Why isn't fscanf() storing this string in this pointer?

Question

Ok, so I am having trouble reading in the string names and storing them in my struct array of type student. The age field is an int and the gpa field is a double and those worked fine. Then when I added the name to be read I started running into issues. How can I successfully read in a name and store it in the student.name field? The code below crashes and I am not sure why. I am new in C so please tell me if there is a better way to do this. Thank you in advance. Oh and the name is consecutive letters with no spaces between characters.

typedef struct student{
     char *name; int age; double gpa;
}student; 

void read(char *filename) {
File *file = fopen(filename,"r");  
if(!file) return; 
student *students = malloc(sizeof(student)*100); 
int num_students = 10; //for example
int i; 
for (i=0;i<num_students;i++) {

    char *n = malloc(MAXLENGTH); 
    fscanf(file,"%s %i %lf", n,&students[i].age,&students[i].gpa); //<---runtime error occurs here
    strcpy(students[i].name,n);
    free(n);
} 

/*code here to do stuff with the array*/
free(students); 
fclosef(file); 
}

Please show the definition of the "student" structure, and what the "crash" error is, and on what line. We are not mind readers. Also, where is num_students defined and assigned a value? Please show **complete** code examples. — OldProgrammer, Feb 04 '14 at 15:12
Are there any spaces in the names? Does the student structure have a built-in array, or just a char pointer? — tabstop, Feb 04 '14 at 15:13
Also: Be sure to check the return value of `fscanf()`. `if (3 != fscanf(...)) Handle_Errror();` — chux - Reinstate Monica, Feb 04 '14 at 15:30

score 1 · Answer 1 · edited May 23 '17 at 10:31

1

Try to replace this line strcpy(students[i].name,n); with students[i].name = strdup(n);

I use strdup to allocate enough memory to hold the string, if you tried = it will work but will make name as read only! strcpy() will work too
Here you are reference for strdup use: strdup() - what does it do in C?

Also I don't see you allocating memory for students[i].name

edited May 23 '17 at 10:31

Community

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answered Feb 04 '14 at 15:13

Ahmed Hamdy

2,547
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tesseract · Accepted Answer · 2014-02-04T15:56:16.893

int n =0;
for (i=0;i<num_students;i++) {

char *n = malloc(MAXLENGTH); 
n = fscanf(file,"%s %i %lf", n,&students[i].age,&students[i].gpa);
if(n!= 3 || n = EOF){
    printf("invald input..");
}
students[i].name = malloc(strlen(n) +1);
if(student[i].name != NULL){
    strcpy(students[i].name,n);
}
free(n);
}

man fscanf

Upon successful completion, these functions return the number of successfully matched and assigned input items; this number can be 0 in the event of an early matching failure. If the input ends before the first matching failure or conversion, EOF is returned. If a read error occurs the error indicator for the stream is set, EOF is
returned, and errno is set to indicate the error.

Yes I forgot to malloc the student.name field for each element in the array. Thank you very much. — Rob L, Feb 04 '14 at 15:25

score 0 · Answer 3 · answered Feb 04 '14 at 15:26

0

Your problem is that the "string" into the file that you want to store into n array does not have null terminator. So %s does not work fine into the fscanf(). Try another option to that, as treat it as a cvs file.

answered Feb 04 '14 at 15:26

Abend

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Why isn't fscanf() storing this string in this pointer?

3 Answers3