I have been working on a small exercise for my CIS class and am very confused by the methods C uses to read from a file. All that I really need to do is read through a file line by line and use the information gathered from each line to do a few manipulations. I tried using the getline method and others with no luck. My code is currently as follows:
int main(char *argc, char* argv[]){
const char *filename = argv[0];
FILE *file = fopen(filename, "r");
char *line = NULL;
while(!feof(file)){
sscanf(line, filename, "%s");
printf("%s\n", line);
}
return 1;
}
Right now I am getting a seg fault with the sscanf method and I am not sure why. I am a total C noob and just wondering if there was some big picture thing that I was missing. Thanks
Say you're dealing with some other delimiter, such as a \t
tab, instead of a \n
newline.
A more general approach to delimiters is the use of getc()
, which grabs one character at a time.
Note that getc()
returns an int
, so that we can test for equality with EOF
.
Secondly, we define an array line[BUFFER_MAX_LENGTH]
of type char
, in order to store up to BUFFER_MAX_LENGTH-1
characters on the stack (we have to save that last character for a \0
terminator character).
Use of an array avoids the need to use malloc
and free
to create a character pointer of the right length on the heap.
#define BUFFER_MAX_LENGTH 1024
int main(int argc, char* argv[])
{
FILE *file = NULL;
char line[BUFFER_MAX_LENGTH];
int tempChar;
unsigned int tempCharIdx = 0U;
if (argc == 2)
file = fopen(argv[1], "r");
else {
fprintf(stderr, "error: wrong number of arguments\n"
"usage: %s textfile\n", argv[0]);
return EXIT_FAILURE;
}
if (!file) {
fprintf(stderr, "error: could not open textfile: %s\n", argv[1]);
return EXIT_FAILURE;
}
/* get a character from the file pointer */
while(tempChar = fgetc(file))
{
/* avoid buffer overflow error */
if (tempCharIdx == BUFFER_MAX_LENGTH) {
fprintf(stderr, "error: line is too long. increase BUFFER_MAX_LENGTH.\n");
return EXIT_FAILURE;
}
/* test character value */
if (tempChar == EOF) {
line[tempCharIdx] = '\0';
fprintf(stdout, "%s\n", line);
break;
}
else if (tempChar == '\n') {
line[tempCharIdx] = '\0';
tempCharIdx = 0U;
fprintf(stdout, "%s\n", line);
continue;
}
else
line[tempCharIdx++] = (char)tempChar;
}
return EXIT_SUCCESS;
}
If you must use a char *
, then you can still use this code, but you strdup()
the line[]
array, once it is filled up with a line's worth of input. You must free
this duplicated string once you're done with it, or you'll get a memory leak:
#define BUFFER_MAX_LENGTH 1024
int main(int argc, char* argv[])
{
FILE *file = NULL;
char line[BUFFER_MAX_LENGTH];
int tempChar;
unsigned int tempCharIdx = 0U;
char *dynamicLine = NULL;
if (argc == 2)
file = fopen(argv[1], "r");
else {
fprintf(stderr, "error: wrong number of arguments\n"
"usage: %s textfile\n", argv[0]);
return EXIT_FAILURE;
}
if (!file) {
fprintf(stderr, "error: could not open textfile: %s\n", argv[1]);
return EXIT_FAILURE;
}
while(tempChar = fgetc(file))
{
/* avoid buffer overflow error */
if (tempCharIdx == BUFFER_MAX_LENGTH) {
fprintf(stderr, "error: line is too long. increase BUFFER_MAX_LENGTH.\n");
return EXIT_FAILURE;
}
/* test character value */
if (tempChar == EOF) {
line[tempCharIdx] = '\0';
dynamicLine = strdup(line);
fprintf(stdout, "%s\n", dynamicLine);
free(dynamicLine);
dynamicLine = NULL;
break;
}
else if (tempChar == '\n') {
line[tempCharIdx] = '\0';
tempCharIdx = 0U;
dynamicLine = strdup(line);
fprintf(stdout, "%s\n", dynamicLine);
free(dynamicLine);
dynamicLine = NULL;
continue;
}
else
line[tempCharIdx++] = (char)tempChar;
}
return EXIT_SUCCESS;
}
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
.
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).
Source: Stackoverflow.com