Weird characters when concatenating strings in c

Question

I'm trying to concatenate multiple strings to create a longer phrase but I get weird, seemingly random, characters in front of the phrase when I try to print it. Here is the code:

char* str2 = argv[2];
int len2 = strlen(str2);
char* str3 = argv[3];
int len3 = strlen(str3);
printf("child PID(%d) receives Y = '%s' and Z= '%s' from the pipe\n",getpid(), str2, str3 );

//get the length of arguments 2 and 3 and create an array that is 1 larger than that (for the space between the two phrases)
//then concatenate them
int catLen = len2+len3;
char conc[catLen + 1];
strcat(conc, str2);
strcat(conc," ");
strcat(conc, str3);
printf("child PID(%d) concatenates Y and Z to generate Y' = '%s'\n",getpid(),conc);

int len1;
//get the length of the first command-line argument in the pipe
read(port[0],&len1,sizeof(len1));

//get the first command-line argument from the pipe
char str1[len1];
read(port[0], &str1,len1);
printf("child PID(%d) reads X from the pipe = '%s'\n",getpid(),str1);


//problems start when concatenating strings here        
int totalLen = len1+catLen+1;
char phrase[totalLen];
strcat(phrase, str1);
printf("%s\n",phrase);
strcat(phrase," ");
printf("%s\n",phrase);
strcat(phrase,conc);
printf("child PID(%d) concatenates X and Y' to generate Z' = '%s'\n",getpid(),phrase);

I only encounter this issue when I try and concatenate the second set of strings, and I don't understand why. What is different between how I did it earlier and at the bottom the program?

They both have the same problem. `strcat` requires that both parameters be strings. But both `conc` and `phrase` are unintialised and hence not valid strings. The first one only "works" by chance and not because it is correct code. Add `conc[0] = 0;` and `phrase[0] = 0;` before passing them to their respective first `strcat` calls. — kaylum, Sep 26 '21 at 20:50

Nate Eldredge · Accepted Answer · 2021-09-26T22:14:12.080

First of all, as Ptit Xav points out, you need one additional byte of space for conc. You need len1 bytes for the first string, plus 1 for the space, plus len2 for the second string, and then one more for the terminating null character. So if catLen = len1 + len2, then you should declare char cond[catLen + 2]; (or change the calculation of catLen in some equivalent way).

Next, when you declare a local array like char conc[catLen + 2]; without initializing it, it contains garbage. If you then attempt to strcat onto it, you concatenate onto garbage. If the garbage happens not to contain any null bytes, then strcat will overrun the array searching for a null byte, causing undefined behavior (which is very bad).

So either make your first use a strcpy instead of strcat:

char conc[catLen + 2];
strcpy(conc, str2);  // note strcpy here
strcat(conc," ");
strcat(conc, str3);

Or else set the first character of your array to be a null character before starting, so that it becomes an empty string:

char conc[catLen + 2];
conc[0] = '\0';     // conc now contains an empty string
strcat(conc, str2);
strcat(conc," ");
strcat(conc, str3);

You can also initialize the array:

char conc[catLen + 2] = "";
strcat(conc, str2);
strcat(conc," ");
strcat(conc, str3);

However this will cause the program to fill the entire array conc with null bytes, where you really only needed one, so it is inefficient.

I think you must add 1 to catlen because you add a space between str2 and str3. So size of conc array should be : len2(str2) + 1(space)+len3(str3)+1(\0 — Ptit Xav, Sep 26 '21 at 22:07

Weird characters when concatenating strings in c

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