C Programming , getc() , stdin , file redirection

Question

My assignment is to redirect a text file and do all sorts of operations on it , everything is working except I have a little problem :

so the main function that reads input is getline1():

char* getline1(){
char *LinePtr = (char*)malloc(sizeof(char*)*LINE);
int i = 0;
for ( ; (*(LinePtr+i) = getc(stdin)) != '\n' ; i++){}
*(LinePtr+i) = '\0';
return LinePtr;
}

it returns a pointer to char array of a single line,

so we know that a new line saparates with '\n' char,

previous problem I had is when I wrote the getline1() function like this :

for (int i = 0 ; Line[i] != '\n' ; i++){
Line[i] = getc(stdin);
}

as it logically it may be authentic the getc() is a streaming function and I saw online answers that this will not work didn't quite understand why.

anyway the big issue is that I need to know how many lines there are in the text so I can stop reading values , or to know from getline1() function that there is no next line left and Im done.

things we need to take for account : 1.only <stdio.h> <stdlib.h> need to be used 2.Im using Linux Ubuntu and gcc compiler 3.the ridirection goes like ./Run<input.txt

also I understand that stdin is a file pointer , didn't found a way that this can help me.

Thank you , Denis

This is prone to overflow, what if the input is larger than `LINE` ? — alex01011, Jan 01 '21 at 19:20
You are allocating more space than you need or expect — use `sizeof(char)` or `sizeof(*linePtr)` as the multiplier, or don't use a multiplier at all since `sizeof(char) == 1`. — Jonathan Leffler, Jan 01 '21 at 19:20
everything should be fixed so LINE = 256 for all inputs ,don't worry about the memory Im in control everything is calculated I just need to know how to know when the text file ends or how many lines there are — architect7992, Jan 01 '21 at 19:23
`int ch; while((ch = getc(stdin)) != EOF) { /* process the character */ }` — Weather Vane, Jan 01 '21 at 19:27

score 1 · Accepted Answer · answered Jan 01 '21 at 19:30

1

You should check for the EOF signal in addition to the newline character, you should also check for that your index-1 is always smaller than LINE to avoid overflow and also save space for the NULL terminator.

#define LINE 100

char *my_getline(void)
{
    size_t i = 0;
    char *str = NULL;
    int c = 0;

     if ((str = malloc(LINE)) == NULL)
     {
         fprintf(stderr,"Malloc failed");
         exit(EXIT_FAILURE);
     }

    while (i+1 < LINE && (c = getchar()) != EOF && c != '\n') /* Saving space for \0 */
    {
        str[i++] = c;
    }
    str[i] = '\0';

    return str;
}

answered Jan 01 '21 at 19:30

alex01011

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hi Alex it doesn't work , tried with the dedicated function getc like this : (blablabla && (c = getc(stdin) != EOF && c!='\n')) also tried (blablabla && (c = getchar() != EOF && c!='\n')) and didn't work :/ – architect7992 Jan 01 '21 at 19:49
@architect7992 By not working, you mean that it only saved the first line of the file? – alex01011 Jan 01 '21 at 19:53
no , the loop doesn't stop , and the text file is not empty , nothing stops the loop – architect7992 Jan 01 '21 at 20:18

score 0 · Answer 2 · answered Jan 01 '21 at 20:53

Thanks for everybody , I just made another function to count line this was the only lazy option available :)

static void linecounter(){
FILE *fileptr;
int count = 0;
char chr;
fileptr = fopen("input.txt", "r");
chr = getc(fileptr);
while (chr != EOF){
if (chr == '\n'){count = count + 1;}
chr = getc(fileptr);}
fclose(fileptr);
count_lines = count;}

C Programming , getc() , stdin , file redirection

2 Answers2