Valgrind error setting a null byte in C

Question

int length = strlen(input_str);
output_s = malloc((sizeof(char*) * totalSentences) + 1);
for (int i = 0; i < totalSentences; i++) {
  output_s[i] = malloc((sizeof(char) * sentences[i]) + 1);
}

free(sentences);

currentSentence = 0;
int currentCharacter = 0;
int firstChar = 1;
for (int i = 0; i < length; i++) {

  if (isalpha(*input_str) && (firstChar == 1)) {
    output_s[currentSentence][currentCharacter] = (char)toupper(*input_str);
    currentCharacter++;
    firstChar = 0;
  } else if (isalpha(*input_str)) {
    output_s[currentSentence][currentCharacter] = (char)tolower(*input_str);
    currentCharacter++;
  } else if (!isspace(*input_str) && !ispunct(*input_str)) {
    output_s[currentSentence][currentCharacter] = *input_str;
    currentCharacter++;
  }

  if (isspace(*input_str)) {
    firstChar = 1;
  }

  if (ispunct(*input_str)) {
    firstChar = 1;
    currentCharacter = 0;
    currentSentence++;
    input_str++;
    if (currentSentence == totalSentences) {
      break;
    }
    continue;
  }

  input_str++;
}

output_s[totalSentences] = NULL;

So I am creating an array of strings in c, and using a for loop and printf I know the strings are being created valid and it prints (null) at the end so I am assuming I am correctly setting the NULL byte at the end with output_s[totalSentences] = NULL; however when running this through valgrind it shows

==18908== Invalid write of size 8
==18908==    at 0x400BB1: camel_caser (camelCaser.c:99)
==18908==    by 0x400D45: test_camelCaser (camelCaser_tests.c:34)
==18908==    by 0x400E17: main (camelCaser_main.c:13)
==18908==  Address 0x5204538 is 24 bytes inside a block of size 25 alloc'd
==18908==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==18908==    by 0x40090C: camel_caser (camelCaser.c:57)
==18908==    by 0x400D45: test_camelCaser (camelCaser_tests.c:34)
==18908==    by 0x400E17: main (camelCaser_main.c:13)

with line 57 being the line where I malloc the malloc(sizeof(char*) * totalSentences + 1) and line 99 being the line where I actually set the last byte in the array to be NULL, so I don't know if this is somehow messing up the memory earlier in the array that has caused the null to be written in a bad spot or am I not allocating enough memory to fit NULL in?

The multiplication and addition in the first malloc() is probably wrongly parenthesized. The odd bytes is of no use in an array of pointers. You probably want ‘(length + 1) * sizeof(char *)`. — Jonathan Leffler
After determining the total number of sentences I made an array sentences that kept track of number of characters per sentence so that I could make a properly sized array to contain them — user3645925

dbeer dbeer · Accepted Answer · 2018-01-30T20:10:20

You definitely need that to be output_s = malloc(sizeof(char*) * (totalSentences + 1)); instead of output_s = malloc((sizeof(char*) * totalSentences) + 1);.

The code - as you have it - allocates space for totalSentences char *s, plus 1 byte. Your code wants space for totalSentences + 1 char *s. That's why valgrind gives the error that you're writing 8 bytes to a space 24 bytes into 25 bytes allocated.

Valgrind error setting a null byte in C

1 Answers