[i], list.student[i]);
    }

    return 0;
}

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