Execution of printf() and Segmentation Fault

Question

#include<stdio.h>  int main() {     char *name = "Vikram";     printf("%s",name);     name[1]='s';     printf("%s",name);     return 0; } 

There is no output printed on terminal and just get segmentation fault. But when I run it in GDB, I get following -

Program received signal SIGSEGV, Segmentation fault. 0x0000000000400525 in main () at seg2.c:7 7       name[1]='s'; (gdb)  

This means program receive SEG fault on 7th line (obviously I can't write on constant char array) . Then why printf() of line number 6 is not executed ?

Answer 1

This is due to stream buffering of stdout. Unless you do fflush(stdout) or you print a newline "\n" the output is may be buffered.

In this case, it's segfaulting before the buffer is flushed and printed.

You can try this instead:

printf("%s",name); fflush(stdout);        //  Flush the stream. name[1]='s';           //  Segfault here (undefined behavior) 

or:

printf("%s\n",name);   //  Flush the stream with '\n' name[1]='s';           //  Segfault here (undefined behavior) 

Answer 2

First you should end your printfs with "\n" (or at least the last one). But that is not related to the segfault.

When the compiler compiles your code, it splits the binary into several section. Some are read only, while other are writeable. Writing to an read only section may cause a segfault. String literals are usually placed in a read only section (gcc should put it in ".rodata"). The pointer name points to that ro section. Therefore you must use

const char *name = "Vikram"; 

In my response I've used a few "may" "should". The behaviour depends on your OS, compiler and compilation settings (The linker script defines the sections).

Adding

-Wa,-ahlms=myfile.lst 

to gcc's command line produces a file called myfile.lst with the generated assembler code. At the top you can see

    .section .rodata .LC0:     .string "Vikram" 

Which shows that the string is in Vikram.

The same code using (Must be in global scope, else gcc may store it on the stack, notice it is an array and not a pointer)

char name[] = "Vikram"; 

produces

    .data     .type name, @object     .size name, 7 name:     .string "Vikram" 

The syntax is a bit different but see how it is in .data section now, which is read-write. By the way this example works.