在简单的C程序中故障排除段错误

Question

I am following an example of a book in writing a function to reverse a string in C. This is the following problem. But when I execute it under ubuntu using gcc. I get a seg fault. I have tried debugging it, but I don't understand how can this line '*start++ = *end;' causing a seg fault.

I appreciate someone can help me understand the seg fault.

#include <stdio.h>
#include <stdlib.h>

void myreverse(char* str) {
    int len = strlen(str);
    char tmp;

    char* start = str;
    char* end = str + (len -1);

    while (start < end) {
        tmp = *start;
        // this is causing Segmentation fault
        *start++ = *end;
        *end-- = tmp;
    }
}

int main(void) {
    char* test = "Hello World";
    puts(test);
    myreverse(test);
    puts(test);
    return EXIT_SUCCESS;
}

Answer 1

String literals are stored in a read only section of your executable. You could avoid that by changing char* test = "Hello World"; to char test[] = "Hello World"; , where "Hello World" will be copied into the array test.

Answer 2

Your string buffer is read only memory. String literals cannot be modified.

Use strcpy or strdup to make a buffer of writeable memory.

char* test = strdup("Hello World");

Remember to free the string when you are done with it.

Answer 3

char* test = "Hello World";

That is a string literal . It's read only; you can't modify it.

You are trying to modify it :)

Answer 4

See the answer here: Modifying a C string: access violation

Basically a C string literal is read only and can't be written to.

Answer 5

char* test = "Hello World";

when you dynamically give a string to a character pointer, the string you give gets saved in Read only part of data segment..Then you are passing this address to your myReverse function where you are trying to change the contents of that location in the line

*start++ = *end;

which cause the segmentation fault..

Just change this line

char* test = "Hello World";

to

char temp[] = "Hello world";
char *test  = temp;

and it wouldn't crash..See this thread for more details..

Answer 6

Based on my experience, seg fault occurs when you are trying to access a memory location outside the scope of your variables. C does not have any bounds checking, so I'm guess somewhere in your code (*start++) is incrementing a pointer to a memory location that you have not reserved in your code. You can't just iterate through memory locations that you haven't allocated in your code because this could interfere with data that is being used for other purposes and will have unexpected results.