Styjun

I am trying to use the C's strtok function in order to process a char* and print it in a display, and looks like that for some reason I don't know the character 'n' is not substituted by '' as I believe strtok does. The code is as follows:

-Declaration of char* and pass to the function where it will be processed:

char *string_to_write = "Some textnSome other textnNewtext";
malloc(sizeof string_to_write);
screen_write(string_to_write,ALIGN_LEFT_TOP,I2C0);

-Processing of char* in function:

void screen_write(char *string_to_write,short alignment,short I2C)

 char *stw;
 stw = string_to_write;
 char* text_to_send;
 text_to_send=strtok(stw,"n");
 while(text_to_send != NULL)
 
 write_text(text_to_send,I2C);
 text_to_send=strtok(NULL, "n");
 

When applying the code, the result can be seen in imgur (Sorry, I am having problems with format adding the image here in the post), where it can be seen that the n is not substituted as it is the strange character appearing in the image, and the debugger still showed the character as well. Any hints of where can the problem be?

Thanks for your help,
Javier

strtok expects to be able to mutate the string you pass it: instead of allocating new memory for each token, it puts characters into the string at token boundaries, then returns a series of pointers into that string.

But in this case, your string is immutable: it's a constant stored in your program, and can't be changed. So strtok is doing its best: it's returning indices into the string for each token's starting point, but it can't insert the s to mark the ends. Your device can't handle ns in the way you'd expect, so it displays them with that error character instead. (Which is presumably why you're using this code in the first place.)

The key is to pass in only mutable strings. To define a mutable string with a literal value, you need char my_string[] = "..."; rather than char* my_string = "...". In the latter case, it just gives you a pointer to some constant memory; in the former case, it actually makes an array for you to use. Alternately, you can use strlen to find out how long the string is, malloc some memory for it, then strcpy it over.

P.S. I'm concerned by your malloc: you're not saving the memory it gives you anywhere, and you're not doing anything with it. Be sure you know what you're doing before working with dynamic memory allocation! C is not friendly about that, and it's easy to start leaking without realizing it.

1.

malloc(sizeof string_to_write); - it allocates the sizeof(char *) bytes not as many bytes as your string needs. You also do not assign the allocated block to anything

2.

char *string_to_write = "Some textnSome other textnNewtext";
char *ptr;
ptr = malloc(strlen(string_to_write) + 1);
strcpy(ptr, string_to_write);
screen_write(ptr,ALIGN_LEFT_TOP,I2C0);