I'm struggling to implement a shell with pipelines for class.

typedef struct {
    char** cmd;
    int in[2];
    int out[2];
} cmdio;

cmdio cmds[MAX_PIPE + 1];

Commands in the pipeline are read and stored in cmds.

cmdio[i].in is the pair of file descriptors of the input pipe returned by pipe(). For the first command, which reads from terminal input, it is just {fileno(stdin), -1}. cmdin[i].outis similar for the output pipe/terminal output. cmdio[i].in is the same as cmd[i-1].out. For example:

$ ls -l | sort | wc

CMD: ls -l 
IN: 0 -1
OUT: 3 4

CMD: sort 
IN: 3 4
OUT: 5 6

CMD: wc 
IN: 5 6
OUT: -1 1

We pass each command to process_command, which does a number of things:

for (cmdi = 0; cmds[cmdi].cmd != NULL; cmdi++) {
    process_command(&cmds[cmdi]);
}

Now, inside process_command:

if (!(pid_fork = fork())) {
    dup2(cmd->in[0], fileno(stdin));
    dup2(cmd->out[1], fileno(stdout));    
    if (cmd->in[1] >= 0) {
        if (close(cmd->in[1])) {
            perror(NULL);
        }
    }
    if (cmd->out[0] >= 0) {
        if (close(cmd->out[0])) {
            perror(NULL);
        }
    }
    execvp(cmd->cmd[0], cmd->cmd);
    exit(-1);
}

The problem is that reading from the pipe blocks forever:

COMMAND $ ls | wc
Created pipe, in: 5 out: 6
Foreground pid: 9042, command: ls, Exited, info: 0
[blocked running read() within wc]

If, instead of exchanging the process with execvp, I just do this:

if (!(pid_fork = fork())) {
    dup2(cmd->in[0], fileno(stdin));
    dup2(cmd->out[1], fileno(stdout));
    if (cmd->in[1] >= 0) {
        if (close(cmd->in[1])) {
            perror(NULL);
        }
    }
    if (cmd->out[0] >= 0) {
        if (close(cmd->out[0])) {
            perror(NULL);
        }
    }

    char buf[6];
    read(fileno(stdin), buf, 5);
    buf[5] = '\0';

    printf("%s\n", buf);
    exit(0);
}

It happens to work:

COMMAND $ cmd1 | cmd2 | cmd3 | cmd4 | cmd5 
Pipe creada, in: 11 out: 12
Pipe creada, in: 13 out: 14
Pipe creada, in: 15 out: 16
Pipe creada, in: 17 out: 18
hola!
Foreground pid: 9251, command: cmd1, Exited, info: 0
Foreground pid: 9252, command: cmd2, Exited, info: 0
Foreground pid: 9253, command: cmd3, Exited, info: 0
Foreground pid: 9254, command: cmd4, Exited, info: 0
hola!
Foreground pid: 9255, command: cmd5, Exited, info: 0

What could be the problem?