How to pass commandline argument as part of a string parameter

Question

This is part of the code I'm facing issue with :

void encrypt(const char *fileIn, const char *fileOut, const unsigned char *key);

int main(int argc, char *argv[]) 
{
     const unsigned char key[100];
     srand(time(NULL));

     aes_init();
     encrypt(argv[1], "/home/python/encrypt/"argv[1]".encrypted", argv[3]);

     return 0;
 }

As you can see, in the encrypt function, I'm asking the user to enter the file name via command line for input. For output of the same function, I wanted the same name to be just appended by '.encrypted'. However, I get the following error whenever I try to compile the code.

In function ‘main’:
error: expected ‘)’ before ‘argv’
error: too few arguments to function ‘encrypt’
note: declared here

What am I doing wrong? Please help.

"/home/python/encrypt/"argv[1]".encrypted" this is how you manipulate it? what you want here? — Jayesh Bhoi
– Jayesh Bhoi, Commented Jul 19, 2014 at 10:20
The code works fine and I get the desired output(encrypted file) if I just name it as "outfile.encrypted" but I want each output file to have a different name, I.e, the original name appended by '.encrypted'. Like "trial.doc" to become "trial.doc.encrypted". — Anki Ju
– Anki Ju, Commented Jul 19, 2014 at 10:26

Jayesh Bhoi · Accepted Answer · 2014-07-19 10:25:33Z

2

I think you want something easy string manipulation like this

snprintf(key,100,"/home/python/encrypt/%s.encrypted",argv[1]);
encrypt(argv[1],key, argv[3]);

answered Jul 19, 2014 at 10:25

Jayesh Bhoi

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5 Comments

Grzegorz Szpetkowski Over a year ago

Note that second parameter of snprintf includes null character, so maximum possible string's length (i.e. strlen) is 99. Longer strings are simply cut-off.

Anki Ju Over a year ago

So, I tried what you said and this is what I got - In function ‘main' warning: passing argument 1 of ‘snprintf’ discards ‘const’ qualifier from pointer target type [enabled by default] /usr/include/stdio.h:387:12: note: expected ‘char * restrict’ but argument is of type ‘const unsigned char *’

alk Over a year ago

Just remove the const modifer from key's definition. @AnkiJu

Anki Ju Over a year ago

@alk ok I did that. Program gets compiled! :) but now, I'm facing something else - Segmentation fault (core dumped)

alk Over a year ago

On how to debug a (small) program, you might like to read here: ericlippert.com/2014/03/05/how-to-debug-small-programs @AnkiJu

Peter Miehle · Accepted Answer · 2014-07-19 10:26:04Z

1

in C, string manipulation is not as smooth as in modern languages. You have to append strings by using library functions.

char buffer[CCHMAXPATH];
sprintf(buffer, "/home/%s.encrypted", argv[1]);
encrypt(argv[1], buffer, argv[3]);

answered Jul 19, 2014 at 10:26

Peter Miehle

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1 Comment

Grzegorz Szpetkowski Over a year ago

This has potential UB (unless input is "trusted"), when strlen(argv[1]) >= CCHMAXPATH - strlen("/home/python/encrypt/" ".encrypted") (sprintf automatically appends null character).

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