C program. Taking string input to char pointer problem

Question

Hello I have a problem with my C code. I'm trying to let user assign a string to a previously defined char *srchstr; and char *repstr; . By looking at other similar threads I've tried implementing it in the following way but it still fails:

char *srchstr;
srchstr = malloc(256);
char *repstr;
repstr = malloc(256);
printf("what are you searching for?:");
scanf("%255s",&srchstr);
fflush(stdin);
printf("\n what do you want to replace it with?:");
scanf("%255s",&repstr);

The whole idea behind the program was to give user ability to chose what text he wants to replace with what (whole code works just fine with srchstr and repstr defined in code but i cannot implement user input) this is how it looked like in the beggining:

char *srchstr = "400,300";
char *repstr = "400,300: (000,000,000) #000000";

How can I fix it so user can type in the srchstr and repstr ?

Leave off the & in the scanf calls. scanf("%s",...) wants a char * which is srchstr but &srchstr is char **. So ... scanf("%255s",srchstr); — Craig Estey
– Craig Estey, Commented Apr 22, 2020 at 23:33
fflush(stdin) is undefined behavior, i.e. don't do that. Instead of scanf, use fgets to read a line of input, and use strcspn to remove the newline that fgets puts in the buffer. — user3386109
– user3386109, Commented Apr 22, 2020 at 23:40

marc_s · Accepted Answer · 2020-06-21 14:20:45Z

1

scanf("%255s",&srchstr); -> scanf("%255s", srchstr);

scanf("%255s",&repstr); -> scanf("%255s",repstr);

The & operator means you are passing the address of a given memory space, effectively a pointer the memory space in which to store the input, identified by a variable name, that would be fine because scanf expects precisely that.

But since srchstr and repstr are already pointers, you are effectively passing the address of the pointers (pointers to the pointers) instead of the address of the memory space (pointers to the memory space) in which to store the input.

Side note:

fflush(stdin)

Invokes undefined behaviour, it's meant to be used with stdout.

You can replace it with:

int c;
while((c = fgetc(stdin)) != '\n' && c != EOF) {}

edited Jun 21, 2020 at 14:20

marc_s

760k186 gold badges1.4k silver badges1.5k bronze badges

answered Apr 22, 2020 at 23:42

anastaciu

24.1k7 gold badges37 silver badges62 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

anastaciu Over a year ago

@PatrykPiwowarczyk, I'm glad I could help, don't forget to accept the answer if you think it correctly addresses your problem.

Collectives™ on Stack Overflow

C program. Taking string input to char pointer problem

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related