CIS241

System-Level Programming and Utilities

C - Reading Input

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Input Types

• stdin
• Command line arguments
• Files

scanf

scanf(format_str, mem_addr_1, mem_addr_2, …);

• format_str is a formatted string like in printf

  • E.g., "%d %f" for an int, space, float

• Each specified var (e.g., %d), will need a memory address

  • Remember, it’s a pointer so the function can change the underlying memory!

• scanf returns a EOF (constant) if EOF (Ctrl+D) is sent via stdin

  • We can use this to know when the user wants to stop

Documentation: https://cplusplus.com/reference/cstdio/scanf/

Example

scanf(format_str, mem_addr_1, mem_addr_2, …);

int x = 0;
int sum = 0;
while (1) 
{
    int res = scanf("%d", &x);
    if(res == EOF) break;
    sum += x;
}
printf("Sum is %d\n", sum);

fgets

fgets(char* s, int size, FILE* stream);

• For now, our stream is stdin (built in)

  • We need to allocate char buffer

• Will read, at most, n-1 chars

  • Null terminator (\0) placed after last char read

• Stops at newline or EOF

• Returns s if successful, NULL if not

Documentation:
https://cplusplus.com/reference/cstdio/fgets/

Example

char s[10];
int main()
{
    while(1)
    {
        char* res = fgets(s, 10, stdin);
        printf("received input: %s\n", s);
        if(res == NULL) break;
    }
}

getline

getline(char** lineptr, size_t *n, FILE* stream);

• For now, our stream is stdin (built in)

• Not in C standard, part of POSIX > 2008

• Delimits by newline

• Reallocates memory if more room is needed

• Allocates memory for you if lineptr is NULL and n=0

• Updates both values

  • Returns the number of chars read

• Or -1 for errors

Documentation:
https://man7.org/linux/man-pages/man3/getline.3.html

Example

getline(char** lineptr, size_t *n,FILE* stream);

void main()
{
    while(1)
    {
        char* s = NULL;
        size_t n = 0;
        int res = getline(&s, &n, stdin);
        printf("Received: %s\n", s);
        free(s);
        if(res == -1) break;
    }
}

Notes

Be careful about newlines

• scanf(" %c", …) is recommended over scanf("%c\n", …)
• This is why printing strings from fgets and getline have empty lines after them!

Be careful mixing and matching these functions

• e.g., fgets and getline consume newlines
• scanf does not

I prefer scanf

• It makes dealing with non-string types easier
• We'll talk later about str->int conversion

Interestingly...

https://www.reddit.com/r/C_Programming/comments/k2js6w/why_does_the_space_before_c_in_scanf_matter_that/

and

https://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html