Reading in a text file in C, separate lines into multiple variables - c

Question

I'm currently working on a program that simulates various CPU scheduling methods. Currently I have the program asking for input:
printf("Enter type of CPU scheduling algorithm (SJF, RR, PR_noPREMP, PR_withPREMP): ");
scanf("%s", typeOf);
printf("Enter number of processes: ");
scanf("%d", &numPro);
struct processStruct structs[numPro];
int burstTimes[numPro];
for (i = 0; i < numPro; i++) {
printf("Enter process number: ");
scanf("%d", &structs[i].pNum);
printf("Enter arrival time: ");
scanf("%d", &structs[i].arTime);
printf("Enter CPU burst time: ");
scanf("%d", &structs[i].cpuBur);
printf("Enter priority: ");
scanf("%d", &structs[i].prio);
}
In addition to the two variables typeOf (an int) and numPro (a char array) I am also using a data structure.
Here is the data structure that is holding the various parameters:
struct processStruct {
int pNum;
int arTime;
int cpuBur;
int prio;
int waitTim;
};
Instead of manual input I could like to use a text file with the same information as input for the program. The text file would look something like this:
SJF
4
1 0 6 1
2 0 8 1
3 0 7 1
4 0 3 1
First line is the name of the scheduling algorithm.
Second line is the number of processes.
The following lines consists of information for each process. So 1 0 6 1 = Process = 1, 0 = Arrival Time, 6 = CPU burst time, 1 = Priority
I unfortunately have little experience using text file input with C. Does anyone have ideas about how I could read in the data from the text file into the variables and data structure?
Thank you
Edit: One of the issues I am having is that the data is not the same for each line. If it was just the rows of 4 numbers then it would be relatively easy. I need the program to read the first line into a char array (string), the second into the numPro variable then the subsequent lines into multiple instances of the data structure (one for each process).

Answer 1

The file can be read fairly simply with fscanf() because everything except the first line identifier is a number. But you do need to check the validity of what is read from the file. I have just used exit(1) on error for illustration, it could be more sophisticated than that (for example an error message).
#include <stdio.h>
#include <stdlib.h>
#define MAX 100
struct processStruct {
int pNum;
int arTime;
int cpuBur;
int prio;
int waitTim;
};
struct processStruct structs[MAX];
int main(int argc, char** args)
{
FILE *fil;
char typeOf[4];
int numPro, i;
if ((fil = fopen("myfile.txt", "rt")) == NULL)
exit(1);
if(fscanf(fil, "%4s", typeOf) != 1)
exit(1);
if(fscanf(fil, "%d", &numPro) != 1)
exit(1);
if(numPro > MAX)
exit(1);
for(i=0; i<numPro; i++) {
if(fscanf(fil, "%d%d%d%d", &structs[i].pNum, &structs[i].arTime,
&structs[i].cpuBur, &structs[i].prio) != 4)
exit(1);
}
fclose(fil);
// test the result
printf("Type: %s\n", typeOf);
printf("Num: %d\n", numPro);
for(i=0; i<numPro; i++) {
printf("%d %d %d %d\n", structs[i].pNum, structs[i].arTime,
structs[i].cpuBur, structs[i].prio);
}
return 0;
}
Program output:
Type: SJF
Num: 4
1 0 6 1
2 0 8 1
3 0 7 1
4 0 3 1

Answer 2

You will want to use fscanf and fprintf instead of scanf and printf to do I/O from/to files instead of standard input/output.
These are widely documented. You will use FILE * variables and fopen & fclose. You can even use stdin, stdout, and stderr as handles for the console.

Answer 3

The best way to read lines in a file with C is to use the getline() function. It is much more reliable than scanf() and can allocate the needed memory automagically.
Here is the code I suggest:
#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, char **argv)
{
FILE *inputfile;
if (argc < 2)
{
fprintf (stderr, "error: missing file name in the arguments\n");
exit (EXIT_FAILURE);
}
inputfile = fopen (argv[1], "r");
if (!inputfile)
{
perror ("myprogram");
exit (EXIT_FAILURE);
}
/* Getting first line */
char *line = NULL;
size_t line_size = 0;
if (!getline(&line, &line_size, inputfile))
{
fprintf (stderr, "error: failed to read first line\n");
exit (EXIT_FAILURE);
}
/* Getting the size of the matrix */
unsigned int size; /* Size of the matrix */
if (sscanf (line, "%zd", &size) != 1)
{
fprintf (stderr, "error: first line has wrong format\n");
exit (EXIT_FAILURE);
}
/* Getting the content of the matrix */
int matrix[size][size];
for (unsigned int i = 0; i < size; i++)
{
if (!getline (&line, &line_size, inputfile))
{
fprintf (stderr, "error: input file has wrong format\n");
exit (EXIT_FAILURE);
}
/* Copying the items of the line to the matrix */
char *ptr = line;
for (unsigned j = 0; j < size; j++)
{
matrix[i][j] = atoi(strtok(ptr, " "));
ptr = NULL;
}
}
free(line);
return EXIT_SUCCESS;
}
Beware, I didn't tried this code... So, if something is not working properly, I'll fix it.