I wrote this function which does the following from scratch w/o looking for ways that it is typically done:

  1. Accepts a char c to search for.
  2. Accepts a char* str to search within.
  3. Accepts a buffer pointer, and stores all indices of c that are in str inside the provided buffer.
  4. Returns the # of times that c was found in str.

Please critique and also - Are there better alternatives to export the indices other than using a buffer ptr?

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
int str_contains(const char c, const char* str, int* output_buffer){
    int i, str_len;
    str_len = strlen(str);
    int *holder = malloc(str_len*sizeof(int));
    int holder_iteration = 0;
    for(i = 0; i < str_len; i++){
        if(str[i] == c){
            holder[holder_iteration++] = i;
    return holder_iteration;

int main(){
    char* myString = "Welcome to the jungle, baby. Do you like to juggle or jump around?";
    int* output = malloc(500*sizeof(int));
    int number = str_contains('a',myString,output);

    printf("# of times: %d\nIndices: \n",number);
    int i;
    for(i = 0; i < number; i++){


1 Answer 1

  • The final memcpy may overflow output_buffer. I recommend to change the signature to

    int str_contains(const char * str, char c, int *indices, size_t indices_size);

    The caller shall compare the return value with the size of buffer it passed and act accordingly.

  • Dynamic memory is uncalled for. You may safely collect indices directly into the passed buffer.

  • There is no need for strlen.

  • When appropriate, use pointers.

All that said,

int str_contains(const char * str, char c, size_t * indices, size_t indices_size)
    size_t occurrences = 0;
    for (char * cursor = str; *cursor; cursor++) {
        if (*cursor == c) {
            if (occurrences < indices_size) {
                indices[occurrences] = cursor - str;
    return occurrences;
  • 2
    \$\begingroup\$ I like everything except the fact thatindices[0] is never filled in because you incremented occurrences beforehand. Also, indices_size shouldn't be a pointer type (your explanation was right but your code was not). \$\endgroup\$
    – JS1
    Commented Feb 16, 2017 at 8:33
  • \$\begingroup\$ @JS1 Thanks, fixed. Shouldn't post an untested code under the influence. \$\endgroup\$
    – vnp
    Commented Feb 16, 2017 at 17:57

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