An unexplained segmentation fault

Question

I'm writing code to reverse the char array in every single line I input. However, when I debugged the program, I encountered a segmentation fault and I don't know why. Here is my code:

#include <stdio.h>
#define DEFAULT 1000

int reverse(char s[]);

main()
{
    int i;
    char line[DEFAULT] = {'\0'};
    while ((i = getline(line, DEFAULT)) != 0) {
        printf("%s", reverse(line));
    }   
    return 0;
}

/*reverse a line*/
int reverse(char s[])
{
    int i = 0, j = 0;
    while (s[i] != '\n')
        ++i;
    for (j = 0; j < i; ++j) {
        s[2 * i - j] = s[j];
    }
    for (j = 0; j < i; ++j) {
        s[j] = s[j + i + 1];
        s[j + i + 1] = '\0';
    }
    return i;
}

/*get a line from input stream*/
int getline(char s[], int lim)
{
     int i, c, j = 0;
     for (i = 0; ((c = getchar()) != EOF) && (c != '\n'); ++i) {        
        if (i < lim - 2) {
            s[j] = c;   // use j to prevent index out of bounds
            ++j;
        }
     }
     if (c == '\n') {
        s[j] = c;
        ++j;
     }
     s[j] = '\0';
     return i;  // return the length of char s[]
} 

When I input "abc\n" and execute reverse(s), the content in s becomes "cba\n" and i refers to 3, everything is okay. The segmentation fault happens when I step out of that function.

Here are some more details:

Complier: GCC 4.9.2 64-bit Release

System: Windows 10

Answer 1

Your function int reverse(char s[]) is returning an int, and then you're trying to print that resulting int as a string:

printf("%s", reverse(line));

If you want to use the function like that, you could change its signature to return a char* instead:

char* reverse(char s[]);

And then return the s that you're giving in as a parameter. Your compiler should issue a warning about giving the wrong type of parameter to printf. If not, try to set it to be more strict with warnings.