get multiple inputs in one line C - c

Question

int main(){
char *inputFile;
char *outputFile;
int numberOfBuffer;
int pageSize;
printf("Enter four inpus, separated by spaces: ");
scanf("%s %s B=%d P=%d", &inputFile,&outputFile,&numberOfBuffer,&pageSize);
readCSV(inputFile,outputFile,numberOfBuffer,pageSize);
return 0;
}
I want to read inputs and run readCSV() method with entering command line to
students.csv test.csv B=5 P=32
that line but my code does not work. Any help?
readCSV() input types
readCSV(char* fileName,char* outputFileName, int numberOfBuffer, int pageSize)

Answer 1

You invoked undefined behavior by passing data having wrong type to scanf(): %s expects char* (pointing at a valid buffer with enough length), but you passed char**.
You should allocate some arrays and pass pointers to them. Arrays in expressions (except for some exceptions) are automatically converted to pointers to its first elements, so you don't need explicit & for them.
Also you should specify the maximum length to read (at most the buffer size minus one for the terminating null-character) to avoid buffer overrun and check if scanf() succeeded to read all required things.
int main(){
char inputFile[1024];
char outputFile[1024];
int numberOfBuffer;
int pageSize;
printf("Enter four inpus, separated by spaces: ");
if(scanf("%1023s %1023s B=%d P=%d", inputFile,outputFile,&numberOfBuffer,&pageSize) != 4){
fputs("read error\n", stderr);
return 1;
}
readCSV(inputFile,outputFile,numberOfBuffer,pageSize);
return 0;
}

Answer 2

Both inputFile and outputFile need to be declared as arrays of char large enough to hold the expected inputs1:
#define MAX_FILE_NAME_LENGTH some-value
...
char inputFile[MAX_FILE_NAME_LENGTH+1]; // +1 for string terminator
char outputFile[MAX_FILE_NAME_LENGTH+1];
Then in the scanf call, do not use the unary & operator for inputFile and outputFile - when you pass an array expression as an argument, it automatically gets converted to a pointer to the first element of the array. You also want to check the result of scanf to make sure you got all your inputs:
if ( scanf( "%s %s B=%d P=%d", inputFile, outputFile, &numberOfBuffer, &pageSize ) != 4 )
{
// bad input somewhere, probably with numberOfBuffer or pageSize,
// handle as appropriate
}
else
{
// process input normally
}
But...
scanf is an awful tool for doing interactive input. It is very hard to make bulletproof, and for stuff like this you're really better off reading the whole thing as a single large string using fgets or something like that, then extracting data from that string.
One thing that may help simplify this for you is that you don't have to read the entire line in a single scanf call. You can read each element individually:
/**
* Start by reading the input file name; we use `fgets` instead
* of `scanf` because it's easier to protect against a buffer overflow
*/
if ( !fgets( inputFile, sizeof inputFile, stdin ) )
{
// error reading input file name, handle as appropriate
}
/**
* Successfully read inputFile, now read outputFile
*/
else if ( !fgets( outputFile, sizeof outputFile, stdin ) )
{
// error reading output file name, handle as appropriate
}
/**
* Now get the number of buffers - the leading blank in the format
* string tells scanf to skip over any leading whitespace, otherwise
* if you have more than one blank between the end of the output file
* name and the 'B' the read will fail.
*/
else if ( scanf( " B=%d", &numberOfBuffer ) != 1 )
{
// error getting number of buffers, handle as appropriate
}
/**
* And finally the page size, with the same leading blank space in the
* format string.
*/
else if ( scanf( " P=%d", &pageSize ) != 1 )
{
// error getting page size, handle as appropriate
}
else
{
// process all inputs normally.
}
Or the memory for them needs to be allocated dynamically, but as you're just learning C that's something to tackle later on.

Answer 3

Most of your issues are caused by misuse of scanf. The solution here is not to fix your usage of scanf, but to avoid it completely. (http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html) Parameters like this should come from the command line arguments, not from the input stream. It is almost always better to leave the input stream clear so that it can be used for collecting data. (eg, write your program as a filter.) For example:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
static void
check_prefix(const char *a, const char *prefix)
{
if( strncmp(a, prefix, strlen(prefix)) ){
fprintf(stderr, "Invalid argument: %s Must start with, %s\n",
a, prefix);
exit(EXIT_FAILURE);
}
}
static void
readCSV(const char *in, const char *out, int n, int p)
{
printf("in = %s, out = %s, n = %d, p = %d\n", in, out, n, p);
}
int
main(int argc, char **argv)
{
if( argc < 5 ){
fprintf(stderr, "Invalid number of arguments\n");
return EXIT_FAILURE;
}
check_prefix(argv[3], "B=");
check_prefix(argv[4], "P=");
char *inputFile = argv[1];
char *outputFile = argv[2];
int numberOfBuffer = strtol(argv[3] + 2, NULL, 10);
int pageSize = strtol(argv[4] + 2, NULL, 10);
readCSV(inputFile, outputFile, numberOfBuffer, pageSize);
return 0;
}