C: creating array of strings from delimited source string - c

Question

What would be an efficient way of converting a delimited string into an array of strings in C (not C++)? For example, I might have:
char *input = "valgrind --leak-check=yes --track-origins=yes ./a.out"
The source string will always have only a single space as the delimiter. And I would like a malloc'ed array of malloc'ed strings char *myarray[] such that:
myarray[0]=="valgrind"
myarray[1]=="--leak-check=yes"
...
Edit I have to assume that there are an arbitrary number of tokens in the inputString so I can't just limit it to 10 or something.
I've attempted a messy solution with strtok and a linked list I've implemented, but valgrind complained so much that I gave up.
(If you're wondering, this is for a basic Unix shell I'm trying to write.)

Answer 1

What's about something like:
char* string = "valgrind --leak-check=yes --track-origins=yes ./a.out";
char** args = (char**)malloc(MAX_ARGS*sizeof(char*));
memset(args, 0, sizeof(char*)*MAX_ARGS);
char* curToken = strtok(string, " \t");
for (int i = 0; curToken != NULL; ++i)
{
args[i] = strdup(curToken);
curToken = strtok(NULL, " \t");
}

Answer 2

if you have all of the input in input to begin with then you can never have more tokens than strlen(input). If you don't allow "" as a token, then you can never have more than strlen(input)/2 tokens. So unless input is huge you can safely write.
char ** myarray = malloc( (strlen(input)/2) * sizeof(char*) );
int NumActualTokens = 0;
while (char * pToken = get_token_copy(input))
{
myarray[++NumActualTokens] = pToken;
input = skip_token(input);
}
char ** myarray = (char**) realloc(myarray, NumActualTokens * sizeof(char*));
As a further optimization, you can keep input around and just replace spaces with \0 and put pointers into the input buffer into myarray[]. No need for a separate malloc for each token unless for some reason you need to free them individually.

Answer 3

Were you remembering to malloc an extra byte for the terminating null that marks the end of string?

Answer 4

From the strsep(3) manpage on OSX:
char **ap, *argv[10], *inputstring;
for (ap = argv; (*ap = strsep(&inputstring, " \t")) != NULL;)
if (**ap != '\0')
if (++ap >= &argv[10])
break;
Edited for arbitrary # of tokens:
char **ap, **argv, *inputstring;
int arglen = 10;
argv = calloc(arglen, sizeof(char*));
for (ap = argv; (*ap = strsep(&inputstring, " \t")) != NULL;)
if (**ap != '\0')
if (++ap >= &argv[arglen])
{
arglen += 10;
argv = realloc(argv, arglen);
ap = &argv[arglen-10];
}
Or something close to that. The above may not work, but if not it's not far off. Building a linked list would be more efficient than continually calling realloc, but that's really besides the point - the point is how best to make use of strsep.

Answer 5

Looking at the other answers, for a beginner in C, it would look complex due to the tight size of code, I thought I would put this in for a beginner, it might be easier to actually parse the string instead of using strtok...something like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
char **parseInput(const char *str, int *nLen);
void resizeptr(char ***, int nLen);
int main(int argc, char **argv){
int maxLen = 0;
int i = 0;
char **ptr = NULL;
char *str = "valgrind --leak-check=yes --track-origins=yes ./a.out";
ptr = parseInput(str, &maxLen);
if (!ptr) printf("Error!\n");
else{
for (i = 0; i < maxLen; i++) printf("%s\n", ptr[i]);
}
for (i = 0; i < maxLen; i++) free(ptr[i]);
free(ptr);
return 0;
}
char **parseInput(const char *str, int *Index){
char **pStr = NULL;
char *ptr = (char *)str;
int charPos = 0, indx = 0;
while (ptr++ && *ptr){
if (!isspace(*ptr) && *ptr) charPos++;
else{
resizeptr(&ptr, ++indx);
pStr[indx-1] = (char *)malloc(((charPos+1) * sizeof(char))+1);
if (!pStr[indx-1]) return NULL;
strncpy(pStr[indx-1], ptr - (charPos+1), charPos+1);
pStr[indx-1][charPos+1]='\0';
charPos = 0;
}
}
if (charPos > 0){
resizeptr(&pStr, ++indx);
pStr[indx-1] = (char *)malloc(((charPos+1) * sizeof(char))+1);
if (!pStr[indx-1]) return NULL;
strncpy(pStr[indx-1], ptr - (charPos+1), charPos+1);
pStr[indx-1][charPos+1]='\0';
}
*Index = indx;
return (char **)pStr;
}
void resizeptr(char ***ptr, int nLen){
if (*(ptr) == (char **)NULL){
*(ptr) = (char **)malloc(nLen * sizeof(char*));
if (!*(ptr)) perror("error!");
}else{
char **tmp = (char **)realloc(*(ptr),nLen);
if (!tmp) perror("error!");
*(ptr) = tmp;
}
}
I slightly modified the code to make it easier. The only string function that I used was strncpy..sure it is a bit long-winded but it does reallocate the array of strings dynamically instead of using a hard-coded MAX_ARGS, which means that the double pointer is already hogging up memory when only 3 or 4 would do, also which would make the memory usage efficient and tiny, by using realloc, the simple parsing is covered by employing isspace, as it iterates using the pointer. When a space is encountered, it reallocates the double pointer, and malloc the offset to hold the string.
Notice how the triple pointers are used in the resizeptr function.. in fact, I thought this would serve an excellent example of a simple C program, pointers, realloc, malloc, passing-by-reference, basic element of parsing a string...
Hope this helps,
Best regards,
Tom.