Segmentation fault when using fixed size array

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct{
    char name[100];
    long size;
    char mode[8];
    char uid[8];
    char ouid[8];
    char size1[12];
    char time[12];
    char checksum[8];
    char type[1];
    char linkname[100];
}HEADER;


int main(int argc, char **argv)
{
    for(int i = 1; i <= argc; i++) argv[0,1,2] are program name, c, tar output file
    {
        HEADER header;
        strcpy(header.name, argv[i]); // saving the name of the file
        FILE *fin = fopen(argv[i], "rb");
        if(fin == NULL)
        {
            fprintf(stderr, "File not found.\n");
            exit(1);
        }
        fseek(fin, 0L, SEEK_END);
        header.size = ftell(fin);
        printf("%s is %ld bytes long\n", header.name, header.size);
        fseek(fin, 0L, SEEK_SET);
        fclose(fin);
    }

    return 0;
}

I want to store the number of bytes and the name of the files provided as command line arguments, but I'm getting a segmentation fault. I ran a debugger and strcpy is the problem, name has fixed size so I don't need to allocate memory. Can you help me?

Some programmer dude · Accepted Answer · 2021-03-16 15:46:07Z

2

The problem is i <= argc as the loop condition. The argc argument is the number of elements in the argv array, not the top valid index. Its semantics is very much similar to the string length returned by strlen.

But what's causing the crash is that the argv element at index argc (i.e. argv[argc]) is a null pointer. And you try to use this null pointer in several places (when i is equal to argc).

You need to use i < argc instead as the loop condition as well.

answered Mar 16, 2021 at 15:46

Some programmer dude

411k36 gold badges420 silver badges655 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

mnikolic · Accepted Answer · 2021-03-16 15:51:04Z

1

argc has the number of parameters passed in the command line, +1 for the program name. This means that if the app is started with

app_name p1 p2

argc will be 3, but the index in the argv array is 0, 1 and 2. In the for loop, you are looping from 1 until argc, it should be from 1 to argc-1, because arrays in C starts from 0. If you replace <= with <, loop will iterate correctly.

for(int i = 1; i < argc; i++) argv[0,1,2] are program name, c, tar output file
    {
        HEADER header;
        strcpy(header.name, argv[i]); // saving the name of the file

answered Mar 16, 2021 at 15:51

mnikolic

6121 gold badge7 silver badges10 bronze badges

Collectives™ on Stack Overflow

Segmentation fault when using fixed size array

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related