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From: Mark Summerfield <mark@qtrac.eu>
Subject: relearning C: why does an in-place change to a char* segfault?
Newsgroups: comp.lang.c
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This program segfaults at the commented line:

#include <ctype.h>
#include <stdio.h>

void uppercase_ascii(char *s) {
    while (*s) {
        *s = toupper(*s); // SEGFAULT
        s++;
    }
}

int main() {
    char* text = "this is a test";
    printf("before [%s]\n", text);
    uppercase_ascii(text);
    printf("after  [%s]\n", text);
}

I know there are better ways to do ASCII uppercase, I don't care about 
that; what I don't understand is why I can't do an in-place edit of a non-
const char*?

I build using scons, which does: 

gcc -o inplace.o -c -Wall -g inplace.c

gcc -o inplace inplace.o

The error with gdb is:

Starting program: /tmp/inplace/inplace 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
before [this is a test]

Program received signal SIGSEGV, Segmentation fault.
0x000055555555516e in uppercase_ascii (s=0x555555556004 "this is a test") 
at inplace.c:6
6	        *s = toupper(*s);