add string to given command line argument in C

Question

Hello i just started to programm in C and i am trying to read a file and give the file name as argument without the ending .txt . I want to add .txt in my code : ./myexample.exe file

if i use file.txt there is no problem but i dont know how to change argv[1] i tried char *n = argv[1] + ".txt"; it doesnt works and i dont know anything else..

int main(int argc, char* argv[]) {
      char *n = argv[1] +".txt";
      FILE *fp1;
      fp1 = fopen(n , "r");

Thats what i get if i use char *n = argv[1]+".txt"

error: invalid operands to binary + (have 'char *' and 'char *')

Possible duplicate of [How to concatenate const/literal strings in C?](http://stackoverflow.com/questions/308695/how-to-concatenate-const-literal-strings-in-c) — UnholySheep, Nov 05 '16 at 17:08
1) A pointer is not a string! 2) A pointer is not an array. 3) C does not have a distinct string type. — too honest for this site, Nov 05 '16 at 17:34

score 3 · Answer 1 · answered Nov 05 '16 at 17:08

In your code,

 char *n = argv[1] +".txt";

does not do what you think it does. In C, the + cannot be used to concatenate strings.

FYI, from C11, chapter §6.5.6

For addition, either both operands shall have arithmetic type, or one operand shall be a pointer to a complete object type and the other shall have integer type. (Incrementing is equivalent to adding 1.)

Both the operands cannot be of pointer-to-object type.

If you meant to concatenate strings, use strcat() but make sure

the destination is modifiable (attempt to modify string literal is UB)
the destination got enough space to contain the final result (shorter destination lead to access out of bound memory invoking UB, again).

yassin · Accepted Answer · 2016-11-05T17:41:34.743

1

You can't concatenate strings with +. Use strcpy and strcat:

char n[256];
strcpy(n, argv[1]);
strcat(n, ".txt");

Make sure that n is large enough to hold the filename + extension.

edited Nov 05 '16 at 17:41

answered Nov 05 '16 at 17:20

yassin

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100 is not big enough to hold any filename. Need at least 256 to hold any filename on Linux for instance. – Chris Nov 05 '16 at 17:25
Thanks for the hint – yassin Nov 05 '16 at 17:42
It would probably be wise to check if the string fits in the buffer before copying, too. – Chris Nov 05 '16 at 17:54

score 1 · Answer 3 · answered Nov 05 '16 at 17:34

This is kind of annoying to do safely: you need to be careful to make a big enough char array to hold the whole string. One way:

size_t arglen=strlen(argv[1]);
char* filename=malloc(arglen+5); //Enough to hold the whole string
strcpy(filename,argv[1]);
strcat(filename,".txt");

Be sure to free the pointer later.

add string to given command line argument in C

3 Answers3