Segmentation fault while reading file

Question

My file looks like:

123456789

My code gives me segmentation fault:

#include <stdio.h>

int main(){

    FILE *f; 
    char ch[5];
    f = open("a.txt", "r");
    fgets( ch, 4, f); 
    ch[4] = NULL;
    printf("%s", ch); //Fixed
    return 0;
}

I am an absolute beginner. What am I doing wrong. My aim is to read first 4 characters of the file using fgets.

`open` returns a file descriptor but not a `FILE*`. use `fopen` instead — starrify, Feb 03 '12 at 05:53

Ernest Friedman-Hill · Accepted Answer · 2012-02-03T05:50:55.447

You'll want to do

    printf("%s", ch);

For the % format, the argument is a pointer to characters; by passing a single character by value, you're telling printf to interpret that character's ASCII value as a pointer, and that's going to blow up on you; i.e., if the character is a 1, which is ASCII 49, then it's going to look at byte 49 in memory for a string -- and looking down there is generally verboten.

But secondly, I see you're calling open() instead of fopen(). You must use fopen() or you won't get a FILE* as you're expecting.

Both of these individually would likely cause a segfault -- you'll need to fix them both.

Looks like I have to wait 5 minutes to accept the answer. Thanks anyways. — yetanotherstacker, Feb 03 '12 at 05:51

score 2 · Answer 2 · answered Feb 03 '12 at 05:49

2

try to use "fopen" instead just "open"

Thanks.

answered Feb 03 '12 at 05:49

rdo

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Right, or the opposite solution : using `int fd` instead of `FILE *f`. – Eregrith Feb 03 '12 at 09:11

Niall Byrne · Answer 3 · 2012-02-03T06:13:28.740

2

Couple of quick changes.

I think you want to use fopen rather than open here, since you used a file pointer.

You need to increase the bytes read to 5, the last one is terminated by a null by fgets.

 int main() {

     FILE *f; 
     char ch[5];
     f = fopen("a.txt", "r");
     fgets( ch, 5, f); 
     printf("%s", ch);
     return 0;

 }

edited Feb 03 '12 at 06:13

answered Feb 03 '12 at 05:58

Niall Byrne

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Hemant Metalia · Answer 4 · 2012-02-03T06:31:38.453

try to use

#include <stdio.h>

int main(){

    FILE *f; 
    char ch[5];
    f = fopen("a.txt", "r"); //use fopen
    fgets( ch, 4, f); 
    ch[4] = NULL;
    printf("%s", ch); // modification here pass the address of an array to printf
    return 0;
}

try following example from the refereed site

/* fgets example */
#include <stdio.h>

int main()
{
   FILE * pFile;
   char mystring [5];

   pFile = fopen ("myfile.txt" , "r");
   if (pFile == NULL) perror ("Error opening file");
   else {
     if ( fgets (mystring , 5 , pFile) != NULL )
       puts (mystring);
     fclose (pFile);
   }
   return 0;
}

refer http://www.cplusplus.com/reference/clibrary/cstdio/fgets/

you can also use

fgetc() :Get character from stream (function)

Segmentation fault while reading file

4 Answers4