printf char array segment fault C

Question

So I have written a code that sorts words in the right order. The words are being stored via pointers and I have initialized another char array in the program to store the char* argv.

The last for loop is what prints segment fault and I can't figure out why.

#include <stdio.h>
#include <string.h>

int main(int argc, char* argv[]) {
  int i, j;
  char *key;
  char a[argc-1];
  for(i=1; i < argc; i++){
    a[i-1]= tolower(argv[i]);
      }

  for (i = 2; i < argc; i++) {
    key = argv[i];

    j = i-1;
    while (j >= 1 && strcmp(argv[j], key) > 0) {
      argv[j+1] = argv[j];
      j--;
    }

    argv[j+1] = key;
  }

  for(i = 1; i < argc; i++){
    a[i-1] = *argv[i];
  }

  for (i = 1; i < argc ; i++){
    puts(argv[i]);
  }

  for(i = 0; i < argc-1; i++){
    printf("%s", a[i]);
}
  return 0;
}

input

./a.out orange banana apple

output

apple
banana
orange
Segmentation fault

tolower(argv[i]) should be rasing a warning from the compiler, otherwise it's because you did not enable them, do not ignore them at all! Warnings are very useful for expert programmers, thy must be a tool for non experts. — Iharob Al Asimi
– Iharob Al Asimi, Commented Nov 21, 2016 at 14:28
@iharob i commented that part out. Yet i am still getting segment fault. — iMataMi
– iMataMi, Commented Nov 21, 2016 at 14:38
Please read my answer ... let the compilation warnings guide you. I mean, try to understand the warnings and infer the way things work from that. — Iharob Al Asimi
– Iharob Al Asimi, Commented Nov 21, 2016 at 14:39

matt · Accepted Answer · 2016-11-21 18:07:27Z

1

Your compiler should also give you a warnging on:

printf("%s", a[i]);

warning: format specifies type 'char *' but the argument has type 'char' [-Wformat]

If you change that to a %cit works fine.

As the warning says, when you use it %s printf is expecting a string or a char* and will treat it as such. a[i] is an integer type that can reference an invalid location in memory. So the correct way to do it would be to either use %c, and print a character; or use %s and pass a char* as the second argument.

Or perchance you want args in a. Change the delcaration.

char *a[argc-1];

Then change the assignment.

a[i-1] = argv[i];

edited Nov 21, 2016 at 18:07

answered Nov 21, 2016 at 15:10

matt

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2 Comments

Michi Over a year ago

Your Answer doesn't seem to explain the actual problem, it offers a Code-Fix. If you change that to a %cit works fine. Is not really an Answer.

matt Over a year ago

@Michi it can be improved. I think the actual problem was they wanted an array of char* but instead they just had a char, so discussing the %c behavior seems a bit irrelevant.

Iharob Al Asimi · Accepted Answer · 2016-11-21 14:37:56Z

There are multiple problems in your code

The line a[i - 1] = tolower(argv[i]) is wrong because tolower() takes int as paramater and you are passing char * so it's converting a pointer to int which is legal in c but does not guarantee defined behavior.
You are not setting the '\0' terminator on the a array which is another cause for problems, specifically when you try to use it as a string, a char array is not a string unless, it's a sequence of printable bytes with a terminating '\0' byte.
Allocating argc - 1 is not going to work because
- The value of argc is the number of passed parameters to the executable.
- If it was the length of the corresponding argv as found by strlen() you would need 1 more byte not less, so it would be argc + 1 in any case.

r-sniper · Accepted Answer · 2016-11-21 14:44:14Z

0

Why are you using %s

for(i = 0; i < argc-1; i++){
printf("%s", a[i]);

try %c

answered Nov 21, 2016 at 14:44

r-sniper

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5 Comments

iMataMi Over a year ago

%c would just print the first letter of each word.

r-sniper Over a year ago

Shouldn't your a be char *a[argc] rather than char a[argc]

iMataMi Over a year ago

No because argc is 4 in case of an input that 3 words. argc also counts main()

r-sniper Over a year ago

No i mean character pointer to array rather than character array. 'char *a[argc-1] ' rather than 'char a[argc-1]' notice the pointer.

iMataMi Over a year ago

That works but my main goal to achieve from this program is to be able to take text and start words into and array and print the words alphabetically without duplicates.

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