CODEWITHSUNDEEP
ciosegmentation-fault
Tesseract_R
Tesseract_R

Reputation: 39

C segmention fault when opening file

This seems to be a really simple one, but I can't figure it out after not touching C programming in four years.

I was trying to open a file in main()

int main(int argc, const char * argv[])
{
    FILE * fp = fopen("data.txt","r");
    ...
    return(0)
}

The program compiled, but when I tried to run it in gdb, the following error occurs.

Program received signal SIGSEGV, Segmentation fault.
0x00000000004016c6 in main ()

when the program is trying to open the file "data.txt". What could cause the error? Thanks!

Upvotes: 0

Views: 159

Answers (2)

paxdiablo
paxdiablo

Reputation: 881093

I suspect your error lies in this bit of code:

...

In other words, there's nothing in the other code shown that appears to be wrong.

The most likely case is that the file doesn't exist, or it doesn't exist in the directory where the program is running (which, if you're in an IDE, usually turns out to be somewhere other than you think it is).

And, in that case, you're getting NULL from the fopen, then later using it, something like:

FILE *fp = fopen ("no_such_file.txt", "r");
int ch = fgetc (fp);

You should generally check return values from all functions that use them to indicate success or failure:

#include <stdio.h>

int main (void) {
    FILE *fp = fopen ("no_such_file.txt", "r");
    if (fp == NULL) {
        perror ("Opening no_such_file.txt");
        return 1;
    }

    // You can use fp here.

    puts ("It worked.");
    fclose (fp);

    return 0;
}

Upvotes: 2

Jeremy Friesner
Jeremy Friesner

Reputation: 73041

What could cause the error?

The most likely cause of the error is that the file data.txt could not be opened (e.g. because it doesn't exist, or it's not in the current directory, or your program doesn't have permission to read it). That will cause fopen() to return NULL. Then if your code (in the ... section) tries to call fread() or fgets() or whatever and passes in the NULL pointer, that will cause a crash. You need to check the value returned by fopen() to make sure it is non-NULL before trying to use it.

Upvotes: 1

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