strtok giving Segmentation Fault

Question

Hi,

Why does the below code give Seg. Fault at last line?

  char* m=ReadName();
  printf("\nRead String %s\n",m); // Writes OK
  char* token;
  token=strtok(m,'-');

As said, read string prints w/o problem, but why cannot split to tokens?

Answer 1

strtok modifies its first argument, hence it should be modifiable.

Maybe ReadName() returns a pointer to a read-only char array.Can you show us your ReadName() function.

If that is the reason for seg-faullt, you can create a copy of the char array before you pass it to strtok using the strdup function like:

char *copy = strdup(m);
token = strtok(copy,'-');
....
....
free(copy); // free the copy once you are done using it.

Answer 2

It's impossible to know for sure without knowing what m points to. But the most likely reason is that m is pointing to readonly memory. So you can print it out, but you can't write to it.

strtok writes to the string, so it's faulting, but printf only reads from it, so it isn't.

Try this

char* m=ReadName();
printf("\nRead String %s\n",m); // Writes OK
char temp = m[0];
m[0] = temp; // I'll bet you segfault here.

Answer 3

Its probably because ReadName() return string. So, the assignment makes m const char* and hence, you cannot alter any of its values (modify the string). So, when 'strtok' tries to modify 'm', Segmentation Fault is there

Remedy:

char *m = malloc(sizeof(char)*MAX);
strcpy(m, ReadName());

OR

char *m = strdup(ReadName());

Answer 4

token=strtok(m,'-'); should generate a compiler warning because the second parameter of strtok() is a const char * pointing to multiple delimiters, not a single char delimiter:

char *strtok(char *str, const char *delim);

The ASCII code of '-' is 0x2D, so passing it as the second parameter of strtok() will cause strtok() to dereference the address 0x0000002D, which will cause a segfault or access violation on most modern operating systems. To fix this, use a string literal instead of a character literal: token=strtok(m,"-");

There's also the issue of how the return value of ReadName() is allocated, which others have addressed in their answers.