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C-Strings

Strings

• Strings are nothing but an array of characters - there is no primitive string type in C.
• Special null character '\0' needed at the end of every string
• "string literal" is legal, compiler will make a const array of chars to hold it for you.
• No string + concatenation like in Java or Python
• #include <string.h> to access library functions; Reference for string.h functions
• strlen(s) returns number of chars before '\0'

String library functions

• These functions show off pointers and const well
• They use pointer types char * for strings, not array types char []
• Memory must previously be allocated by you if you are passing a reference where the function will put a result.
• The functions use const to indicate arguments they don't change

stdlib.h string conversion functions

• double atof(const char *s)
• int atoi(const char *s)
• double strtod(const char *s, char **endp)

stdio.h string i/o functions

• puts(s)
• gets(s) - very insecure, don't use - may overwrite memory not allocated to your program!
• fgets(s, MAXSIZE, stdin) - use this instead; must allocate memory for s first!

Common string.h string manipulation functions

• strlen(const char *str) - length of string
• char *strcpy(char *dest, const char *orig)
• copy from string orig to string dest
• return dest
• strncpy(dest, orig, howmany) only copies howmany characters
• char *strcat(char *orig, const char *more)
  • concatenate more to end of orig (orig = orig + more)
  • return orig
• int strcmp(const char *s1, const char *s2)
  • compare s1 to s2
  • return 0 if same, <0 if s1 < s2, >0 if s1 > s2

Dynamically allocating space for strings and putting them in an array:

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

void addstring(char **stringarray, char *newstring, int * numptr)
{   printf("allocating %lu bytes for %s\n",strlen(newstring)+1,newstring);
    stringarray[*numptr] = malloc(strlen(newstring)+1);
    strcpy(stringarray[(*numptr)++],newstring);
}

int main()
{
    int numstrings = 0;
    char *sarray[100];

    addstring(sarray, "hi", &numstrings);
    addstring(sarray, "ho", &numstrings);
    addstring(sarray, "wee", &numstrings);
    for (int i=0; i < numstrings; i++) {
       printf("%s\n",sarray[i]);
       // must also free the strings to prevent memory leaks
       free(sarray[i]);
    }
}

See also the Pointers & Dynamic Memory Allocation notes resource.