# Split a string and write the i-th element into a buffer with C

### Question

Recently I've written the following splitv() function, without using strtok. Any way to make it shorter and more efficient?

### Descrption

The function calculates the index-th element of the string str split by delim. The element is written into buff and 0 is returned. When an index error is occurred, any non 0 value is returned. This function was inspired by the python split function: buff = str.split(delim)[index], but I think it works too hard and can be improved.

int splitv(char buff[], char str[], char delim[], int index) {
// returns 0 if an element is written. Else non 0.
char *start = str, *end;
int i = 0, buff_i = 0;
// check if delim not in str
if ((end = strstr(str, delim)) == NULL) {
if (index == 0) strcpy(buff, str);
return index;
}
// delim is in str
for (i = 0; i < index; i++) {
start = end + strlen(delim);
end = strstr(start, delim);
if (end == NULL) {
// reached the last element
if (i == index - 1) {
end = str + strlen(str);
break;
}
// index error: index >= elements_count
return -1;
}
}
// copy element to buffer
while (start != end) {
buff[buff_i] = *start;
start++;
buff_i++;
}
buff[buff_i] = '\0';
return 0;
}


Before we talk about improving the efficiency of your function, lets talk about safety:

1. You never check the validity of your function parameters.
2. You have no way of knowing how big buff is.

For the first issue, if str or delim is null, passing them directly in to strstr will cause a segfault and crash your program.

The second issue is worse: if buff is smaller then str, your function may end up clobbering memory when it tries to copy from str to buff.
This is undefined behavior, and the results are unpredictable.

Now for some efficiency suggestions:

1. Calculate string length only once.
Since strlen works by running over the string looking for '\0' terminator, it is more efficient to call it only once at the beginning of your function and store the result in a variable.
This can also allow you to implement a simple shortcut for edge cases such as where str or delimiter are empty strings, or delimiter is longer then str.

2. You don't need your own while loop at the end and the buff_i variable.
You can simply use memcpy or strncpy to copy the element in to the buffer.
memcpy would be slightly more efficient, since it does not check for '\0', while strncpy does.

3. Since your logic is that if delimiter is never found, but the requested element is 0 the result is "success", you don't need to handle this as a separate edge case.
That means you can get rid of the first if block, and handle everything in one loop.
You can also get rid of the special checks in the if loop and just focus on the search.

The following code example does not include the suggested validity checks:

int i;
char *start = str;
char *end = start;
size_t delim_len = strlen(delim);
size_t copy_len;

for (i = 0; i <= index && end != NULL; i++) {
end = strstr(start, delim);
if (end != NULL && i != index) start = end + delim_len;
}

if (i != index) return index;

if (end == NULL) {
copy_len = strlen(start);
} else {
copy_len = (ssize_t)(end - start);
}

memcpy(buff, start, copy_len);
buff[copy_len] = '\0';

return 0;

• @chux-ReinstateMonica typo on my side, thanks for pointing it out, fixed. Feb 10, 2021 at 20:14
int splitv(char buff[], char str[], char delim[], int index) {
// returns 0 if an element is written. Else non 0.


• Shouldn't delim be a pointer to const char? I think the function ought not to be modifying it. Similarly, if we can avoid modifying the input string (which is one major advantage to not using strtok()), then take that as pointer-to-const, too.
• Please accept an argument to indicate the capacity of buff() so that we can write code without buffer overflows!
• index ought to be unsigned, as negative values make no sense here.
• We could make the return value more useful by returning the length of the match (or negative on failure). Like the interface to snprintf(), this is very useful in the event the buffer is too small. I'd even go as far as accepting NULL, 0 for the buffer arguments in the same manner.
• It's more conventional to write the character pointer arguments as char *arg than with the indefinite array syntax. The two are identical as far as the compiler is concerned, of course, but keeping with convention makes code easier for other programmers to read.

Let's see how we'd use the existing interface:

#include <stdio.h>
int main(void)
{
char list[] = "Alice, Bob, Charlie, Don";
char delim[] = ", ";
char buff[10];              /* is that enough?? */

if (splitv(buff, list, delim, 2)) {
fputs("Couldn't split the string.\n", stderr);
return 1;
}

printf("Found %s\n", buff);
}


See how much easier the new interface is:

int splitv(char *buff, size_t buff_len,
const char *str, const char *delim,
unsigned int index);

#include <stdio.h>
int main(void)
{
const char *list = "Alice, Bob, Charlie, Don";
char buff[10];

int len = splitv(buff, sizeof buff, list, ", ", 2);
if (len < 0) {
fputs("Couldn't split the string.\n", stderr);
return 1;
}
if ((size_t)len >= sizeof buff) {
fputs("Substring too long.\n", stderr);
return 1;
}

printf("Found %s\n", buff);
}


We don't have to guess if our buffer is big enough (and adaptive code can use the result to allocate a big enough buffer), and we can pass string literals as arguments.

const char *start = str, *end;
int i = 0, buff_i = 0;


Prefer to declare one variable per line, and prefer to declare where you can also initialise:

const char *start = str;
// check if delim not in str
const char *end = strstr(str, delim);
if (!end) {

for (int i = 0;  i < index;  ++i) {


We have two bits of code doing the same thing when we reach the last element. It may help to rethink what this code does:

1. Skip over zero or more substrings ending in delim.
2. Find the next occurrence of delim, or the end of string.
3. Copy the substring.

Here's a simplified implementation that conforms to my suggested interface, and is structured according to those steps:

#include <stdint.h>
#include <string.h>

int splitv(char *buff, size_t buff_len,
const char *str, const char *delim,
unsigned int index)
{
/* returns length of the substring if it exists */
/* returns negative on failure */

/* useful constant */
const size_t delim_len = strlen(delim);

while (index--) {
/* advance past next delimiter */
const char *next = strstr(str, delim);
if (!next) {
/* not enough delimiters */
return -1;
}
str = next + delim_len;
}

/* okay, we've found the right substring; now find its end */
const char *end = strstr(str, delim);
if (!end) {
end = str + strlen(str);
}

/* decide how much to copy, and write it */
size_t len = (size_t)(end - str);
if (buff && buff_len > 0) {
size_t write_size = len < buff_len  ? len :  buff_len - 1;
/* write the output */
memcpy(buff, str, write_size);
buff[write_size] = 0;
}

/* return the substring length (not including null terminator) */
return (int)len;
}


Note that I've hoisted strlen(delim) into a constant, as we know this doesn't need to be recomputed every time round the loop. I've also used standard memcpy function instead of the hand-rolled loop to write to buff.

• Further than making index unsigned, it should be a size_t, as this function won't work on most (all?) 64-bit systems with more than 4B occurrences Feb 6, 2021 at 4:07
• Yes, I think so. In fact, in an early iteration I did exactly that, and I'm not sure why I changed it back. And int as return type is wrong, too (probably should be size_t, with (size_t)-1 used as the error sentinel. Can I argue that my code isn't the finished item, just a step on the way? Feb 6, 2021 at 11:14
• Yes, quite right. Thanks @chux Feb 10, 2021 at 7:15