What does this seg fault mean

Question

I have a segmentation fault when i run my C program and i do not understand it. I am reading the header from a binary file that contains student structs.

this is the seg fault i get when i run it with gdb

Program received signal SIGSEGV, Segmentation fault. 0x0804850f in main () at aidb.c:49 49 }

I am under the impression that the segmentation fault is at line 49, however there's only the closing bracket of my main () method at line 49. This is my code, just in case it helps clarify things:

#include<stdio.h>

typedef struct {
    char id_chars[4];
    int file_size;
    int section_table_offset; 
    int section_count;
} Header;


typedef struct {
    int offset;
    int num_entries;
    int type;  // legal value above
} SectionHeader;


    int main(void) {


      FILE *infile = fopen("file.bin", "r");

      Header aidbheader;


        //Reads the aidb file header
       // fread(aidbheader, sizeof(Header),16, infile);
       fread(&aidbheader.id_chars, sizeof(char),4, infile);
       fread(&aidbheader.file_size, sizeof(int),1, infile);
       fread(&aidbheader.section_table_offset, sizeof(int),1, infile);
       fread(&aidbheader.section_count, sizeof(int),1, infile);

       SectionHeader table[4];
       fread(table, sizeof(SectionHeader), 48, infile);


      printf("\nSectionHeader offset: %d \n", table[3].offset);




      return 0;
    }   // this is line 49

int id_chars[4]; --> char id_chars[4];, fread(table, sizeof(SectionHeader), 48, infile); : 48 --> 4 — BLUEPIXY
Crashes on the closing bracket of a function (and in general on return statements) are usually due to overwriting of the function return address, which is often caused by going out of boundaries of local arrays. — Matteo Italia

R Sahu R Sahu · Accepted Answer · 2016-05-23T05:13:40

You declare table as:

SectionHeader table[4];

Then you try to read into table 48 objects of size sizeof(SectionHeader).

fread(table, sizeof(SectionHeader), 48, infile);

There isn't enough space in table to hold that much data. Because of that, you write over memory that you are not supposed to. That is cause for undefined behavior. In your case, the undefined behavior manifests as segmentation fault when the program returns from main.

You can fix that problem by changing the size of table or changing the fread line. Make sure that table has enough space to read the data.

The other error in your code is that the id_chars member of Header is defined as:

int id_chars[4];

When you read data into it, you are using

fread(&aidbheader.id_chars, sizeof(char), 4, infile);

This by itself won't cause segmentation fault. It is a symptom of possibly buggy code. Make sure to change the definition of id_chars to

char id_chars[4];
^^^ char  not int

or change the fread line to use:

fread(&aidbheader.id_chars, sizeof(int), 4, infile);
                                  ^^^^ int not char

What does this seg fault mean

1 Answers