Searching char array for string in C

Question

I have a string that looks like this:

"show net\r\r\nSSid=roving1\r\nChan=1\r\nAssoc=FAIL\r\nRate=12,   24Mb\r\nAuth=FAIL\r\nMode=NONE\r\nDHCP=FAIL\r\nBoot=0\r\nTime=OK\r\nLinks=0\r\n<4.00> "

This is data I receive from a peripheral device and I want to scan it for the string "Auth=FAIL". I'm querying it using the following command:

res = strstr(uart_rd, "Auth=FAIL");

however, it returns a value of 0. But if I simply issue the command:

res = strstr("nnAuth=FAILn", "Auth=FAIL");

it returns a value. What could be going wrong when it tries to search my char array uart_rd?

EDIT:

It looks like the issue was with a memset I was doing to clear the array before putting new data into it. I was filling it with 0s using the command:

memset(uart_rd,0,sizeof(uart_rd));

whereas I've now changed it to

memset(uart_rd,"",sizeof(uart_rd));

and it all seems to be working. Thanks for the help in diagnosing where my problem was though!

strstr() certainly does work. If it can't find the substring in your array, then the substring isn't there. Maybe an encoding issue, or you accidentally pasted "invisible" characters either in the haystack or in the needle, or perhaps you have some other error at another place in your code (e. g. a buffer overflow that overwrites the contents of your array or whatnot). — user529758
– user529758, Commented Jan 9, 2014 at 9:31
Please give a sscce example which can be used to reproduce your problem. — Lee Duhem
– Lee Duhem, Commented Jan 9, 2014 at 9:40

Mehmet Sedat Güngör · Accepted Answer · 2014-01-09 09:42:49Z

2

Well, this code

    char uart_rd[] = "show net\r\r\nSSid=roving1\r\nChan=1\r\nAssoc=FAIL\r\nRate=12,   24Mb\r\nAuth=FAIL\r\nMode=NONE\r\nDHCP=FAIL\r\nBoot=0\r\nTime=OK\r\nLinks=0\r\n<4.00> ";
    char *res;
    res = strstr(uart_rd, "Auth=FAIL");
    printf("%s", res);

gives this output:

Auth=FAIL
Mode=NONE
DHCP=FAIL
Boot=0
Time=OK
Links=0
<4.00>

So you better check your declarations of strings.

answered Jan 9, 2014 at 9:42

Mehmet Sedat Güngör

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3 Comments

chadianscot Over a year ago

I agree, i tried the same thing and it worked. I'm wondering if it's to do with the size of my array? My char array is declared as char uart_rd[1000], which gets filled with data from the peripheral. At some times, there is a lot of data, hence the large size. So when it gets filled with the data above, it puts it about 90 positions in, and the rest of the array is filled with 0s. Is it possible the size of the array is throwing up issues or something?

Mehmet Sedat Güngör Over a year ago

No, I don't think so. In the code above, uart_rd with size 1000 will give the same output. How did you declare res variable? Can you show it?

chadianscot Over a year ago

It looks like the issue was with a memset I was doing to clear the array before putting new data into it. I was filling it with 0s using the command: memset(uart_rd,0,sizeof(uart_rd)); whereas I've now changed it to memset(uart_rd,"",sizeof(uart_rd)); and it all seems to be working. Thanks for the help in diagnosing where my problem was though!

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