in C the correct way to test nth character == "x" in a string passed to a function

Question

Given the following I am getting a segmentation fault and I am not sure if it is because I am testing against a pointer or other issue.

What is the correct way to test if the 4th character is a comma?

string is read from fifo abc,def,xyz

char in[BUFFER] = {'\0'};
if ((in_fl = open(*fifofile, O_RDONLY)) == -1)
    {
     while (read(in_fl, in, BUFFER)>0) {
      doParseInput(&in);
    }


void *doParseInput(char *inputread){
//copy string to use later....
 char* theParsedString = calloc(strlen(inputread)+1, sizeof(char));
 strcpy(theParsedString , inputread);
 if (strcmp(theParsedString[3], ",") == 0){ //causes seg fault

I have also tried using the passed string directly, but also seg fault

if (strcmp(inputread[3], ",") == 0){ //causes seg fault

`if (inputread[3] == ',') {` is how you compare a *char*. `strcmp()` is for comparing *strings* (sequence of chars terminated with a null byte). — P.P, Jan 18 '17 at 14:38
See this related [question](https://stackoverflow.com/questions/10490636/compare-between-a-char-in-a-string-to-a-given-char). Not really a duplicate since you are asking for a C solution but the accepted answer there is also C. — qwattash, Jan 18 '17 at 14:41
Possible duplicate of [Compare between a char in a string to a given char](http://stackoverflow.com/questions/10490636/compare-between-a-char-in-a-string-to-a-given-char) — qwattash, Jan 18 '17 at 14:42
Be sure you read enough into `in` (your buffer): if you read only 3 chars, you can't test the 4th. Then, use `doParseInput(in)` without ampersand. Then, compare like `inputread[3] == ','`. — linuxfan says Reinstate Monica, Jan 18 '17 at 14:55

score 1 · Accepted Answer · answered Jan 18 '17 at 14:38

1

To pass a buffer to a function, don't use &.
Instead:

doParseInput(in);

To compare the 4th character of a buffer (index == 3):

if (theParsedString[3] == ','){

(Notice the single-quotes, meaning Character-Value, rather than the double-quotes, which would mean "String")

answered Jan 18 '17 at 14:38

abelenky

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can you expand or point to some docs on "why" not to use the & to pass a buffer. Using both with `&` and without works, I am interested to understand the good practice behind not using it, thx Art – art vanderlay Jan 18 '17 at 14:50

score 0 · Answer 2 · answered Jan 18 '17 at 14:40

First, you're passing the wrong type of argument to doParseInput. It expects a char * but you're passing it a char (*)[]. Your compiler should have warned you of this.

The other issue is that you're using a string comparison to check a single character. You need to use a character constant (i.e. use single quotes not double quotes) and compare that against the array member directly.

if (theParsedString[3] == ',') {

in C the correct way to test nth character == "x" in a string passed to a function

2 Answers2