Going through a text file line by line in C

Solution 1:

So many problems in so few lines. I probably forget some:

• argv[0] is the program name, not the first argument;
• if you want to read in a variable, you have to allocate its memory
• one never loops on feof, one loops on an IO function until it fails, feof then serves to determinate the reason of failure,
• sscanf is there to parse a line, if you want to parse a file, use fscanf,
• "%s" will stop at the first space as a format for the ?scanf family
• to read a line, the standard function is fgets,
• returning 1 from main means failure

So

#include <stdio.h>

int main(int argc, char* argv[])
{
    char const* const fileName = argv[1]; /* should check that argc > 1 */
    FILE* file = fopen(fileName, "r"); /* should check the result */
    char line[256];

    while (fgets(line, sizeof(line), file)) {
        /* note that fgets don't strip the terminating \n, checking its
           presence would allow to handle lines longer that sizeof(line) */
        printf("%s", line); 
    }
    /* may check feof here to make a difference between eof and io failure -- network
       timeout for instance */

    fclose(file);

    return 0;
}

Solution 2:

To read a line from a file, you should use the fgets function: It reads a string from the specified file up to either a newline character or EOF.

The use of sscanf in your code would not work at all, as you use filename as your format string for reading from line into a constant string literal %s.

The reason for SEGV is that you write into the non-allocated memory pointed to by line.

Solution 3:

In addition to the other answers, on a recent C library (Posix 2008 compliant), you could use getline. See this answer (to a related question).