Reading from stdin using fgets()

问题:

I'm new to C programming and I'm currently trying to read a line from stdin using fgets(), but I'm having trouble with memory allocation since I'm using a char* to point to the string I want to read. When I execute the file it reports a segmentation fault.

This is the function I'm using:

char *read_line(char *line){
    printf("%s",PROMPT);
    line = (char*)malloc(sizeof(char)*500);
    fgets(line,sizeof(line),stdin);
    printf("%s","pasa el fgets");
    return line;
}

And my main:

void main(){
    char line0;
    char *line=&line0;
    while(read_line(line)){
        execute_line(line);
    }
}

回答1:

The main mistake is to pass the pointer line to the function read_line (by value) and try to modify it in that function.

read_line allocates the memory and actually creates the pointer value. So it should be able to change the value of line in main:

char *read_line(char **line){
    ...
    *line = malloc(500);
    fgets(*line, 500, stdin);
    ...
    return *line;
}

int main(void) {
    char *line;
    while(read_line(&line)){
        ...
    }

}

Or, you use the return value of read_line in order to modify main's line. In that case you don't need the parameter at all:

char *read_line(void) {
    char *line;
    ...
    line = malloc(500);
    fgets(line, 500, stdin);
    ... 
    return line;
}

int main(void) {
    char *line;
    while(line = read_line()){
        ...
    }
}

Additional errors (pointed out by Jonathon Reinhart) and remarks:

1. sizeof does not "work" for pointers (array decayed to pointers).
2. You malloc many strings line but you do not free them.
3. sizeof(char) is always 1.
4. Some people (me too) think that casting the result of malloc should be avoided.