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I am trying to write an argument to a string. What am I doing wrong?
How would it be more correct to write the argument from getopt to a string?
#include <string.h>
#include <unistd.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
char fsrt[100], sstr[100], tstr[100];
int rez = 0;
while ( (rez = getopt(argc, argv, "f:s:l:")) != -1 )
{
if (rez == 'f') {
strcat(fsrt, optarg);
}
if (rez == 's') {
strcat(sstr, optarg);
}
if (rez == 'l') {
strcat(tstr, optarg);
}
}
printf("%s %s %s", fsrt, sstr, tstr);
return 0;
}
Update:
strange symbol in output.
./test -f one -s two -l three
output: https://i.stack.imgur.com/kksVh.png
char fsrt[100] = "", sstr[100]= "", tstr[100]= "";
If you don't initialize the char arrays they randomly point somewhere in memory.
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Closed 2 years ago.
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I run the program with this command:
./word_search oi < text.txt
and got segmentation fault when running it.
This program is aiming to find where the word (giving as command line arg) exists in a file and print out those lines.
#include <stdio.h>
#include "substring.c"
int main(int argc, char ** argv) {
if(argc == 2) {
char *str;
while(fgets(str, 100, stdin)) {
if(substring(str, argv[1]) != -1) {
printf("Found: %s", str);
}
}
}
return 0;
}
If I change char *str into char str[100] then it works pretty good. Could anyone please tell me why?
The contents in substring.c:
#include <stdio.h>
#include <string.h>
int substring(const char *line, const char *substr) {
int i, j;
int result;
for(i = 0; i <= strlen(line)-strlen(substr); i++) {
result = 0;
if(line[i] == substr[0]) {
int c = i;
for(j = 0; j < strlen(substr); j++) {
if (line[c] != substr[j]) {
result = -1;
}
c++;
}
if(result != -1)
return i;
}
}
return -1;
}
The contents in test.txt are just several lines of meaningless characters.
char *str is an unitialized pointer, it cannot hold the string you are trying to copy into it, either allocate memory to it:
#include <stdlib.h>
#define SIZE 100
char *str = malloc(SIZE); //char has the size of 1 across platforms
Or simply declare it with the size you need:
char str[SIZE];
Pass the size of str to fgets
while(fgets(str, SIZE, stdin))
Of fgets:
Your container will be null terminated, it can only hold a string of SIZE - 1 characters.
All characters above SIZE - 1, including '\n' will remain unread and therefore in the buffer, you might need to clear it.
I suggest you take some time to learn basic C. Especially read about pointers, they are bit hard to get right at first.
In your example, str is a pointer to an undefined memory location.
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I have a program that does two separate things. But I want to the program to only run one specific part based on what I enter in the command like in the format
program -a
program -s
where the a or s would be programmed to tell it to do something specific.
For example, if my program was something like:
# program -a entered
z = x + y
# program -s entered
z = x - y
Now I could easily do this with an if statement after running the program, but is it possible to directly skip to a statement like I said from the command prompt?
int main(int argc, char* argv[]) {
if (argc >= 2) {
if (strcmp(argv[1], "-a") == 0) {
z = x + y;
} else if (strcmp(argv[1], "-s") == 0) {
z = x - y;
}
}
return 0;
}
Determine if the value of argv[1] equal to "-a" or "-s"
You can use a switch statement on command-line arguments (in this case argv[1]).
argv[1] is a string containing the options you have given ("-a", "-s" etc).
But you have to perform certain checks for the validity of the command-line arguments before you can use them like this.
Check if argc == 2
Check if strlen(argv[1]) == 2
Check if argv[1][0] == '-'
An MCVE would look like this:
#include <stdio.h>
#include <string.h>
int main(int argc, char* argv[]) {
if(argc == 2 && strlen(argv[1]) == 2 && argv[1][0] == '-')
{
switch(argv[1][1])
{
case 'a':
//do something
break;
case 's':
//do something else
break;
default:
//do the default thing
break;
}
}
else
{
//print appropriate error message
}
}
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When prompting the user to enter there name, it is necessary that the user indeed entered a name and not just whitespace.
Output ex:
Name:
Your name is: /user entered only whitespace/
How would one validate that characters were actually implemented, and not just spaces? Im using fgets() to retreive my input however i am unable to post the code now for I am at work and doing this off my mobile device. If code is nessesary i will post what im working on late tonight (pacific standard time). Thank you!
One way to test is to check all characters if they are space.
#include <stdio.h>
#include <stdlib.h>
// TODO: you should use bool instead
int is_all_space(const char *s)
{
char *tmp;
tmp = (char *) s;
// you might check for '\n' instead in your case
// or even '\r'
while (*tmp != '\0') {
// TODO: this check will fail if the input has zero length
if (*tmp != ' ') {
return 0;
}
tmp++;
}
return 1;
}
// don't forget to put the argument in quotes if it contains spaces
int main(int argc, char **argv)
{
if (argc < 2) {
fprintf(stderr, "Usage: %s somestring\n", argv[0]);
exit(EXIT_FAILURE);
}
if (is_all_space(argv[1])) {
puts("Input is all-space and nothing else");
} else {
puts("Input it not all-space");
}
exit(EXIT_SUCCESS);
}
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Closed 8 years ago.
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How do I write a replace function which takes a pointer to a string as a parameter and replaces all spaces in that string with minus signs?
example :
input = "I love pies", output = "I-love-pies"
This is what you need:
#include <stdio.h>
int change_string(char *input) {
for (char *p = input; *p; p++) {
if (*p == ' ') *p = '-';
}
return 1;
}
int main() {
char input[] = "I love pies";
printf("%s\n", input);
change_string(input);
printf("%s\n", input);
return 0;
}
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I am new to this and I can't seem to understand can you please help me identify the buffer that can be overflowed and why?
#include <stdio.h>
#include <string.h>
#define S 100
#define N 1000
int main(int argc, char *argv[]) {
char out[S];
char buf[N];
char msg[] = "Welcome to the argument echoing program\n";
int len = 0;
buf[0] = '\0';
printf(msg);
while (argc) {
sprintf(out, "argument %d is %s\n", argc-1, argv[argc-1]);
argc--;``
strncat(buf,out,sizeof(buf)-len-1);
len = strlen(buf);
}
printf("%s",buf);
return 0;
}
The problem is here:
sprintf(out, "argument %d is %s\n", argc - 1, argv[argc - 1]);
When you face a problem like this one, it's always a good idea to go through your code commenting lines until the program stops crashing. Then you will know where the bug is :D